Driving with homonymous visual field loss: a review of the literature

Alex R Bowers

doi:10.1111/cxo.12425

. Author manuscript; available in PMC: 2017 Sep 1.

Published in final edited form as: Clin Exp Optom. 2016 Aug 17;99(5):402–418. doi: 10.1111/cxo.12425

Driving with homonymous visual field loss: a review of the literature

Alex R Bowers ¹

PMCID: PMC5012957 NIHMSID: NIHMS800863 PMID: 27535208

Abstract

Driving is an important rehabilitation goal for patients with homonymous field defects (HFDs); however, whether or not people with HFDs should be permitted to drive is not clear. Over the last 15 years there has been a marked increase in the number of studies evaluating the effects of HFDs on driving performance. This review of the literature provides a much-needed summary for practitioners and researchers, addressing the following topics: regulations pertaining to driving with HFDs, self-reported driving difficulties, pass rates in on-road tests, the effects of HFDs on lane position and steering stability, the effects of HFDs on scanning and detection of potential hazards, screening for potential fitness to drive, evaluating practical fitness to drive, and the efficacy of interventions to improve driving of persons with HFDs. Although there is clear evidence from on-road studies that some people with HFDs may be rated as safe to drive, others are reported to have significant deficits in skills important for safe driving including taking a lane position too close to one side of the travel lane, unstable steering and inadequate viewing (scanning) behaviors. Driving simulator studies have provided strong evidence of a wide range in compensatory scanning abilities and detection performance, despite similar amounts of visual field loss. Conventional measurements of visual field extent (in which eye movements are not permitted) do not measure such compensatory abilities and are not predictive of on-road driving performance; thus, there is a need to develop better tests to screen for visual fitness to drive of people with HFDs. We are not yet at a point where we can predict which HFD patient is likely to be a safe driver. Therefore, it seems only fair to provide an opportunity for individualized assessments of practical fitness to drive either on the road and/or in a driving simulator.

Homonymous field defects (HFDs) affect the same side (left or right) of the visual field in each eye. They result from post-chiasmal damage to the visual pathway and are contralateral to the side of the brain injury. The most common cause is stroke followed by traumatic brain injury, tumors, and brain surgery.¹ In a population-based survey of an urban community near Sydney, Australia, HFDs were found to affect nearly 1% of people over 49 years of age.² The field loss can range from small areas of homonymous paracentral field defects, to loss of one quadrant (homonymous quadranopia), or complete loss of one hemifield (homonymous hemianopia). The majority (at least 60%) of HFDs are incomplete with part of the vision in the affected hemifield spared.¹ Although spontaneous recovery, either complete or partial, may occur within the first three months after the injury,^3-6 improvement after six months is rare³ and about 30% of stroke patients still have HFDs after nine months.⁷

Driving is the primary mode of transportation in many countries. Cessation of driving decreases independence and employment opportunities, and increases the risk of depression.^8-11 Thus, returning to driving following a stroke or other brain injury is an important rehabilitation goal for many people with HFDs who were previously licensed to drive.¹¹ People with HFDs usually have relatively good visual acuity; it is the hemifield loss and compensating for that loss which present significant challenges when driving. In particular, HFDs may impair visuo-motor control, which could adversely affect steering stability and lane position, and may also impair the ability to detect and respond in a timely manner to hazards on the side of the field loss. The underlying brain injury may also cause other perceptual, attentional, cognitive and motor impairments that could adversely affect driving performance.^12-15 Motor impairments, such as hemiparesis, may be addressed by car modifications (such as adding a steering knob). By comparison, however, significant cognitive or attentional impairments are clear contraindications for driving. In particular, anosognosias (unawareness of the deficit) and hemispatial neglect, a failure to attend to the contralesional side of space,¹⁶ present safety concerns. Indeed, on-road studies of HFD drivers usually exclude people with significant cognitive decline or hemispatial neglect (see Table 1).

Table 1.

Summary of studies of HFD drivers that have included an on-road driving assessment

Study	Location	Exceptional case program	Sample characteristics	Pass n (%)	Reasons for failing and/or comments about driving skills	Other details

Tant 2001⁹⁷	Groningen, Netherlands	No	17 HH	2 (12)	No details given	All participants had previously failed a road test.
			Age 58 (range 29-76) y			They completed in-lab scanning training and on-road driving instruction and then re-took the road test; only 2 passed.
			> 6 mo since onset
			Stroke main aetiology (13/17)			One unmasked certified driving examiner rated performance
			No neglect
			All non-current drivers

Tant 2002²⁵	Netherlands	Yes	28 HH (complete and incomplete)	4 (14)	Unstable steering (11/28)	Referred to the Dutch Licensing Authority for a fitness-to-drive test where there was a concern about whether they should return to driving
					Unacceptable lateral deviations (8/28)
					Inadequate overview of traffic situation (5/28)
			Age 53 (range 24 – 76) y		Viewing behavior problems (7/28)
			> 6 mo since onset		Speed too high (8/28); speed too low (10/28)	Unmasked certified driving examiners scored performance
			Stroke main aetiology (23/28)		Uncertain driving behavior (7/28)
			No neglect		Driving too close on affected side (right HH) (4/28)
			MMSE ≥ 24		Driving too wide on right turns (right HH) (5/28)
			All non-current drivers

Racette 2005³⁴	Toronto, Canada	No	7 HH	4 (57)	No details given	Retrospective review of 1350 records (1976 – 2004) from a driving assessment and rehabilitation program
			4 HQ	4 (100)
			Stroke main aetiology (11/11)			Heterogeneous sample including 20 HFDs, 86 peripheral field loss and 25 monocular vision; but 9 HFDs not included in analysis as the road test outcome was unknown
			No neglect
			No significant cognitive decline
						Road test conducted by unmasked certified driving instructors
						People with significant cognitive or motor impairment excluded (OT assessment; standardized tests not used)
						Age of HFD subjects and duration of HFD unknown

Wood 2009³⁰	Alabama, USA	No	22 HH	16 (73)	In general HFD drivers were worse than NV for:	Recruited from Neuro-ophthalmology clinic
					Lane position
			8 HQ	7 (88)	Steering stability	Two masked evaluators in back seat rated driving skills (5-point scales) and determined safe/unsafe rating
			30 NV	30 (100)	Gap judgment
			Complete and incomplete HH/HQ			Pass rates are for the non-interstate part of the driving route (only 19 HFD participants completed the interstate section)
			Age 52 ± 20 y
			> 6 mo since onset
			Stroke main aetiology (18/30)
			No neglect
			No hemiparesis
			MMSE ≥ 24
			Current drivers: 24 of 30 HFD

Elgin 2010³²	Alabama, USA	No	22 HH	17 (77)	In general HFD drivers were worse than NV for:	Unmasked certified driving rehabilitation specialist rated driving skills (5-point scale), determined safe/unsafe, and noted interventions
					Vehicle control skills
			8 HQ	7 (88)	Adjustment of speed to traffic conditions
			30 NV	30 (100)	Driving style (margin of anticipation)
					Reactions to unexpected events
			Same cohort as Wood 2009³⁰		Bad driving maneuvers
					Verbal interventions:
					10/22 HH, 4/8 HQ, 5/30 NV
					Braking/steering interventions:
					9/22 HH, 1/8 HQ, 1/30 NV

Wood 2011³⁵	Alabama, USA	No	22 HH	HFD overall	Unsafe drivers:	Lane position rated from video data (3 external cameras of road ahead); Sudden braking and acceleration quantified from an accelerometer and inertial sensors; Speed quantified from GPS data.
					Less stable lane position than safe drivers
			8 HQ	23 (77)	Lane position bias toward affected side (6/7)
			30 NV		Drove more slowly than safe drivers	Head and eye movements scored from video data (one camera pointed toward subject)
					More sudden braking events than safe drivers
			Same cohort as Wood 2009³⁰		Similar number of head movements to both sides
					Less excursive eye movements than safe drivers	Two masked observers used 5-point scales to rate lane position and amount of eye scanning, and to count number of lateral head movements
					Safe drivers:
					Lane position bias toward affected side (4/23)
					More head movements to affected side

Parker 2011²⁹	Alabama, USA	No	17 HH	14 (82)	HFD drivers reported greater difficulty than NV drivers for driving situations relying on peripheral vision and involving independent mobility	Unmasked certified driving rehabilitation specialist rated driving skills (5-point scale), determined safe/unsafe, and noted interventions
			7 HQ	7 (100)
			24 NV	24 (100)	The levels of self-reported driving difficulties did not differ between HFD drivers rated as safe and unsafe	Self-reported driving difficulties and driving patterns evaluated with a modified version of the Driving Habits Questionnaire²⁹
			Subset of the cohort from Wood 2009³⁰

Dow 2011²¹	Québec, Canada	Yes	21 HH	19 (90)	No details given	Retrospective review of all requests (103) in a 4-month period (Jan – April 2009) for exemptions from the 100° field standard in Québec, Canada
			32 HQ	25 (78)
			50 other types of field loss	47 (94)		Heterogeneous sample including 53 HFDs, 35 “scotomas”, 14 “central vision only”, and 1 monocular vision
			Age range 20 – 90 y
			No neglect			Road test conducted by SAAQ (Société de l’assurance automobile du Québec)
						Applicants needed to have a favorable recommendation from an eye specialist; neglect was a specific exclusion criterion

Bowers 2012²⁴	Ghent, Belgium	No	12 complete HH	Not reported	Inadequate scores mostly for:	Recruited from people who had applied to the Belgian Road Safety Institute for a fitness-to-drive evaluation or annual re-evaluation
					Unstable steering
			Age 49 (range 29 to 68) y		Driving too slowly
			> 3 mo since onset		Path too wide or too tight on left turns	Masked observer in the back seat rated driving skills, responses to potential hazards, and noted interventions.
			Stroke main aetiology (8/12)		Poor gap judgment (hesitating) at intersections
			No neglect			2 hours of supervised driving practice with prisms.
			3 hemiparesis
			MMSE ≥ 24		Interventions:	Test drives with real and sham oblique peripheral prisms in counterbalanced order. Responses to unexpected hazards were better with real than sham prisms
					Total 29 interventions: 15 braking; 8 steering; 5 verbal advice; 1 accelerator
			Current drivers: 4 of 12 HH		Non-current drivers more interventions than current

deHaan 2014²³	Netherlands	Yes	21 HH	10 (48)	Unfit drivers had lower scores than fit drivers for:	All persons (n = 86) with HH referred to the Dutch licensing authority (Jan 2010–July 2012) for fitness-to-drive test for re-licensure; 60 did not meet the legal requirements for the exceptional-case provision
					Judgment when overtaking
			5 HQ	4 (80)	Anticipatory viewing behaviors
			Age 52 ± 12 y		Tactical anticipation of changing traffic situations
			> 6 mo since onset		Steering stability
			Stroke main aetiology (18/26)		Speed adaptation	15 unmasked certified driving examiners scored performance
			No neglect
			MMSE ≥ 24		5/5 right HFD passed; 9/ 21 left HFD passed	Non-current drivers were allowed to practice vehicle control during one lesson before the road test.
			Current drivers: 3 of 26 HFD		Problems with steering and lane position:
					Lane choice not sufficient 8/21 left HH
					Unstable steering 11/21 left HH
					Inadequate lane position in 7/21 left HH (3 too much to right, 1 too much to left, and 3 too much fluctuation in lateral position)
					Interventions:
					Braking or steering interventions for 5/12 unfit
					Looking toward affected side caused inappropriate lane position 3/12 unfit

