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. Author manuscript; available in PMC: 2007 Oct 1.
Published in final edited form as: Vision Res. 2006 Aug 22;46(22):3853–3861. doi: 10.1016/j.visres.2006.06.014

Identification of contrast-defined letters benefits from perceptual learning in adults with amblyopia

Susana TL Chung a,*, Roger W Li b, Dennis M Levi b
PMCID: PMC1852540  NIHMSID: NIHMS13516  PMID: 16930666

Abstract

Amblyopes show specific deficits in processing second-order spatial information (e.g. Wong, Levi, & McGraw (2001). Is second-order spatial loss in amblyopia explained by the loss of first-order spatial input? Vision Research, 41, 2951–2960). Recent work suggests there is a significant degree of plasticity in the visual pathway that processes first-order spatial information in adults with amblyopia. In this study, we asked whether or not there is similar plasticity in the ability to process second-order spatial information in adults with amblyopia. Ten adult observers with amblyopia (five strabismic, four anisometropic and one mixed) were trained to identify contrast-defined (second-order) letters using their amblyopic eyes. Before and after training, we determined observers’ contrast thresholds for identifying luminance-defined (first-order) and contrast-defined letters, separately for the non-amblyopic and amblyopic eyes. Following training, eight of the 10 observers showed a significant reduction in contrast thresholds for identifying contrast-defined letters with the amblyopic eye. Five of these observers also showed a partial transfer of improvement to their fellow untrained non-amblyopic eye for identifying contrast-defined letters. There was a small but statistically significant transfer to the untrained task of identifying luminance-defined letters in the same trained eye. Similar to first-order spatial tasks, adults with amblyopia benefit from perceptual learning for identifying contrast-defined letters in their amblyopic eyes, suggesting a sizeable degree of plasticity in the visual pathway for processing second-order spatial information.

Keywords: Amblyopia, Letter recognition, Perceptual learning, Second-order

1. Introduction

Amblyopia is a developmental disorder of spatial vision often accompanied by the presence of strabismus, anisometropia or form deprivation early in life (Ciuffreda, Levi, & Selenow, 1991). The hallmark of amblyopia is the presence of visual deficits in one eye (often referred to as the “amblyopic eye”) that cannot be attributed to an identifiable ocular pathology. The visual deficits usually include, but are not limited to, a reduction in visual acuity (Gstalder & Green, 1971; Hess, Campbell, & Greenhalgh, 1978; Levi & Klein, 1982), contrast sensitivity (Bradley & Freeman, 1981; Hess & Howell, 1977; Levi & Harwerth, 1977), Vernier acuity (Levi & Klein, 1982, 1985; Rentschler & Hilz, 1985) and other position acuities (Levi & Klein, 1983, 1986), as well as spatial distortion (Bedell & Flom, 1981, 1983; Bedell, Flom, & Barbeito, 1985).

Traditionally, it is believed that the visual deficits in amblyopia, most notably, visual acuity, can only be reversed if amblyopia treatment is implemented before the critical period for visual development is over (von Noorden, 1981). In humans, this critical period is around age 6–8 years (von Noorden, 1981). However, over the past decade, there is strong evidence showing that visual performance in the amblyopic eye can be improved through repeated practice or perceptual learning, even in adult amblyopes, suggesting that the plasticity of the visual system extends well beyond the critical period. For example, adults with amblyopia demonstrate significant perceptual learning of contrast sensitivity for gratings (Polat, Ma-Naim, Belkin, & Sagi, 2004; Zhou et al., 2006) and letters (Levi, 2005), Vernier acuity (Levi & Polat, 1996; Levi, Polat, & Hu, 1997) and other position discrimination tasks (Li & Levi, 2004).