Kasneci 2014³¹	Tübingen, Germany	No	10 mixed HFD	6 (60)	HFD reasons for failing:	Recruited from Neuro-ophthalmology and Glaucoma clinics; sample included 10 HFDs and 10 Glaucoma;
					Difficulties with scanning (3/4)
			20 NV	17 (85)	Difficulties with lane keeping (2/4)
			Complete and incomplete HH/HQ		Difficulties with speed (2/4)	1 camera recording road; 1 camera recording head and shoulders and mobile eye tracker
					Difficulties with gap judgment (1/4)
			Age 53 ± 13 y		Difficulties with left turns (1/4)	Masked evaluator in backseat determined pass/fail
			> 6 mo since onset		Inadequate reactions to pedestrians, traffic (1/4)
			No neglect		Difficulty following traffic lights (1/4)	Two unmasked observers scored lane position, lane keeping, speed, gap judgment, and head and shoulder movements on 6-point rating scales from the recorded video data
			No hemiparesis
			MMSE ≥ 24		NV reasons for failing:
			> 6 mo since onset		Difficulties with lane keeping (2/3)
					Inadequate reactions to pedestrians, traffic (2/3)
			Current and non-current drivers (numbers of each not specified)		Difficulties with scanning (1/3)
					Speeding (1/3)
					Difficulties with left turns (1/3)
					Skill ratings of HFD that failed:
					Poorer lane keeping score than NV who passed;
					Fewer head movements than HFD and NV who passed

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Abbreviations: HH - Homonymous hemianopia; HQ - Homonymous quadranopia; NV - Normal vision; HFD - Homonymous field defect; MMSE - Mini-Mental State Examination; mo – months

Whether or not people with HFDs should be permitted to drive is not clear. Moreover, regulations pertaining to visual requirements for licensing are largely without scientific basis. Currently there are no data on accident rates of HFD drivers. However, the last 15 years has seen a surge in the number of studies which have evaluated performance of HFD drivers in on-road and simulated driving. The purpose of this review, therefore, is to summarize the current status of research relevant to driving with HFDs. The focus is on HFD drivers without significant cognitive decline or hemispatial neglect. The following topics are addressed: regulations pertaining to driving with HFDs, self-reported driving difficulties, pass rates in on-road tests, the effects of HFDs on lane position and steering stability, the effects of HFDs on scanning and detection of potential hazards, screening for potential fitness to drive, evaluating practical fitness to drive, and the efficacy of interventions to improve driving of persons with HFDs.

Regulations pertaining to driving with HFDs

People with HFDs are denied licensure in many jurisdictions because they do not meet the visual field requirements. For example, in Australia,¹⁷ Europe,^{18, 19} Canada,^{20, 21} and many states in the USA,²² they are unlikely to meet the minimum horizontal peripheral visual field extent requirement (Table 2). Furthermore, in countries with regulations pertaining to the integrity of the central visual field, including Australia,¹⁷ the UK¹⁸ and Canada,²⁰ people with paracentral HFDs may fail the central visual field requirements even if they meet the minimum horizontal extent (Table 2). However, there are some states where drivers with HFDs do meet the field requirements and may legally drive (e.g., New Hampshire has no field requirement²²). Furthermore, the European Driving Licence Directive¹⁹ permits all member countries to make exceptions for persons with stable vision loss who do not meet the minimum field requirements. The process typically involves a thorough medical and vision examination before the applicant is permitted to take a specialised road test to demonstrate their practical fitness to drive.^{18, 23} It appears, however, that this provision has been implemented in only a minority of European Union countries including Belgium,²⁴ the Netherlands^{23, 25} and the UK.¹⁸ In addition, some countries outside the European Union also operate exceptional-case programs (e.g., Switzerland and Canada^{20, 21}) and the vision standards for driving in Australia¹⁷ include a provision that a conditional licence may be considered when a driver does not meet the visual field requirements for an unconditional licence.

Table 2.

Visual field requirements for driving in selected countries

Country	Peripheral field requirements	Central field requirements	Exceptional case provisions (Drivers that do not meet the requirements)

Australia¹¹⁸	≥ 110° horizontal extent within 10° above and below the horizontal midline	No significant field loss (scotoma) within a central 20° radius or other scotoma likely to impede driving performance.	A conditional licence may be considered “taking into account the nature of the driving task and information provided by the treating optometrist or ophthalmologist.”
Australia¹¹⁸	Complete HH or HQ unlikely to meet the requirement	Incomplete HH or HQ or paracentral HFDs might fail to meet the requirement

Canada^{21, 119}	≥ 120° continuous horizontal extent (110° in Quebec)and 15° continuous above and below fixation	Continuous horizontal extent implies that there cannot be any signifcant scotomas in the central field	Recommended that the “individual undergo a special assessment for fitness to drive by the appropriate licensing authorities and that the following be taken into consideration: (1) favourable reports from the ophthalmologist or optometrist; (2) good driving record; (3) stability of the condition; (4) no other significant medical contraindications; (5) other references (e.g. professional, employment, etc); (6) functional assessment.”
Canada^{21, 119}	Complete HH or HQ unlikely to meet the requirement	Incomplete HH or HQ or paracentral HFDs might fail to meet the requirement

Europe¹⁹	≥ 120° horizontal extent,	No defects within a central 20° radius	“The driver should undergo examination by a competent medical authority to demonstrate that there is no other impairment of visual function, including glare, contrast sensitivity and twilight vision. The driver or applicant should also be subject to a positive practical test conducted by a competent authority.”
	≥ 50° left and right
	≥ 20° above and below	Incomplete HH or HQ or paracentral HFDs might fail to meet the requirement
	Complete HH or HQ unlikely to meet the requirement

United States of America²²	Minimum horiziontal extent varies, most commonly 70° to 140°; some states have no minimum requirement	No requirements	Some states (e.g., Alabama) might permit a restricted license for a driver that doesnot meet the visual field requirement
United States of America²²	Complete HH or HQ unlikely to meet the requirement in states with a minimum ≥ 90°:	No requirements

United Kingdom¹⁸	≥ 120° horizontal extent	No significant defect within 20° of fixation above or below the horizontal meridian	“Drivers who have previously held full driving entitlement may be eligible to reapply to be considered as exceptional cases on an individual basis. The defect must have been present for at least 12 months, caused by an isolated event or a non-progressive condition, and there must be no other condition or pathology present which is regarded as progressive and likely to be affecting the visual fields. In order to meet the requirements of European law, the Driver Vehicle Licensing Authority will, in addition, require clinical confirmation of full functional adaptation. If reapplication is then accepted, a satisfactory practical driving assessment, carried out at an approved assessment centre, must subsequently be completed.”
	≥50° left and right
	Complete HH or HQ unlikely to meet the requirement	Incomplete HH or HQ or paracentral HFDs might fail to meet the requirement
		HFDs “which come close to fixation, whether hemianopic or quadrantanopic, are not normally accepted as safe for driving”

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Self-reported driving difficulties and habits

Persons with HFDs have reported difficulties with driving on vision-related quality of life questionnaires,^{26, 27} such as the National Eye Institute Visual Function Questionnaire,²⁸ and a modified version of the Driving Habits Questionnaire.²⁹ In a recent study by Parker et al.,²⁹ current HFD drivers reported significantly greater difficulty than age-matched normally-sighted drivers in situations relying on peripheral vision (e.g., left-hand turns across traffic, driving through intersections without lights and seeing objects off to the side) and involving independent mobility (e.g., driving alone, in unfamiliar areas and navigating to places), but not low visibility conditions (driving at night, at dusk and in rain). Specifically, for maneuvers relying on peripheral vision, the mean difficulty scores were 4.42 ± 0.61 and 4.95 ± 0.16 for HFD and normally-sighted drivers, respectively, rated on a scale from 2 (extreme difficulty) to 5 (no difficulty). Mean difficulty scores for independent mobility were 4.35 ± 0.80 and 4.92 ± 0.27, and for low visibility conditions were 4.22 ± 0.85 and 4.69 ± 0.36, for HFD and normally-sighted drivers, respectively. HFD drivers were also more likely to indicate that someone had suggested they stop or limit their driving in the previous year.²⁹ However, self-ratings of driving quality were similar for the two groups with the majority rating their driving as good or excellent.²⁹ On average, HFD drivers drove substantial distances (173 miles per week), but made fewer trips and drove fewer miles than normally-sighted drivers (281 miles).²⁹

Pass/fail rates in on-road studies

A number of studies have evaluated driving performance of persons with HFDs on open-road test routes along public roads, either for research purposes or as part of an exceptional-case program (Table 1). The tests were usually conducted in a dual-control car with a driving examiner or rehabilitation specialist in the front passenger seat.^{23, 30, 31} The test routes were typically about 45 minutes in duration with a variety of traffic environments (residential, city, highway) and maneuvers.^{23, 30, 31} The terms used to describe overall performance on the road test varied across studies, including pass/fail,^{21, 25, 31} safe/unsafe,^30-32 and fit/unfit.^{21, 23, 25, 31} Despite these differences in terminology, passing the test, being rated as safe to drive, or being rated as fit to drive meant that the participant would have been permitted to drive based on their road test performance. Therefore, in this review the terms pass/safe/fit are used interchangeably to mean the person would have been granted a license to drive while fail/unsafe/unfit are used interchangeably to mean the person would not have been granted a license to drive. For a detailed review of methods of evaluating on-road driving, please see Owsley et al..³³

Pass rates reported in on-road studies varied widely, from as low as 14%²⁵ to as high as 90%²¹ for drivers with hemianopia, and around 80%^{21, 23} to 100%³⁴ for drivers with quadranopia (Table 1). This wide variability is not surprising given that the studies all had relatively small samples of HFD drivers (from 10 to 53 participants) and differed in recruitment strategies, sample characteristics and geographic location (Table 1). Differences in on-road test methodology, test route difficulty and the purpose of the road test (whether for research or an exceptional-case evaluation) may also have contributed to between-study differences in pass rates. While road tests conducted purely for research^{24, 30, 31} used the same masked evaluators for all tests, those conducted for practical-fitness-to-drive assessments in exceptional-case evaluations^{21, 23, 25} used a number of different driving examiners who were not masked to participants’ visual status. Furthermore, two of the studies^{21, 34} were retrospective reviews while the remainder^{23-25, 30-32, 35} used a prospective study design. Despite such differences, results of on-road studies clearly suggest that some people with HFDs may be considered safe to drive.