To date, all studies of perceptual learning with amblyopes have involved visual stimuli that are defined by variations in local luminance, often referred to as first-order (luminance-defined) stimuli. In the absence of local luminance variations of an object, the visual system is still capable of detecting the object against its background based on other stimulus attributes such as variations in local contrast or texture. These stimuli are usually referred to as second-order stimuli. Second-order processing has been widely modeled as a filter–rectifier–filter pathway (Chubb & Sperling, 1988), where the first-stage filters are linear, with a possible neural substrate in the early cortical areas (Mareschal & Baker, 1998; Schofield, 2000). The output from this first-stage linear filters then undergoes nonlinear processing, possibly rectification, before feeding onto the second-stage filter. The neural site(s) of this second-stage filter in humans and primates have yet to be identified, but brain-imaging experiments suggest a possible extrastriate locus, at least for motion processing (Dumoulin, Baker, Hess, & Evans, 2003; Smith, Greenlee, Singh, Kraemer, & Hennig, 1998). Psychophysical studies have provided ample evidence suggesting that first- and second-order spatial information can be processed independently (Schofield & Georgeson, 1999, 2003; Willis, Smallman, & Harris, 2000). There is also physiological (Baker & Mareschal, 2001; Mareschal & Baker, 1998) and brain-imaging (Smith et al., 1998) evidence corroborating that first- and second-order stimuli are processed by distinct processing streams.

Several studies of second-order visual processing in amblyopia have shown that the amblyopic deficit is larger for second- than first-order stimuli, for stationary (Mansouri, Allen, & Hess, 2005; Simmers, Ledgeway, & Hess, 2005; Wong, Levi, & McGraw, 2001) as well as motion stimuli (Simmers, Ledgeway, Hess, & McGraw, 2003; Simmers et al., 2005). These results are interpreted as an amplified amblyopic deficit in the extrastriate cortex, which presumably is the site for processing second-order stimuli. Because visual performance for first-order stimuli often improves through perceptual learning (e.g. Ball & Sekuler, 1982, 1987; Beard, Levi, & Reich, 1995; Chung, Legge, & Cheung, 2004; Chung, Levi, & Tjan, 2005; Fahle & Edelman, 1993; Fiorentini & Berardi, 1980, 1981; Karni & Sagi, 1991; Li, Levi, & Klein, 2004; McKee & Westheimer, 1978; Poggio, Fahle, & Edelman, 1992; Saarinen & Levi, 1995), even in amblyopic eyes (Levi, 2005; Levi & Polat, 1996; Levi et al., 1997; Li & Levi, 2004; Polat et al., 2004; Zhou et al., 2006), in this study, we asked the question of whether or not performance for second-order stimuli would also benefit from perceptual learning in amblyopic eyes. Given that the amblyopic deficit is more pronounced for processing second- than first-order spatial information, we predicted that amblyopic observers would show significant perceptual learning of second-order stimuli and that the magnitude of improvement might even be greater than that following perceptual learning of first-order stimuli.

2. Methods

To examine the question of whether or not perceptual learning leads to an improvement in identifying contrast-defined letters with the amblyopic eye, and whether or not the improvement transfers to the task of identifying luminance-defined letters, and to the fellow non-amblyopic eye, we compared contrast thresholds for identifying luminance-defined and contrast-defined letters before (pre-test) and after (post-test) a period of intensive training in a group of amblyopic observers. Pre- and post-test measurements were obtained monocularly in each eye of each observer. Training consisted of repeated measurement of thresholds for identifying contrast-defined letters in the amblyopic eye of each observer.

2.1. Basic experimental design

The basic experimental design and training schedule are represented schematically in Fig. 1. Given that acuities varied across the amblyopic eye of our observers, we first determined the acuities for luminance-defined and contrast-defined letters by measuring the size thresholds for identifying such letters of a fixed high-contrast (0.7 contrast for luminance-defined letters and a differential contrast [see below for definition] of 0.7 for contrast-defined letters) in each eye of each observer. We used a letter size (specified as the height of the letter ‘x’, or the ‘x-height’) equivalent to 1.3× the size threshold for identifying contrast-defined letters in the amblyopic eye in subsequent testing.1 The acuities for identifying contrast-defined letters are shown in Fig. 2 (ordinate) and the sizes used for each observer in the main experiment are given in Fig. 3.

Fig. 1.

Fig. 1

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A schematic cartoon illustrating the basic experimental design of the study.

Fig. 2.

Fig. 2

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Acuity, or letter size threshold (in deg), for identifying contrast-defined letters is plotted as a function of that for identifying luminance-defined letters, for the non-amblyopic (NAE: unfilled symbols) and the amblyopic (AE: filled symbols) eye of each amblyopic observer. Data from four normal observers obtained at the fovea (+ symbols) and 10° eccentricity (∗ symbols) are also included for comparison. The diagonal lines represent threshold ratios between contrast-defined and luminance-defined letters of 1:1 and 6:1. In this and subsequent figures, observers are color-coded according to the type of amblyopia they exhibited (strabismic, red; anisometropic, green; both, blue).