The lowest pass rates (14%) were reported for drivers with hemianopia referred to the Dutch licensing authority for a practical-fitness-to-drive test when there was a question/concern about whether they should return to driving.²⁵ By comparison, much higher pass rates (90%) were found for drivers with hemianopia who had already received a favorable recommendation from an eye specialist and were requesting an exception to the visual field requirement in Québec, Canada.²¹ High pass rates (73%) were also found in a study in Alabama where participants with hemianopia were mostly current drivers or had stopped driving no more than two years previously and wished to return to driving.³⁰

Drivers that failed a road test were most commonly reported to have problems with lane position, steering stability, and inadequate viewing (scanning) behaviors (see Table 1),^{23, 25, 30-32} which in some cases required a steering or braking intervention by the driving examiner.^{23, 24}

Effect of HFDs on lane position and steering stability

The ability to steer a vehicle so that it remains on a steady course within the travel lane is an important aspect of safe driving. Steering behaviors that result in incursions into adjacent travel lanes endanger both the driver and other road users. On-road studies^{23, 24, 30-32, 35, 36} of HFD drivers have used observer ratings of steering stability and lane position as measures of steering control, whereas driving simulator studies^{37, 38} have used positional data recorded during the drive to quantify lateral offset of the virtual vehicle from the center of the lane, variability in lateral lane offset and number of lane boundary crossings.

While some HFD drivers in on-road studies were reported to have vehicle control skills that did not differ from those of normally-sighted drivers,^{30, 31}other HFD drivers were noted to have deficits in steering stability and lane positioning.^{23, 25, 30, 31} Possible reasons for these deficits include impairments in visuo-motor control, errors in spatial judgments, and/or strategic compensation.

Impaired visuo-motor control

Steering involves use of visual feedback to minimize the vehicle’s lane-position error and to provide guidance about future road trajectory.^39-41 Given that drivers with HFDs may lack visual feedback from both peripheral and central vision on one side, it might, therefore, be expected that they would have impaired visuo-motor control of steering. In a simulated steering task, using a joystick to keep a randomly moving target at the center of a screen, participants with hemianopia had greater target position error along the horizontal axis than participants with a full field of vision, but similar performance along the vertical axis.⁴² These findings suggested impaired visuo-motor control only along the axis of the field defect. Consistent with these results, HFD drivers had significantly greater lateral lane position variability³⁷ and made more lane boundary crossings^{37, 38} than normally-sighted drivers in a driving simulator. Moreover, problems with unstable steering and unacceptable lane positioning were more common among HFD drivers who failed a road test than those who passed.^{23, 25, 30, 32} In a few cases, unstable steering was reported to occur when participants rated as unsafe on the road test attempted to compensate for the field loss by visual scanning toward the affected side.^{22, 23}

Errors in spatial judgments

In addition to impaired visuo-motor control, drivers with HFDs may have impaired perception of space, which could result in lane position biases when driving. Patients with HFDs (without hemispatial neglect) typically misperceive the midpoint of a horizontal line to be shifted slightly toward the affected hemifield, e.g., in line bisection tasks.^{43, 44} These small visuo-spatial misjudgments have been found for purely perceptual tasks,^{45, 46} as well as visuo-motor tasks, such as pointing,⁴⁶ moving a hand between two closely-spaced obstacles⁴⁶ and using a joystick to keep a moving target at screen center.⁴² Consistent with these visuo-spatial misjudgments, HFD participants in a driving simulator adopted an average lane position on straight road segments toward their unaffected side, whereas controls maintained a more central lane position.³⁷

Similar lane position biases have been observed in some on-road studies; however, the biases were mostly for drivers rated as unsafe. Wood et al.³⁵ reported that 6 of 7 unsafe drivers with HFDs and 4 of 23 safe drivers had a lane position bias toward the unaffected side. De Haan et al.²³ reported that of 12 unsafe drivers with left HFDs, 3 held a lane position too much toward the unaffected side and 1 too much toward the affected side. By comparison, Tant et al.,²⁵ reported that 4 of 13 unsafe drivers with right HFD drove too close to the right (affected) side of the road. Unlike in the driving simulator study,³⁷ none of these on-road studies directly measured vehicle position relative to the lane center; instead, lane position errors were based on observer ratings of driving skills during the road test^{23, 25} or from subsequent analysis of video recordings of the forward roadway.³⁵ Thus small biases in lateral lane position that could be measured in the simulator might not have been noticed by raters during an on-road test where only biases of a relatively large magnitude that raised a safety concern were likely to have been observed.

Strategic compensation

Rather than impaired perception of space, it is possible that spatial biases in lane position might represent strategic compensation. Drivers with HFDs might purposefully hold a lane position toward the seeing hemifield to increase the safety margin on their affected side. Indeed, both safe and unsafe drivers in on-road studies^{23, 35} have exhibited the tendency to take a lane position toward the unaffected side, which may be suggestive of strategic compensation. If HFD drivers hold a position too close to the edge of the travel lane on the unaffected side, they should be aware of either the oncoming traffic on the opposite side of the road, or the proximity of the road edge or other traffic on their side of the road (depending on the side of the HFD, the side of the road on which they are driving and the number of lanes in each direction) and should be able to correct their error. However, that is not the case on the affected side. Results from on-road studies^{23, 35} suggest that HFD drivers rated as safe have adequate lane position control and do not put themselves or other road users in danger by going too far toward one side of their travel lane. However, it appears that some HFD drivers rated as unsafe take a lane position too far toward the unaffected side or, in some cases, take a lane position too far toward the affected side, such that they become a danger to other traffic, requiring a steering intervention by the driving examiner.^{23, 24}

Other factors

Other factors, such as hemispatial neglect or motor impairments, might affect steering and could have contributed to inconsistencies^{23, 25, 35, 37} in lane position biases across studies. For example, patients with left hemispatial neglect tend to veer to one side when walking or driving a wheelchair (in fact, deviations to both sides having been reported).⁴⁷ However, severe neglect was unlikely to be a factor affecting lane position in studies reporting biases for HFD drivers, because testing positive for neglect on paper-and-pencil tests was an exclusion criterion in all studies.^{23, 25, 35, 37} Nevertheless, the possible effects of subclinical neglect cannot be discounted.⁴⁸ Hemiparesis, which might affect steering, was an exclusion criterion in only one³⁵ of the three on-road studies that reported lane position biases. Interestingly, two participants with left HFD and hemiparesis demonstrated opposite lane offsets in a driving simulator;³⁷ one had the most rightward lane position on straight road segments of the six left HFD drivers, while the other was the only left HFD driver with a leftward lane offset.

It is also possible that lack of driving experience after the onset of the HFD could contribute to impaired lane position and steering instability of non-current HFD participants. Two studies^{23, 24} reported that non-current drivers were permitted to undertake a small amount of supervised driving practice before the road test; other studies^{25, 30-32} did not address this detail in the methods. However, the extent to which lack of recent driving experience might affect lane position and steering stability has not been systematically investigated.

Effects of HFDs on scanning and detection of potential hazards

Timely detection of potential hazards, such as a pedestrian stepping into the road or a speeding vehicle approaching an intersection, is critical when driving. Depending on the environment (e.g. rural vs. city center), such events might be encountered only rarely when driving, but any failure to detect a hazard could have fatal consequences. In order to detect a potential hazard on the side of the field loss, drivers with HFDs have to scan far enough toward that hemifield to bring the object of interest into a seeing area of their visual field. For drivers with complete hemianopia, this means scanning toward the affected side until the object at least falls on their fovea. The scan might involve only eye movements, or both eye and head movements, especially when scanning far toward the affected side. Although this might seem a straightforward method of compensating for the field loss, there is no cue from peripheral vision as to when to scan or how far to scan. Scans toward the affected field are volitional not reflexive. Patients might exhibit good scanning in simple tasks in the clinic, but fail to remember to scan under conditions of high attentional load when driving. Even if they remember to scan and notice a potential hazard, the scan toward the affected side might be delayed relative to the appearance of the hazard so there might not be sufficient time to respond by braking or steering.

Detection performance when driving

Evaluating responses to potential hazards during an open-road driving test provides the greatest real-world validity,³³ but there are a number of limitations that have to be considered, including safety concerns and the lack of control over when and if an unexpected hazard might occur.²⁴ By comparison, driving simulators offer safe, repeatable conditions in which the effects of HFDs can be systematically investigated.^{49, 50} However, simulations do not necessarily replicate all of the complexities of real-world situations.³³

The first driving simulator studies (Table 3) to investigate detection performance of HFD participants included only a small number of unexpected events in a brief test^{38, 51} (e.g., the appearance of a single deer in 3 minutes of test driving⁵¹) or used peripheral detection targets which appeared at fixed positions on the simulator display.^{52, 53} These targets would have been perceived as moving with the vehicle (as if on the windshield), rather than moving separately from it, as would be the case for real hazards. The next generation of studies^{49, 50} used numerous presentations of detection targets (either pedestrians⁴⁹ or vehicles⁵⁰) that were part of the driving scene (Table 3). However, there were still limitations, either the detection target did not present an imminent collision risk,⁴⁹ or the participant did not steer the vehicle⁵⁰ and had only limited control of speed.⁵⁰ The frequency of events (e.g. about one per minute^{49, 54, 55}) was higher than would be the case when driving on quiet rural roadways where an HFD driver would need to detect only infrequent peripheral targets. However, the use of numerous presentations enabled a robust assessment of detection performance within a reasonable time frame. More recent studies^54-57 have used scenarios in which realistic potential hazards posed an imminent collision risk and participants controlled vehicle steering^54-57 and speed,^{54, 56} or used cruise control,⁵⁵ or were driven at a predefined speed without being able to use either the accelerator or the brake⁵⁷ (Table 3).

Table 3.

Summary of driving simulator studies of HFD drivers

Study	Sample characteristics	Simulated driving task	Results and comments

Lovsund 1991⁵²	26 mixed HFDs	Evaluated response times (brake) for detection of static flickering stimuli at 24 fixed locations; 10 presentations at each location	Detection performance on the affected side
			Only 3 HFD subjects had reaction times within the range of NV subjects
	> 1 y since onset
			Eye movements
	5 other field loss	Recorded eye movements of 4 HFD subjects with a head-mounted IR eye tracker (NAC Eye Mark Recorder model V)	One HQ subject with better (shorter) reaction times made a greater proportion of fixations to the blind side than did one HQ subject with poorer (longer) reaction times
	20 NV
		2 hours driving; 20 minutes practice	Comments
		Subjects controlled speed and steering	Detection stimuli would have been perceived as moving with the vehicle (as if on the windshield) rather than moving separately from it.
		Field of view: 120° H by 30° V	Age, cognitive status, aetiology and driving status not specified for HFD subjects

Szlyk 1993³⁸	4 HH & 2 HQ	Evaluated lane position, speed, and brake responses to traffic signals	Lane position and steering
			HFD subjects made more lane boundary crossings and had greater lane position variability than NV
	Age 71 y (53 – 80)
	2 mo since onset	Single camera video-recorded subjects; eye and head position quantified manually from the recordings	Eye and head movements
	3 had neglect		The 3 HFD subjects without neglect had similar amount of eye position variability but greater head position variability than NV
	Stroke main aetiology (6/6)	5 minutes driving; 15 minutes practice
	All non-current drivers		Other driving skills
		Subjects controlled speed and steering	No differences between HFD and NV subjects in braking responses or speed
	7 older NV (62-83 y)	Field of view: 160° H by 35° V
			Comments
			Very short drive, small sample size and 3 subjects had neglect;
			Eye and head position sampled only once every 4 s.

Schulte 1999⁵¹	6 mixed HFDs (from complete HH to paracentral scotomas)	Evaluated speed, number of accidents/errors and response times (brake) to a suddenly-appearing deer;	Detection performance
			No difference in response times between HFD and NV subjects
		5.2 km drive and 2.6 km practice	Other driving skills
	Age 45 y (29–74)		No differences in speed or number of accidents/errors between HFD and NV subjects
	> 6 mo since onset	Subjects controlled speed and steering
		Field of view: 21° H by 16° V	Comments
	Current drivers: 3 of 6 HFD		Very short drive with only one unexpected event
			Small sample with heterogeneous HFDs
	3 other field loss		Deer crossed from right to left; therefore detection on the affected side was evaluated once for subjects with right HFDs and not at all for subjects with left HFDs
	13 NV		“Errors” not defined

Bowers 2009⁴⁹	12 complete HH	Evaluated detection rates and response times (horn press) for stationary pedestrians appearing at 4° or 14° on right or left from car heading direction; total 72 pedestrian events at each session	Detection performance on the affected side; HH subjects had:
			Very wide range of detection rates
	Age 50 ± 13y		Lower detection rates at the 14° than the 4° eccentricity
	≥ 1 y since onset		Lower detection rates and longer reaction times than NV subjects
	Stroke main aetiology (10/12)
	No neglect	2 sessions; each 60 minutes driving and 20 minutes practice	Lower detection rates on the affected side associated with:
	MMSE ≥ 24		Older age, but not driving status (current or non-current), side of HH or duration of HH
		Subjects controlled speed and steering
	Current drivers:6 of 12 HH	Field of view: 225° H by 32° V	Detection performance on the unaffected side, HH subjects had:
	12 age-matched NV		Detection rates that did not differ from the NV subjects, but reaction times were longer

Bowers 2010³⁷	Same cohort as Bowers 2009⁴⁹	Evaluated lane position and steering stability on straight road segments, curves and turns	Lane position; HH subjects:
	12 complete HH		Lane position toward the seeing side on straight road segments; more noticeable in drivers with right than left HH; NV maintained a central lane position
	12 age-matched NV	2 sessions; each 60 minutes driving and 20 minutes practice	Lane position on curves and turns which would provide a safety margin on their affected side; NV cut curves (to left on left curve, right on right curve)
		Subjects controlled speed and steering
	2 HH subjects had hemiparesis
		Field of view: 225° H by 32° V	Steering stability; HH subjects:
			More variable lane position than NV subjects and were out of lane more often
			Factors affecting lane position and steering of HH subjects
			Non-current drivers had more variable lane position than current drivers
			Age was not related to lane position or steering stability

Bronstad 2011⁵⁹	Case series: 3 paracentral HFDs	Evaluated detection rates and response times (horn press) for pedestrians that walked or ran toward the road appearing at 4° or 14° on right or left from car heading direction; total 52 pedestrians events per session	Detection performance on the affected side; HFD subjects had:
	Ages: 38, 60 and 76 y		Lower detection rates, longer response times and lower timely response rates (would have been able to stop in time to avoid a collision) than NV subjects
	No neglect		Comments
	All current drivers	2 sessions; each 60 minutes driving and 20 minutes practice	More realistic detection task than used in Bowers 2009; pedestrians were programmed to move on a collision course (maintaining a constant eccentricity) with respect to the subject’s vehicle, but stopped before entering the subject’s travel lane.
	3 age-matched NV	Subjects controlled speed and steering
		Field of view: 225° H by 32° V

Papageorgiou 2012a⁵⁰	20 HH & 10 HQ (complete and incomplete)	Measured number of virtual collisions in an intersection collision avoidance task; participants “drove” along a 172.5 m straight road and through an intersection; they adjusted their speed (18 – 61 km/h) using a joystick to avoid collisions with cross traffic.	Collision avoidance on the affected side; HFD subjects had:
			More collisions than NV subjects at both traffic densities
	Age 46 ± 16 y		No difference in numbers of collisions on affected and unaffected sides a low traffic density
			Significantly more collisions on affected than unaffected side at high traffic density
	> 6 mo since onset
	Stroke main aetiology (30/30)		Collision avoidance on unaffected side; HFD subjects had:
	No neglect	Cross traffic moved at a constant speed of 50 km/h at two traffic densities (low and high).	More collisions than NV subjects at the lower density but not the higher traffic density.
	No cognitive decline		More collisions associated with:
	Current drivers: 8 of 30 HFD⁵⁸	15 trials for each traffic density; 5-10 minutes practice	Older age (highly significant)
	30 age-matched NV	Subjects did not control steering, only speed.	Lesser amounts of spared (remaining) field in the affected hemifield (weak association)
		Field of view: 150° H by 70° V	Side of HFD and duration of HFD were not predictive of the number of collisions
			Comments
			Subjects could not adjust speed from 22.5m before the intersection

Papageorgiou 2012b⁵⁸	Subset of subjects from Papageorgiou 2012a⁵⁰:	Evaluated gaze behaviors while subjects performed the intersection collision avoidance task	Gaze movements
	14 HFDs	Head mounted IR eye tracker (Applied Science Laboratory) and separate IR head tracker (ARTtrack/Dtrack)	Relative to HFD subjects with inadequate performance, those with adequate performance showed more active gaze scanning in both traffic densities with longer scanpaths, more gaze shifts, more fixations on virtual vehicles, less ‘‘straight-ahead’’ fixations on the intersection and larger saccadic gaze amplitudes towards both hemifields. There were no significant differences between the two HFD groups for total number of fixations, fixation durations, proportion of fixations and proportion of gaze eccentricity to the blind hemifield
	19 age-matched NV	Using a median split, HFD subjects were divided into “adequate” (n = 5) and “inadequate” (n = 9) performance groups based on the number of collisions (unequal numbers in each group because some subjects with insufficient gaze tracking data were excluded)	In comparison to NV subjects, adequate and inadequate HFD subjects exhibited a higher proportion of fixations and gaze eccentricity to the blind hemifield, and shorter saccades to the blind hemifield
		Field of view: 150° H by 70° V

Bahnemann 2014⁵⁵	14 HH (complete and incomplete)	Evaluated detection rates and response times (pressing brake or using indicator, or both) for potentially hazardous objects (coloured balls and wild boars) crossing from the right or left of the road; 24 hazards in total, but data for only the first 16 reported	Detection performance on the affected side
	Age 57 ± 14 y		Low performance HFD group detected fewer objects than both the high performance group and the NV group (high performance and NV groups did not differ)
	≥ 6 mo since onset		Reaction times of both HFD groups were longer than those of the NV group
	Stroke main aetiology (14/14)
	No neglect		Detection performance on the unaffected side
	No cognitive impairment	Head-mounted IR tracker (EyeSeeCam) pupil tracker; analysed eye and head movements separately	HFD subjects detected all objects and reaction times were not different to NV
	11/14 HH held a license (unclear whether currently driving)		Eye movements
		Using a median split, HFD subjects were divided into “high” (n = 6) and “low” (n = 8) performance groups based on the number of missed objects (high < 2 missed; low ≥ 2 missed)	Low performance group had a narrower horizontal spread of eye fixations than the high performance and NV groups, and also smaller amplitude eye saccades overall.
	14 age-matched NV		The high performance group, but not the low performance group, made a greater percentage of fixations in the affected hemifield than the NV group in the right hemifield
		1 session with 30 minutes of test drives; training sessions were completed before the test drives	Head movements
		Speed (70 km/h) maintained by cruise control; subjects controlled steering	No difference between HFD and NV subjects in number or magnitude of head movements; also no difference in proportion of head movements toward the affected (HFD) or right (NV) hemifield
		Field of view: 58° H by 44° V	Comments
			Eccentricity at which hazards appeared was not reported
			Saccade amplitudes were not reported separately for the affected and unaffected sides

Alberti 2014⁵⁴	12 complete HH	Evaluated detection rates and response times (horn press) for (a) stationary pedestrians and (b) approaching pedestrians that walked or ran toward the road on a collision course, appearing at 4° or 14° on right or left from car heading direction; total 52 pedestrians events per session (one session for approaching and one for stationary pedestrians)	Detection performance on the affected side
			Very wide range of detection rates
	Age 39 ± 18 y		Detection rates were higher but reaction times were longer for approaching than stationary pedestrians on the affected side, especially at the larger eccentricity;
	≥ 6 mo since onset
	Stroke main aetiology (8/12)		The overall proportions of potential collisions (missed detections and responses that were too late to avoid a collision) were not different for the two types of pedestrians.
	No neglect
	MMSE ≥ 24
		Recorded gaze movements using a 6-camera IR remote eye-and-head tracking system (Smart Eye Pro)	Lower detection rates on the affected side associated with:
	Current drivers: 2 of 12 HH		Older age and a shorter HH duration
		2 sessions; each 60 minutes driving and 30 minutes practice	Comments
		Subjects controlled speed and steering	Age-matched NV group not included because the purpose was to compare detection performance of HH subjects for stationary and approaching pedestrians
		Field of view: 225° H by 37° V	For gaze data results, please see Bowers et al., 2015⁶⁰ and Alberti et al. 2016⁷³