Fig. 3.

Fig. 3

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Differential contrast threshold (ΔC) for identifying contrast-defined letters is plotted as a function of training block, for each individual observer. Filled symbols in each panel represent thresholds obtained at the pre- and post-tests. The solid line in each panel represents the best-fit regression line to each set of data. The slope of this line, if different from 0, represents significant amount of learning (p-value given in each panel). Acuity and letter size used are given in each panel.

The pre-test consisted of measurement of contrast threshold for identifying contrast-defined and luminance-defined letters, at a background rms noise contrast of 0.07, in the non-amblyopic and the amblyopic eyes. The two eyes of each observer were tested in a random order, but the luminance-defined letters were always tested before the contrast-defined letters. Details for generating luminance- and contrast-defined letters are described elsewhere (Chung, Levi, & Li, 2006). In brief, contrast-defined letters were generated by assigning a different contrast to the white noise2 that made up the letter, with respect to the contrast of the background (see Fig. 1). The mean luminance of the letter and its background were the same. Thus, contrast threshold for identifying contrast-defined letters is specified as the differential contrast (ΔC) that defined the letter from its background. Luminance-defined letters were generated by assigning a different luminance value to the letter, compared with its mid-gray background (see Fig. 1). An array of white noise (generated in exactly the same way as in the case of contrast-defined letters) covered both the letter and its background. Hence, contrast threshold for identifying luminance-defined letters is specified as the Weber contrast between the letter and its background, (letter luminance − background luminance)/background luminance. Threshold reported for each condition (eye x type of letters) was the average value of two blocks of trials (100 trials per block).

Training consisted of 80 blocks of trials (100 trials per block, ten blocks per day for 8 days) of identifying contrast-defined letters at a background rms noise contrast of 0.07 in the amblyopic eyes. Each training session lasted approximately 1–1.5 h. The post-test, identical to the pre-test, followed the last training session.

2.2. Apparatus

Stimuli were generated on a Macintosh G4 computer with software written in Matlab (The MathWorks, MA) using the psychophysics tool-box extensions (Brainard, 1997; Pelli, 1997), and were displayed on a Sony 17″ monitor (model number G400) at a mean luminance of 23 cd/m2 (Berkeley) or a Mitsubishi Diamond Plus 15″ monitor (model number N0701) at a mean luminance of 26 cd/m2 (Houston). The luminance of the display was measured using a Minolta photometer. By combining the red and blue output of the display using a video attenuator (Pelli & Zhang, 1991) and the use of custom-built software (Tjan, personal communication), we obtained an effective 10 bit resolution on luminance after correcting for the gamma of the display. Observers sat at 42 cm from the display during testing (except when letter size thresholds were being determined). At this viewing distance, each pixel subtends 2.5 arc min.

2.3. Testing and psychophysical procedures

Testing and psychophysical procedures were identical for pre-test, training and post-test sessions. Each condition was tested in a separate block of trials. In each block of trials, we used the Method of Constant Stimuli to present the stimulus letter at five stimulus levels (five differential contrast (ΔC) for contrast-defined letters or five Weber contrast levels for luminance-defined letters), with each stimulus level presented 20 times within the block. On each trial, a single letter of x-height that corresponded to 1.3x the threshold letter size for identifying contrast-defined letters in the amblyopic eye, was presented for 150 ms in the center of the display monitor. Letters were randomly chosen with equal probability from the 26 lowercase letters of the Times-Roman alphabet. The task of the observers was to indicate the letter identity using the keyboard. Audio feedback was given to indicate whether or not the response was correct. Testing was monocular, with the non-tested eye covered with a standard black eye-patch. We defined threshold as the ΔC that yielded 50%-correct performance (after correction for guessing) on the psychometric function, constructed based on the data from each block of trials.

2.4. Observers

Ten adults with amblyopia aged between 21 and 58 participated in this study. Table 1 summarizes the visual characteristics of these observers. Two of the observers had previous experience with second-order stimuli (JT and RH). Both had participated in earlier studies examining second-order stimulus detection (Wong et al., 2001, Wong, Levi, & McGraw, 2005 and unpublished studies) several years prior to this study. Observers JS, JT and RH had participated in perceptual learning studies involving luminance-defined targets. All observers had best-corrected visual acuity of 20/ 20 or better in the non-amblyopic eye, and acuity difference between the two eyes ranging from 1 to 10 lines. All observers wore their best optical correction (glasses or trial frame) during the experiment. Written informed consent was obtained from each observer after the procedures of the experiment were explained, and before the commencement of data collection.