Bowers 2014⁷⁰	13 complete HH (11 from Bowers 2009⁴⁹)	Evaluated detection rates for stationary pedestrians at T-intersections; total 10 intersection pedestrians (among total 144 other pedestrian events, Bowers 2009)	Detection rates; HH subjects had:
			Lower detection rates than NV subjects for intersection pedestrians at large eccentricities on the affected side (about 80° with respect to straight ahead)
	Age 54 ± 9 y
	≥ 4 mo since onset	Head movements recorded with a head mounted IR tracker (TrackIR 3)	Head movements; HH subjects:
	Stroke main aetiology (11/13)		Tended to make the first head scan to the affected side when approaching an intersection
	No neglect		Did not differ from NV subjects in the total number of head scans, but made more to the affected side than did the NV subjects to the corresponding side.
	MMSE ≥ 24	2 sessions; each 60 minutes driving and 20 minutes practice
		Subjects controlled speed and steering	Made smaller magnitude head scans than NV subjects
	Current drivers: 6 of 13 HH		Did not differ from NV subjects in rates of failing to head scan
		Field of view: 225° H by 32° V
	12 age-matched NV		Detection failures of HH subjects on the affected side were associated with
			Failing to head scan in the direction of the pedestrian
			Head scanning toward the pedestrian, but not far enough
			Comments
			Eye movements were not recorded
			Pedestrians at intersections were stationary and did not present an imminent threat

Kübler 2014⁵⁶	8 mixed HFDs (incomplete and complete HH, HQ, and paracentral scotomas)	Evaluated responses to nine hazards along a 37.5km route, including pass/fail for each hazard and whether the hazard was fixated;	Responses to hazards
			Majority of failures were for the first of the nine hazards
			4 HFD, 3 glaucoma and 1 NV subject failed the driving test
	Age 52 ± 12 y	Pass/fail determined by a masked driving instructor according to the criteria of the official driving test in Germany	Results of eye and head tracking were reported in a subsequent paper (see Kubler 2015⁷²)
	> 6 mo since onset
	Stroke main aetiology (6/8)		Comments
	No neglect⁷²	Eye and head movements were recorded (see Kubler 2015⁷²)	Galvanic skin conductance (GSC) and heart rate (ECG) were also recorded and evaluated as stress indicators in response to each hazard
	MMSE ≥ 24	About 36 minutes driving (practice time not reported)
	No current drivers	Field of view: 360°
	7 glaucoma
	15 age-matched NV

Kübler 2015⁷²	Same cohort as Kübler 2014⁵	Evaluated responses to nine hazards along a 37.5km route, including pass/fail for each hazard and whether the hazard was fixated;	Responses to hazards
			4 HFD, and 1 NV subject failed the driving test (inadequate response to at least one hazard)
	8 mixed HFDs (incomplete and complete HH, HQ, and paracentral scotomas)		No tendency for higher collision rates with hazards on the affected side
		Pass/fail determined by a masked driving instructor according to the criteria of the official driving test in Germany	Head movements
	8 age-matched NV		Two of the four HFD subjects who passed appeared to make more head movements than NV subjects and the other HFD subjects (see Figure 4 in Kübler et. al 2015⁷²);
		Eye movements recorded by a head-mounted video tracker (Dikablis 1.0.9); Head movements recorded with an optical laserBIRD tracker (Ascension Technology Coorporation)	Eye movements; HFD who passed:
			Longer saccades than those who failed
		About 36 minutes driving (practice time not reported)	Average eye position distribution did not differ from the NV subjects, but those who failed had an eye position bias to right (but does not take account of head position)
		Field of view: 360°	Fixation durations and number of fixations per minute did not differ from NV subjects
			Lane position and other driving measures
			There were no clear trends in the data, probably because sample size was limited
			Comments
			The small sample size places severe limitations on conclusions that can be drawn for comparisons of differences between HFD subjects that passed and failed.
			Eye and head movements to the affected and unaffected sides were not analysed separately
			Some of the eye and head movement measures lack a clear definition (e.g., 148 fixations per minute seems impossible)

Smith 2015⁵⁷	12 mixed HFDs (complete and incomplete HH, HQ and paracentral scotomas)	Evaluated detection of pedestrian orientation at the side of the road while performing a brief lane change driving task; pedestrians appeared at about 13° eccentricity, on the left or right and either faced the subject and walked along the sidewalk or were orthogonal and walked into the road; subjects pressed a response pad to indicate the orientation of the pedestrian;	Detection of pedestrian orientation on the affected side
			Accuracy rates of the adequate performance group did not differ from those of the NV group, but the inadequate group had lower accuracy rates
			Response times of HFD subjects were longer than those of NV subjects
	Age 63 ± 7 y
	≥ 3 mo since onset		Detection of pedestrian orientation on the unaffected side
	Stroke main aetiology (12/12)		Accuracy rates and response times of HFD subjects did not differ from those of NV subjects
	2 HFD had hemiparesis		Comments
	All non-current HFD drivers	Each trial was about 30 seconds and included 2 pedestrians before the lane change and 2 after; total of 4 trials and 16 pedestrians.	Neither head nor eye movements were recorded
			Very brief trials (30 seconds) for a total of 2 minutes of driving
	12 age-matched NV		Very high frequency of pedestrian events
		HFD subjects were split into adequate (n = 5) and inadequate (n = 7) performance groups based on number of correct responses in a visual search task; the cutoff criterion (≥ 72 of 96) was based on the lowest score of the NV subjects (range 72 to 89)	Neglect and cognitive status of HFD subjects not reported
		Total of about 2 minutes of “driving”; practice trials were given prior to experimental data collection
		Vehicle moved at a constant speed of 12 m/s (26.8 mph); accelerator and brake were not used.
		Subjects controlled steering
		Field of view: 89° H by 71° V

Bowers 2015⁶⁰	Same cohort as Alberti 2014⁵⁴	Evaluated detection rates for stationary pedestrians at T-intersections; total 10 intersection pedestrians (among total 104 other pedestrian events⁵⁴)	Detection rates:
	12 complete HH		Similar to Bowers et al. 2014,⁷⁰ HH subjects had low detection rates for intersection pedestrians on the affected side at large eccentricities (at about 80°)
	Age 39 ± 18 y
	≥ 6 mo since onset	Recorded gaze movements using a 6-camera IR remote eye-and-head tracking system (Smart Eye Pro)	Detection failures of HH subjects on the affect side were associated with
	Stroke main aetiology (8/12)		Failing to gaze scan toward the pedestrian
	No neglect	2 sessions; each 60 minutes driving and 30 minutes practice	Gaze scanning toward the pedestrian but not far enough
	MMSE ≥ 24	Subjects controlled speed and steering
	Current drivers: 2 of 12 HH	Field of view: 225° H by 37° V

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Abbreviations: HH - Homonymous hemianopia; HQ - Homonymous quadranopia; NV - Normal vision; HFD - Homonymous field defect; IR – infra red; MMSE - Mini-Mental State Examination; mo - months

Notwithstanding differences in the level of realism and the specifics of the simulated driving tasks, the strong and very consistent finding from the majority of these driving simulator studies has been the wide between-subject variation in detection performance for objects on the side of the field loss.^{49, 50, 54, 55, 57} Despite similar amounts of visual field loss (e.g. complete hemianopia), some HFD participants had detection rates on the affected side that were indistinguishable from normally-sighted, age-similar drivers,^{49, 57, 58} though response times were longer,^{49, 57} while other HFD participants failed to see many hazards and/or had responses that were too slow to have avoided a collision.^{49, 50, 54, 55, 57} Even participants with paracentral HFDs exhibited impaired detection of objects on the side of the field loss.⁵⁹ As might be expected, detection performance was more impaired for hazards that appeared at larger than smaller eccentricities on the affected side,^{49, 54} especially large eccentricities at intersections.^{49, 60}

It is possible that scanning toward the affected side to compensate for the hemifield loss might impair detection performance on the unaffected side. However, the results of simulated driving studies^{49, 50, 55, 57} suggest, at most, only mild impairment in detection performance on the unaffected side. Two studies^{49, 55} reported high detection rates on the unaffected side (100% or close to 100%), which were not significantly different to those of normally-sighted participants. Another study⁵⁰ using an intersection collision-avoidance task reported collision rates on the unaffected side that were only slightly higher than those of normally-sighted drivers at low traffic density, but did not differ from the collision rates of normally-sighted drivers at high traffic density. Two studies^{55, 57} reported reaction times on the unaffected side that did not differ from those of normally-sighted participants while another study⁴⁹ reported reaction times on the unaffected side that were only about 0.1 s longer than those of normally-sighted drivers (unaffected side 0.8 s, affected side 1.3 s, normally-sighted 0.7 s).

To date, only one on-road study²⁴ of HFD drivers has used detection of potential hazards as an outcome measure. In that study, the majority of interventions by the driving examiner (about 60%) were for failures to respond to hazards, including hazards approaching from both the affected and unaffected sides. The study was conducted on busy city streets in Ghent, Belgium, with a high density of traffic, pedestrians and cyclists. Two other studies^{23, 32} of HFD drivers have also reported braking interventions. Although no details were provided, it is possible that some of those braking interventions might have been a result of failures to respond in a timely manner to hazards. Indeed, 27% of HFD drivers in one of those studies³² were rated as having problems with reactions to unexpected events.

It might be inferred that detection problems are less evident in on-road studies than driving simulator studies. However, detection failures might only be noticed in an on-road test when there is a safety concern, whereas in simulated driving, every detection failure can be identified based on a failure to respond to a programmed event (e.g., a failure to press the horn⁴⁹ or a response button⁵⁷, or a failure to apply the brake⁵⁵). Furthermore, hazards may be less frequent in on-road tests (e.g. about 0.1 hazard per minute²⁴) than in recent simulated driving studies with a high frequency of detection events (e.g. about 1^{49, 54, 55} or even 8⁵⁷ events per minute).

Head and eye movements (scanning) when driving

The wide range in detection performance on the affected side measured in simulated driving tasks suggests that HFD drivers differ greatly in their ability to compensate for their field loss by scanning. The first studies to investigate scanning patterns of HFD patients used laboratory tasks, such as viewing simple patterns,⁶¹ or performing visual search,^{62, 63} or dot-counting tasks.^{36, 64} Even in these highly-constrained paradigms, a range of scanning abilities and search performance was documented.^64-66 Other more recent laboratory studies have examined scanning patterns for viewing natural images,⁶⁷ performing a block-building task,⁶⁸ performing a comparative visual search task,⁶⁶ and during detection of virtual moving basketballs when seated and walking.⁶⁹ The scanning behaviors and the extent to which compensatory scanning patterns were used depended on the task, suggesting that oculomotor behaviors observed during laboratory-based tasks might not necessarily generalize to on-road driving.

On-road studies of visually-impaired drivers frequently use observer ratings of eye and head movements.^{23-25, 30} However, it is not clear how well observers either in the front passenger seat or the back seat can evaluate head and eye scanning movements while also scoring other aspects of driving (such as lane position and speed). Another approach has been to record the head and face of the driver with a single video camera and subsequently score head and shoulder movements based on observation of those recordings^{31, 35} (e.g., manual counting of the number of small and large head movements, or rating of the amount of scanning on a 5-point scale). Only one on-road study of HFD drivers has used mobile eye tracking equipment to quantify eye scanning behaviors.³¹ By comparison, there are several 12 studies which have recorded either head,⁷⁰ or head and eye movements^{38, 55, 58, 60, 71, 72} in the more controlled environment of driving simulators where ambient illumination is usually relatively constant.

Reports of whether HFD drivers make more head movements than normally-sighted drivers are conflicting. An early simulator study,³⁸ in which head position was quantified from videotape recordings frozen only once every 4 s (thus, many head movements may have gone undetected), reported that HFD participants had greater head position variability than control drivers. Subsequent studies reported that HFD participants made fewer head movements,⁷¹ a similar number of head movements,⁷⁰ or tended to make more head movements than normally-sighted drivers.³⁵

Some studies analyzed head movements with respect to the side of the field loss. Using observer counts of head movements from video footage, Wood et al.³⁵ reported that HFD drivers rated as safe on a road test made significantly more head movements to the affected than the unaffected side, but there was no such difference for unsafe drivers, possibly suggestive of compensatory head scanning by the safe drivers. Using a lightweight optical tracker to record head scanning at T-intersections in a simulator, Bowers et al.⁷⁰ reported that HFD drivers had head scanning behaviors that tended to be dominated by large scans from the straight ahead position toward their affected side. Although HFD drivers made a similar total number of head movements to normally-sighted drivers, their first head scan was usually toward the side of their field loss and they made more scans in that direction (and fewer scans toward the unaffected side) than did the normally-sighted drivers to the corresponding side,⁷⁰ again suggestive of compensatory head scanning. HFD drivers with right field loss tended to scan right-left-right or right-right-left, whereas HFD drivers with left field loss tended to scan left-right-left or left-left-right. Normally-sighted drivers scanned left-right-left, as expected when driving on the right.

Only two on-road studies have specifically reported eye scanning behaviors of HFD drivers. Based on observer ratings from video footage, HFD drivers rated as safe were reported to make more excursive eye movements in general (ratings of eye movements to the affected and unaffected sides were not reported) than drivers rated as unsafe.³⁵ By comparison, in a study using a lightweight head-mounted eye tracker, no difference in the standard deviation of the eye fixation positions along the horizontal meridian was found between those who passed and failed.³¹ However, those who passed made more head and shoulder movements in general than those who failed (as rated by observers from video recordings). Whether there were any differences in the distribution of eye fixations or in the number of head/shoulder movements to the affected and unaffected sides was not reported. Recently, a number of studies using virtual environments have recorded both eye and head movements and examined the relationship between gaze scanning and detection, as summarized in the next section.

Relationship between scanning and detection in virtual driving tasks

In the simulated intersection collision avoidance task developed by Papageorgiou and colleagues,⁵⁰ HFD participants who had few collisions exhibited more active gaze (eye + head) scanning behaviors, including larger saccades to both sides, greater mean gaze eccentricity and more frequent gaze shifts than HFD participants who had more collisions.⁵⁸ Similar results were reported by Bahnemann et al.⁵⁵ and Alberti et al.⁷³ for driving simulator hazard detection tasks. In the Bahnemann et al.⁵⁵ study, HFD participants with better detection performance (< 2 missed detections on the affected side) exhibited more active eye scanning behaviors with larger saccades and a wider distribution of fixations along the horizontal meridian than HFD participants with poorer detection performance (≥ 2 missed detections on the affected side).

In some simulated driving studies^{55, 58} HFD participants were reported to make a greater proportion of fixations on the affected than the unaffected side. Similar behaviors have also been observed for HFD participants in many laboratory tasks.^{36, 64, 66, 67} In the simulated driving studies,^{56, 58} however, there were no significant differences in the proportions of fixations on the affected side between HFD participants with poor and good detection performance,⁵⁸ suggesting that simply making more fixations on the affected side is not sufficient for good detection performance. Rather, it is the extent of scanning toward the affected side which is important with larger movements and a wider distribution of fixations associated with better detection performance.^{55, 58, 73}

HFD participants in simulated driving studies^{55, 58} made a greater proportion of fixations on the affected than the unaffected side. Similar behaviors have also been reported for HFD participants in many laboratory tasks.^{36, 64, 66, 67} However, the results of the simulated driving studies^{56, 58} suggest that simply making more fixations on the affected side is not sufficient for good detection performance; there were no significant differences between HFD participants with poor and good performance in the proportions of fixations on the affected side.⁵⁸ Rather, it is the extent of scanning toward the affected side which is important with larger movements and a wider distribution of fixations associated with better detection performance.^{55, 58, 73}

The wide field of view that needs to be scanned at intersections is a challenging situation for drivers with HFDs. Bowers and colleagues^{60, 70} conducted two driving simulator studies to evaluate whether drivers with hemianopia scanned sufficiently far at intersections. For example, at stop-controlled T-intersections,⁷⁴ drivers with hemianopia would need to make a gaze scan of about 85° to view the whole of the intersection on the affected side, requiring both head and eye movements. In the first study,⁷⁰ head movements of HFD drivers and age-similar, normally-sighted drivers were tracked. Some HFD drivers demonstrated good head scanning. However, overall rates of failing to head scan to the affected side were similar to those of normally-sighted drivers, which was surprising because lower failure-to-scan rates might have been expected. For HFD drivers, failing to head scan to the affected side was associated with failures to detect pedestrians on that side. In addition, even when they did head scan toward the affected side, the results suggested that they frequently did not scan far enough to view all of the intersection and failed to detect pedestrians on the near sidewalk at about 80° eccentricity (relative to straight ahead gaze). Head scan magnitudes of HFD drivers were smaller when pedestrians were not detected than when they were detected. The second study⁶⁰ with gaze (eye + head) tracking confirmed that the gaze scan amplitude of HFD drivers toward the affected side was often insufficient to cover the full area of the intersection on that side. For normally-sighted drivers, failures to head scan toward the pedestrian were also associated with detection failures when the pedestrian was on the near sidewalk at about 80° eccentricity.⁷⁰

Results of both the Bowers et al.⁷⁰ and Papageorgiou et al.⁵⁸ studies revealed that, on average, the head and gaze scan amplitudes of HFD drivers were smaller when looking toward the affected than the unaffected side at intersections, and that scans toward the affected side were smaller than those of normally-sighted drivers. Nevertheless, larger scans to the affected side were associated with better detection performance in the Bowers et al. study⁷⁰ and fewer collisions in the Papageorgiou et al. study.⁵⁸ It may seem surprising that HFD drivers do not make larger scans to the affected than the unaffected side. However, this behavior is consistent with the lack of increased scan magnitudes (relative to normally-sighted observers) demonstrated by people with severe peripheral visual field restrictions (due to retinitis pigmentosa) when walking^{75, 76} and may be because they do not know how far to scan, as there is no guidance from peripheral vision. With respect to the unaffected side, HFD participants with good performance (few collisions) in the Papageorgiou et al.⁵⁸ study made gaze scans similar in magnitude or slightly larger than those of normally-sighted observers while HFD participants with poorer performance (more collisions) made smaller scans in that direction.

In summary, HFD drivers show a range of compensatory eye and head movement behaviors in both simulated and on-road driving. They tend to make more scans toward the affected than the unaffected side, with the first scan often toward the affected side when approaching an intersection. However, scans toward the affected side are usually smaller than those toward the unaffected side. Better detection performance is associated with a greater extent of scanning toward the affected side, including larger gaze movements and a wider distribution of fixations.

Screening of persons with HFDs for potential fitness to drive

When assessing whether a person with an HFD might be fit to drive, it would be useful if potential fitness to drive could be predicted from patient characteristics (e.g., age, duration of HFD, lesion location, field extent, or cognitive status) and/or performance on clinical screening tests. However, most studies to date have only evaluated predictors as a secondary aspect of the investigation.^{49, 50, 54,21, 23, 30} Furthermore, sample sizes were relatively modest (e.g., n = 12⁴⁹ to 26²³ or 30^{30, 50}) and sample characteristics differed across studies (see Tables 1 and 3).

Predicting blind side detection performance from patient characteristics

The only consistent predictor across studies of blind side detection performance appears to be age; older HFD participants tended to have poorer detection performance than younger HFD participants in simulated driving tasks.^{49, 50, 54} The same was also true for performance on a visual search task.⁷⁷ In a study which included participants with heterogeneous HFDs (ranging from complete hemianopia to incomplete quadranopia), the size of the remaining field was, at best, only weakly related to performance in a simulated intersection collision avoidance task.⁵⁰ Furthermore, several studies^{49, 50, 54} have demonstrated that HFD observers with similar amounts of remaining visual field can have very different levels of detection performance (e.g. from 0 to 100%⁵⁴). Duration of the field loss was not predictive in a number of studies,^{49, 50, 54} possibly because participants had field loss for at least 6 months (usually longer). Side of the field loss was also not predictive.^{49, 50, 54} It might be expected that patients with left-sided field loss (right brain damage) would perform less well than those with right-sided field loss (left brain damage), but that was not the case, possibly because those who were positive for hemispatial neglect (more common following right brain damage) on standard paper-and-pencil tests (Bells test and line bisection tests) were excluded.^{47, 48, 54} Brain lesion size or location might be a relevant factor, but only one study⁷⁸ (the simulated intersection collision avoidance task study) examined lesion location. Cortical structures associated with impaired performance were the parieto-occipital region and posterior cingulate gyrus in the right hemisphere and the inferior occipital cortex and parts of the fusiform (occipito-temporal) gyrus in the left hemisphere.⁷⁸

Predicting on-road driving performance from patient characteristics

As expected, driving status was predictive of road test performance; non-current HFD drivers were less likely to pass a road test than current HFD drivers³⁰ and had more interventions.²⁴ In a similar vein, HFD drivers for whom a long period of time had elapsed since they last drove were less likely to pass than those who had stopped driving only recently.²³ In two studies,^{23, 30} which included heterogeneous HFDs (hemianopia and quadranopia), road test failure was weakly associated with a smaller extent of remaining visual field, but was not associated with time since onset. Age was not a significant predictor of road test outcome in two studies of HFD drivers.^{23, 30} However, older age was associated with poorer performance on visual aspects of on-road driving (scanning, perception of traffic lights) in one study.²⁵ In another study²¹ with a very wide age range (approximately 20 – 90 years), drivers over 70 years of age were more likely to have their license suspended than those under 70 years. Of the 12 participants whose license was suspended, 11 were 70 years or older; however 29 of the 91 who were permitted to drive were over 70 years of age. (Note that the study included drivers with other types of field loss, such as central scotomas, as well as drivers with HFDs.)