Table 1.

Visual characteristics of the amblyopic observers

Observer Gender Age Type Visual Acuity Refractive Errors Eye Alignment Stereoacuity (if any)
AA F 29 Strab 20/25 − 2 −2.00/−2.25 × 180 >30Δ Alt ET
20/20 + 2 −3.75/−2.00 × 005 10Δ const RHyperT
AP F 21 Strab 20/12.5 − 2 −1.50/−0.50 × 180 3-4Δ LET
20/50 −0.75/−0.25 × 180 2Δ LHyperT
GK M 25 Strab 20/20 − +0.50/−2.25 × 010 12Δ RET
20/15 +0.50/−2.25 × 170 10Δ RHyperT
JS F 21 Strab 20/16 +1.25 6-8Δ LET
20/40 +1.00 4-6Δ LHyperT
JT F 58 Strab 20/12.5 −1.00/−0.50 × 010 4-6Δ LET
20/63 − 1 −0.75/−0.50 × 090
RH M 41 Strab 20/15 −1.00/−0.50 × 170 Microtropia 2Δ LET 200″
20/60 −1.50/−1.50 × 010
CJ M 22 Aniso 20/100 + 1 −12.25/−1.75 × 160
20/12.5 − 1 −5.75
ED M 39 Aniso 20/32 +2.75/−1.00 × 080 25″
20/16 + 2 +0.25/−0.25 × 110
SC M 30 Aniso 20/16 + 2 +0.50 70″
20/40 − 1 +3.25/−0.50 × 155
VC F 21 Strab + Aniso 20/200 +4.75/−1.50 × 010 5Δ RXT with ARC
20/20 pl/−0.50 × 180
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3. Results

3.1. Acuity for identifying contrast-defined letters

Acuity for contrast-defined letters is coarse—in normal foveal vision, about a factor of 4–6 lower than for first-order letters (Oruc, Landy, & Pelli, 2006; Regan, 2000). We find a similar loss of resolution in the amblyopic fovea. Fig. 2 compares acuity (letter size thresholds) for identifying contrast-defined letters with those for identifying luminance-defined letters for the two eyes of each amblyopic observer (unfilled symbols for non-amblyopic eyes and filled symbols for amblyopic eyes). These size thresholds were obtained before training. For comparison, we include data from four observers with normal vision, obtained at the fovea (+ symbols) and 10° eccentricity in the lower visual field (∗ symbols). Averaged across our amblyopic observers, the acuity ratio for contrast-defined vs. luminance-defined letters was 5.9 (SD = 2.8) and was virtually identical between the two eyes (mean ± SD: non-amblyopic eyes = 5.8 ± 2.8; amblyopic eyes= 5.9 ± 2.8). In other words, the letter size threshold for contrast-defined letters is close to six times larger than that for luminance-defined letters, independent of whether or not the eye is amblyopic. Observer JS, however, had acuity ratios that were close to 10 for both of her eyes, the significance of which will be addressed in Section 4.

3.2. Effect of training

Eight of our ten observers showed significant improvements in thresholds for identifying contrast-defined letters in the amblyopic eye following training (Fig. 3, unfilled symbols). For comparison, pre- and post-test thresholds are also plotted, as filled symbols. Each panel of Fig. 3 presents data of one observer. Improvement due to training would be represented by a lowering in thresholds as training progressed. To quantify the improvement, we fit a linear regression function to each set of data that included thresholds for all training blocks, along with thresholds obtained during pre- and post-tests. A t-test was performed to determine if the slope of each regression function differed from a slope of 0, an indication that there was no improvement due to training. The p-value for the t-test is given in each panel in Fig. 3. Except for observers AA and JT, all observers showed a statistically significant improvement in identifying contrast-defined letters following training. Applying a conservative (Bonferroni) correction for the number of observers results in seven (rather than eight) of the 10 observers reaching statistical significance. We also performed an ANOVA on the slope of the regression lines for the group. Compared to a slope of 0 (no learning), the mean slope of the regression lines (mean improvement in thresholds across training sessions) is highly significant: t(df = 9) = −4.49, p = .0015 (two-tailed test).