Finally, one study¹²⁰ evaluated how well two neuro-ophthalmologists could predict either safe or unsafe on-road driving based on neuro-imaging reports containing information about the site and extent of the brain lesion. The level of agreement between the neuro-ophthalmologists was relatively poor and neither was able to accurately predict either safe or unsafe driving as defined by various outcome measures. The study concluded that clinical information on neuroimaging, available in standard clinical practice, is not sufficient for prediction of driving safety of HFD drivers.

Screening tests for evaluating visual fitness to drive

In most jurisdictions, screening for visual fitness to drive is based on measures of static visual acuity and visual field extent. In the case of HFDs, however, conventional visual field measures (detection of a white light on a plain uniform background while looking at a central point) provide little relevant information, other than confirming the presence of the HFD, because they do not reflect the ability of the patient to use compensatory scanning.

Recordings of head and eye movements could be used to quantify scanning behaviors while patients performed driving-related tasks; but, specialist equipment and analysis software would be required, which is usually not practical for the clinic, and information about which scanning behaviors were considered efficient would be needed. However, if the purpose is to screen for visual fitness to drive, it might not be necessary to measure scanning. Given that patients with HFDs will only see an object on the affected side if they look far enough in that direction, it should be sufficient to measure detection performance in a driving-relevant task. Quantification of scanning behaviors could, however, provide additional information to guide development of individualized training plans to address scanning deficits during rehabilitation.

Thus, for screening purposes, a test is needed in which the patient has to scan, although scanning behaviors would not actually be recorded, and which is predictive of hazard detection in a more complex, interactive driving task. Two studies have started to address this need. Bowers et al.⁷⁹ developed a video test in which participants watched short clips of driving scenes recorded from the driving simulator displayed on a screen subtending 65° horizontally by 40° vertically (equivalent to the size of the central monitor of the driving simulator). They pressed a response button whenever a pedestrian was seen and free eye and head movements were permitted. Although the participants did not have to drive, a wide range of detection rates and response times were measured in participants with hemianopia and quadranopia. Detection rate test-retest repeatability was good. Moreover, detection rates measured in the video test were predictive of those measured in a more complex driving simulator pedestrian detection task. Smith et al.⁵⁷ developed a fairly demanding visual search task in which a target (square) was either larger or smaller than 31 distractors (also squares). Participants with HFDs who performed poorly on this task (< 75% correct), had more impaired detection of pedestrian hazards in a brief driving simulator test than those who performed well on the search task (≥ 75% correct). The results of these two studies^{57, 79} provide preliminary evidence that it may be possible to develop a clinical test to be used as a tool for screening of visual fitness to drive in HFD patients, although the relationship to on-road driving performance has yet to be examined.

Another important consideration in evaluating visual fitness to drive is whether a person with HFDs is able to compensate for their hemifield loss when visual attention has to be deployed to more than one task, e.g., monitoring the car ahead in busy traffic while also scanning for potential hazards approaching from the side. Neither the video detection test nor the visual search task described in the previous paragraph required visual attention to be divided between two tasks. The Useful Field of View test^{80, 81} is the most well-established test of visual attention in the field of driving research. It measures visual processing speeds under conditions of increasing difficulty, including a divided attention subtest in which a discrimination task displayed at screen centre is performed concurrently with a peripheral localization task. There is a growing body of literature to suggest that slower visual processing speeds, regardless of etiology, increase the risk for unsafe driving (for a review, see Owsley and McGwin⁸²). However, the Useful Field of View test has limitations as a test of processing speed under divided attention conditions for HFD drivers because it does not involve active scanning. Fixation is constrained by the central discrimination task; therefore, peripheral targets that fall in areas of visual field loss will not be seen. The Trail-Making test,⁸³ on the other hand, measures visual processing speed and involves visual scanning. Part B of the test involves divided attention. The participant has to draw lines as quickly as possible to connect numbers and letters randomly positioned across a page in an alternating numeric and alphabetic sequence (i.e., 1-A-2-B, etc.). In two studies,^{25, 31} HFD drivers with slower processing speeds on the Trail-Making test had poorer on-road driving performance than HFD drivers with faster processing speeds. Thus the Trail-Making test may be a relevant screening test for HFD drivers; ^{25, 3025, 3025, 3025, 30}however, the evidence base is very limited (only two small-sample studies^{25, 30}) and further investigation is clearly needed.

Evaluating practical fitness to drive of persons with HFDs

On-road tests by certified driving rehabilitation specialists,^{33, 84} or certified driving examiners,^{21, 23} are considered the clinical gold standard when determining practical fitness to drive by persons with functional or medical impairments. However, whether road tests should be considered a gold standard is debatable.^{85, 86} Surveys of driving assessors (mostly occupational therapists) in the USA and Canada revealed that about 80%^{84, 87} of driving assessment programs used a standard test route but only 24%⁸⁴ to 67%⁸⁷ applied standardized scoring procedures. Although road tests can never be totally standardized and there is little control over the behavior of other road users, the obvious advantage is that driving is evaluated in a natural traffic environment. However, the route must include sufficient situations to enable an assessment of the person’s ability to adequately compensate for their impairment when driving.^{82, 83,88, 89} For drivers with HFDs, this review underscores the importance of evaluating responses to potential hazards as well as evaluating other driving skills such as lane position and steering steadiness. The route would ideally include situations with many potential hazards as well as situations with less frequent hazards because HFD drivers need to be able to scan effectively to detect potential hazards in both busy and less busy traffic environments.

However, using a route with a high likelihood of unexpected hazards places the vehicle occupants and other road users at risk and there is no control of whether or when such hazards might appear. As such, a standardized test in a driving simulator^{90, 91} including detection of potential hazards may be a useful adjunct or precursor to a road test, providing controlled, repeatable conditions with many opportunities to evaluate performance. Driving simulators can also be used to evaluate a range of other driving skills (lane position, steering, speed control, gap judgments, scanning etc.) in a controlled manner with scenarios designed to probe specific aspects of performance likely to be affected by the vision impairment.⁹² A significant disadvantage, however, is that some people experience simulator sickness, similar to motion sickness,⁹³ reported to be about 7% in two studies of HFD drivers.^{49, 50} Furthermore the braking and steering characteristics of the simulator might not fully replicate those of a real car and participants will be aware that it is only a simulation, which could affect their driving behaviors. (For a more detailed consideration of the advantages and limitations of driving simulators, please see other sources^{33, 85, 94}) To date, there have not been any studies of HFD drivers which have evaluated the relationship between simulator and on-road driving.

Interventions for HFD drivers

HFD drivers might benefit from interventions to address deficits in lane position and steering stability as well as deficits in viewing behaviors and reactions to potential hazards. However, relatively few studies^{24, 95-97} have evaluated interventions for HFD drivers. Studies evaluating driving rehabilitation interventions for post-stroke patients are more numerous,¹² but HFD patients were frequently excluded so the results are of limited relevance to this review. Notably, only four studies of interventions for post-stroke drivers met the criteria for inclusion in a recent Cochrane Review,¹² which concluded “there was insufficient evidence to reach conclusions about the use of rehabilitation to improve on-road driving skills after stroke”. Unfortunately, the same is also true of interventions for HFD drivers.

Lane position and steering stability

To date, there do not appear to have been any studies which have evaluated interventions to improve lane position and steering stability of HFD drivers. This may seem surprising given that problems with steering were commonly reported for HFD drivers with unsafe on-road performance.^{23, 25, 30-32} HFD drivers need to learn to maintain a stable and appropriate lane position. It is possible that rehabilitation training involving driving simulator lessons followed by on-road driving lessons with feedback about steering and lane position errors might be helpful and improve visuo-motor control.

Visual scanning training

A range of visual scanning training paradigms have been developed for rehabilitation of HFD patients,^{64, 97-103} but only one study⁹⁷ has specifically evaluated the effects on driving performance. Seventeen participants with hemianopia who had previously failed an on-road test underwent about 12 hours of laboratory-based scanning training followed by 6 hours of on-road driving instruction.⁹⁷ Only two of the participants subsequently passed a road test after the training. It was an open-label study without a control group; therefore, it is unknown whether the small improvement in pass rates was due to the scanning training, the additional driving instruction, or simply repetition of the driving test. The Dynavision visual scanning training system (Performance Enterprises, Ontario, Canada) has been evaluated for post-stroke patients in two studies. The first study,¹⁰⁴ a small, open-label investigation, included a heterogeneous group of ten post-stroke patients some of whom had “mild” visual field loss, but mild was not defined. The second study,¹⁰⁵ a small randomized controlled clinical trial with 26 participants, excluded people with hemianopia. Thus, these studies,^{104, 105} which showed no benefit of the training for post-stroke patients, provide no data on whether Dynavision training is a beneficial intervention for HFD drivers.

Visual restorative function training

Bergsma et al.⁹⁵ evaluated the effects of ‘visual restorative function training’ on eye movements in a driving simulator. This training is similar to vision restoration therapy,¹⁰⁶ which aims to promote visual field recovery. Participants completed the training at home 5 days a week for 15 weeks. They were told that eye movements were not permitted; however, fixation was not formally monitored. After training, there was a trend for fewer collisions with other traffic and pedestrians while driving in a simulator with two of the six patients making more eye movements toward the affected hemifield than before training. However, sample size was very limited, unmasked observers counted the eye movements from video recordings, and there was no control group of patients who either did not receive training or underwent placebo training. Hence it is unknown whether the very minor improvements in performance were really a result of the training or simply due to repetition of the driving simulator assessment.

Prism glasses

Prism glasses are applied as a rehabilitation treatment for hemianopia to optically shift images of objects located in the blind field into regions of the seeing field so that they can be seen.¹⁰⁷ Of the various methods of fitting prisms,^{107, 108} only two have been evaluated for driving: unilateral sector prisms of the Gottlieb¹⁰⁹ style and unilateral peripheral prisms in the oblique design.^{110, 111} Gottlieb-style sector prisms use relatively low power 18.5 Δ prisms mounted on the part of the lens that would be in the affected hemifield when the eyes are in the primary gaze position;^{96, 107} they have no effect unless the patient is actively looking into the prism.¹⁰⁸ By comparison, peripheral prisms¹¹² use high-powered 40 Δ or 57 Δ prism segments placed above and below the primary line of sight on the spectacle lens on the side of the field loss providing visual field expansion (enlargement of the visual field) for a range of lateral gaze positions.¹¹³ The oblique design^{110, 111} provides expansion in paracentral areas of the field, in regions used when looking through a car windshield.¹¹⁴ The prism images fall on more peripheral retina, so there is no central diplopia, unlike with unilateral Gottlieb sector prisms.¹⁰⁸

Szlyk et al.⁹⁶ evaluated the effects of 18.5 Δ Gottlieb¹⁰⁹ sector prisms compared to 20 Δ Fresnel sector prisms of similar size and position on the lens. A cross-over design was used in which ten participants were fitted first with one type of prism glasses and then the other, in counterbalanced order. They received 3 months of training with each type of prism glasses. Performance on a large battery of tests was evaluated twice without prisms (at the start of the study) and then once with each type of prisms after training. There were no significant differences in the amount of improvement with the two types of prisms, most likely because they were essentially identical except that one was a permanent ophthalmic prism ground into the lens while the other was a temporary press-on Fresnel prism placed on the surface of the lens (which would have poorer optical quality). Unfortunately the effects of the prisms, either on overall driving performance or specific driving skills, is unknown because data were collapsed across all tasks in the large test battery, irrespective of whether they were indoor, outdoor, or driving tasks. At the two-year follow-up, three of the ten participants were licensed current drivers and two used the Gottlieb sector prisms when driving.

A pilot, sham-controlled study of the oblique peripheral prisms (40Δ temporary press-on Fresnel prisms) was conducted in Ghent, Belgium.²⁴ After acclimation to the prisms, each of the twelve participants completed an on-road test on busy city-center streets with real prisms (oblique 40Δ) and sham prisms (oblique 5Δ) in counterbalanced order. The proportion of satisfactory responses to unexpected hazards on the side of the field loss (scored by a masked evaluator) was significantly higher with the real than the sham prisms. However, on the unaffected side, the proportion of satisfactory responses to hazards did not differ for real and sham prisms. There was no evidence that the real prisms had any adverse effects on other aspects of driving such as lane position or steering stability. The early results of a driving simulator study of the oblique prisms (57Δ rigid Fresnel) are also promising; detection rates for pedestrian hazards on the affected side are better with than without the prism glasses.¹¹⁵ A recent case study¹¹⁶ reported two young patients (24 and 31 years) with acquired hemianopia who were fitted with oblique peripheral prisms (57Δ rigid Fresnel). Both were licensed to drive in Minnesota (field extent requirement 105°) and had operated a motor vehicle without any restrictions or accidents for 1 and 5 years, respectively, at the time the report was written. Thus there is emerging evidence that oblique peripheral prisms may improve responses of HFD drivers to hazards on the affected side; however, to date, the prisms have only been evaluated in small sample studies (n ≤ 12).

Conclusions

On-road studies over the last 15 years have confirmed that some people with HFDs, without significant cognitive decline and without neglect, may be rated as safe to drive. However, pass rates varied widely across studies, which may, in part, be a result of the relatively small sample sizes as well as differences in recruitment strategies and sample characteristics (see Table 1).

Drivers with HFDs who failed a road test were commonly reported to have deficits in skills important for safe driving including unstable steering and taking a lane position too close to one side of the travel lane (see Table 1). Impaired visuo-motor control may contribute to unstable steering, while errors in spatial judgments and strategic compensation (i.e., holding a lane position toward the seeing hemifield to increase the safety margin on the side of the field loss) may contribute to lane position biases. Currently, however, there is insufficient evidence to draw any conclusions about the extent to which each of these might contribute to the unstable steering or lane position biases observed in on-road studies.

Drivers with HFDs who failed a road test were also reported to have inadequate viewing (scanning) behaviors and problems with responses to unexpected hazards (see Table 1). These findings were mostly based on observer ratings of participants’ eye and head movement behaviors and interactions with other traffic and road users. However, it is really within the controlled conditions of driving simulators that a number of research groups have started to study the relationship between eye/head movement behaviors and detection performance of HFD drivers (see Table 3). The main finding^{49, 50, 54, 55, 57} has been a very wide range in detection performance on the affected side, from HFD drivers who performed almost as well as normally-sighted drivers to those who failed to detect the majority of hazards, suggesting widely differing abilities to compensate by eye/head scanning. More active gaze scanning toward the affected side, including larger gaze movements and a wider distribution of fixations on that side, was associated with better detection performance.^{55, 58, 60, 70, 73} However, which scanning strategies are more effective have yet to be determined. Furthermore, the conclusions which could be drawn were limited by small sample sizes in some studies,^{38, 72} and eye/head movement behaviors were not analyzed separately for the affected and unaffected sides in other studies.^{31, 72}

From a clinical standpoint when evaluating a patient with an HFD, it would be convenient if potential fitness to drive could be predicted from characteristics such as age, duration of HFD, lesion location, field extent, or cognitive status, and/or performance on clinical screening tests. However, to date, there have been only limited investigations of predictors of both on-road driving performance and detection performance in simulated driving tasks. The main conclusion which can be drawn is that field extent is, at most, only a very weak predictor of detection performance.⁵⁰ There is clear evidence from several driving simulator studies^{49, 50, 54} that two people with similar amounts of remaining visual field may have very different scanning and detection abilities.

As we are clearly not yet at a point where we can predict from patient characteristics, visual field extent or other clinical tests which person with HFD is likely to be a safe driver, it seems only 23 reasonable to provide an opportunity for individualized assessments of practical fitness to drive. An on-road test by a certified driving rehabilitation specialist,^{33, 84} or a certified driving examiner,^{21, 23} is currently considered the clinical gold standard when determining practical fitness to drive by persons with HFDs. However, driving simulators may provide a useful adjunct to a road test for the evaluation of responses to potential hazards under safe, controlled and repeatable conditions.

Efficacy of interventions for drivers with HFDs was the final topic considered in the review. Surprisingly, only four studies^{24, 95-97} have evaluated interventions for HFD drivers. They were all small sample studies and only one²⁴ included a sham control condition. Oblique peripheral prism glasses are the only rehabilitation strategy for which there is preliminary evidence of efficacy in improving responses to hazards on the road²⁴ and in a driving simulator.¹¹⁵ There is no evidence as yet that scanning training is a beneficial intervention and there does not seem to have been any formal evaluations of training (on road or in a simulator) to address deficits in lane positioning or steering stability.

Although progress has been made in the last 15 years, this review highlights the need for further research in many areas: Development of more relevant tests of vision to evaluate the ability of HFD patients to compensate by scanning in tasks related to driving; Systematic investigations to understand more about the scanning behaviors of HFD drivers in realistic driving situations and to identify whether any scanning strategies are more effective than others; Development and evaluation of training programs to address specific scanning deficits, including scanning at intersections; Development and evaluation of more effective prismatic corrections; Development and evaluation of driving simulator or on-road training to address lane position and steering problems; and Large-sample, well-designed, randomized controlled studies to evaluate the effects of promising driving rehabilitation interventions.

In conclusion, the findings of this review underscore the need for individualized assessments of practical fitness to drive for HFD patients. This applies in jurisdictions where HFD patients do not meet the horizontal visual field extent requirement as well as those jurisdictions where they do meet the field extent requirement (because even if they do meet the requirement, they might not be a safe driver). A specific recommendation in the recent Driver Fitness Medical Guidelines produced by the USA National Highway and Traffic and Safety Administration¹¹⁷ states “Drivers with hemianopia or quadrantanopia should be given the opportunity for a comprehensive on-road evaluation by a driving specialist, and if judged fit to drive, should be given the opportunity to take the jurisdiction’s road test” (p. 46). Results from two recent studies that included participants who had applied to exceptional-case programs in Québec, Canada²¹ and the Netherlands,²³ provide evidence in support of the more widespread implementation of similar programs in other jurisdictions.

Acknowledgments

Disclosure of funding sources

Supported in part by National Institutes of Health grant R01-EY025677

Footnotes

Disclosure of potential conflict of interest

No conflicts of interest.

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PERMALINK

Driving with homonymous visual field loss: a review of the literature

Alex R Bowers, PhD MCOptom

Abstract

Table 1.

Regulations pertaining to driving with HFDs

Table 2.

Self-reported driving difficulties and habits

Pass/fail rates in on-road studies

Effect of HFDs on lane position and steering stability

Impaired visuo-motor control

Errors in spatial judgments

Strategic compensation

Other factors

Effects of HFDs on scanning and detection of potential hazards

Detection performance when driving

Table 3.

Head and eye movements (scanning) when driving

Relationship between scanning and detection in virtual driving tasks

Screening of persons with HFDs for potential fitness to drive

Predicting blind side detection performance from patient characteristics

Predicting on-road driving performance from patient characteristics

Screening tests for evaluating visual fitness to drive

Evaluating practical fitness to drive of persons with HFDs

Interventions for HFD drivers

Lane position and steering stability

Visual scanning training

Visual restorative function training

Prism glasses

Conclusions

Acknowledgments

Footnotes

References

ACTIONS

PERMALINK

RESOURCES

Cite

Add to Collections

PERMALINK

Driving with homonymous visual field loss: a review of the literature

Alex R Bowers, PhD MCOptom

Abstract

Table 1.

Regulations pertaining to driving with HFDs

Table 2.

Self-reported driving difficulties and habits

Pass/fail rates in on-road studies

Effect of HFDs on lane position and steering stability

Impaired visuo-motor control

Errors in spatial judgments

Strategic compensation

Other factors

Effects of HFDs on scanning and detection of potential hazards

Detection performance when driving

Table 3.

Head and eye movements (scanning) when driving

Relationship between scanning and detection in virtual driving tasks

Screening of persons with HFDs for potential fitness to drive

Predicting blind side detection performance from patient characteristics

Predicting on-road driving performance from patient characteristics

Screening tests for evaluating visual fitness to drive

Evaluating practical fitness to drive of persons with HFDs

Interventions for HFD drivers

Lane position and steering stability

Visual scanning training

Visual restorative function training

Prism glasses

Conclusions

Acknowledgments

Footnotes

References

ACTIONS

PERMALINK

RESOURCES

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