3.3. Transfer of learning

To determine whether or not the improvement in performance transferred to the task of identifying luminance-defined letters (an untrained task), and to the fellow non-amblyopic eye (the untrained eye), we compared contrast thresholds for identifying luminance-defined and contrast-defined letters for each eye, before (pre-test) and after (post-test) training. Fig. 4 plots the ratios of post- and pre-test contrast thresholds for the four conditions (non-amblyopic eye vs. amblyopic eye; and luminance- vs. contrast-defined letters), and for all observers. Ratios <1 represent improvements. Averaged across all observers, the mean ratios (± 95% confidence intervals) differ significantly from a value of 1 for both luminance-defined and contrast-defined letters in the amblyopic eyes. These ratios are not statistically different from a value of 1 for either type of letters in the non-amblyopic eyes.

Fig. 4.

Fig. 4

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Threshold ratios (post-test/pre-test) are compared for the four conditions (non-amblyopic eye (NAE) vs. amblyopic eye (AE), luminance-defined vs. contrast-defined letters). The trained condition (AE cont) is indicated by an asterisk. The group-averaged ratio for each condition is given under the label for each condition. Observers are ordered according to the decreasing magnitude of their ratios (improvement = 1 − ratio) for the trained condition. Error bars represent ± 1 SEM.

Although the mean ratio for luminance-defined letters in the amblyopic eyes (untrained task, trained eye) is statistically different from a value of 1, the magnitude of the improvement is tiny (mean ratio = 0.95, range = 0.88–1.05), and only two of the observers had post/pre ratios that are significantly lower than 1. In contrast, even though the mean ratio for contrast-defined letters in the non-amblyopic eyes (trained task, untrained eye) is not statistically different from a value of 1, half of the observers (three strabismic and two anisometropic) showed post/pre ratios that are significantly different from 1 (range = 0.64–0.70), suggesting a sizeable transfer of learning to the untrained fellow eye. Note that the two anisometropic observers who showed this interocular transfer of learning had measurable stereoacuity (25″ and 70″, respectively), although none of the strabismic observers who also showed the transfer effect demonstrated any measurable stereoacuity.

4. Discussion

4.1. Identification of contrast-defined letters benefits from perceptual learning in adults with amblyopia

The primary question of this study was whether or not performance for processing second-order stimuli would benefit from perceptual learning in amblyopic eyes, as for first-order stimuli. We found that following 80 blocks (8000 trials) of intensive training on identifying contrast-defined (second-order) letters in the amblyopic eye, eight of the 10 observers improved in their ability to identify such letters in the trained (amblyopic) eye (see Fig. 3). The improvement did not seem to depend on the type of amblyopia (strabismic vs. anisometropic). For the two observers who did not show any improvement (AA and JT), there was little in common between them other than the fact that both were strabismic. AA, a novice psychophysical observer, had an alternating esotropia. Consequently, the acuities in her two eyes were highly similar, with a very mild acuity loss in her worse (“amblyopic”) eye. JT, an experienced psychophysical observer, had a unilateral esotropia in her left eye, and a larger acuity difference between the two eyes. JT had participated in previous studies involving second-order tasks (unpublished). Interestingly, observer RH participated in two earlier studies that involved detecting second-order stimuli (Wong et al., 2001, 2005) at least four years prior to the present study. His performance showed a modest improvement in the present study (p = .035). Further, observers JS and RH had participated in previous studies on perceptual learning of first-order targets, yet they demonstrated improvements in learning contrast-defined letters in this study. JT had also participated in previous studies on perceptual learning of first-order targets but did not show any improvement in learning contrast-defined letters. Thus, the differences in the amount of improvement following perceptual learning of second-order stimuli among our amblyopic observers is likely to be due to both prior experience (e.g. JT and RH) and individual differences. Note that substantial individual differences have been reported for perceptual learning of first-order stimuli in normal observers (Fahle & Henke-Fahle, 1996).

Using a similar paradigm, we previously showed that the performance for identifying contrast-defined letters improved in peripheral vision (10° eccentricity) in a group of normal observers following perceptual learning of identifying such letters (Chung et al., 2006). In that study, we measured observers’ contrast thresholds for identifying luminance-defined and contrast-defined letters before and after training, for letters that were embedded at three background noise contrasts (r.m.s. = 0, 0.07, and 0.14). Averaged across the eight observers and the three background noise contrasts, the threshold improvement (1 − post/pre ratio) for identifying contrast-defined letters was 32% in the trained eye, and 28% in the untrained eye. Here, the threshold improvements for identifying contrast-defined letters in the trained amblyopic eye of our observers ranged between −4% and 73%, with a mean of 33%. This magnitude of improvement is virtually identical to that found in normal peripheral vision (Chung et al., 2006) suggesting that the plasticity of adult amblyopic vision may be similar to that in normal (peripheral) vision. Also note that the magnitude of improvement of 33% is highly similar to that found following perceptual learning of first-order letters in adult amblyopes (Levi, 2005).

4.2. Specificity of learning

Perceptual learning, at least for first-order stimuli, is known to be stimulus- and task-specific (e.g. Ball & Sekuler, 1982, 1987; Fahle & Edelman, 1993; Fiorentini & Berardi, 1980, 1981; Poggio et al., 1992). Nevertheless, several studies have reported a transfer of learning to an untrained task for observers with normal vision (Beard et al., 1995; Chung et al., 2004; Sireteanu & Rettenbach, 2000). For observers with amblyopia, whether or not perceptual learning generalizes to untrained tasks is still unclear, and could be related to whether or not the observers are novice or experienced observers. For example, Levi et al. (1997) reported that the improvement following perceptual learning on a Vernier acuity task did not transfer to an untrained detection task (also see Levi & Polat, 1996), but two (novice) observers showed an improvement in the untrained Snellen acuity task. Polat et al. (2004) trained their amblyopic observers to detect a small grating patch with and without flanking collinear high-contrast patches, and found improvement in detecting the grating patch transferred to the untrained tasks of contrast sensitivity function and letter acuity measurements. Li and Levi (2004) trained their amblyopic observers on the task of position discrimination and found a transfer of learning to other untrained higher-level visual tasks such as stereopsis and visual counting. Zhou et al. (2006) trained their amblyopic observers in detecting sine-wave gratings at a single spatial frequency. As expected, contrast sensitivity for detecting the trained spatial frequency improved following training, but the improvement also generalized to a range of other spatial frequencies, as well as to a visual acuity task (as measured using Tumbling E). In this study, the improvement following training in identifying contrast-defined letters transferred only minimally to the untrained task of identifying luminance-defined letters in the same trained (amblyopic) eye (threshold improvement = 5%, range = −5 to 12%). This magnitude of improvement is highly similar to that observed in normal peripheral vision (mean threshold ratio = 7.7%: Chung et al., 2006). As we concluded in our previous paper, the lack of a sizeable transfer of learning from the task of identifying contrast-defined to luminance-defined letters is consistent with the notion that first- and second-order visual stimuli are processed by separate and distinct pathways.

Recently, Dosher and Lu (2006) compared perceptual learning for discriminating the orientation (forward or mirror-reversed) of the letter K when defined by luminance contrast (a first-order stimulus) or by texture contrast (a second-order stimulus) at the fovea of normal observers. They found no systematic improvement for the luminance-defined stimulus. In contrast, there was a significant improvement in the texture-defined stimulus, especially when the letters were embedded in low rather than high levels of external noise. Dosher and Lu interpreted their findings to imply that the site of learning is likely to occur after the rectification stage of the second-order pathway, and provide further support of independent processing of first-and second-order information. However, it seems quite likely that the failure to learn with the luminance-defined stimulus reflects the fact that luminance-defined letters are already over-learned at the normal fovea. Unlike the normal fovea, the amblyopic fovea shows similar learning for luminance-defined (Levi, 2005) and contrast-defined (present study) letters.

Previous studies of perceptual learning of first-order stimuli in amblyopes have reported a transfer of learning from the trained amblyopic eye to the untrained fellow (non-amblyopic) eye in some amblyopic observers and for some tasks. For example, Levi et al. (1997) reported a transfer that averaged approximately 60% of the direct learning effect following perceptual learning of Vernier acuity [see also Levi and Polat (1996) for a similar magnitude of transfer for Vernier acuity, but see Li and Levi (2004) who did not find any transfer of learning of a position discrimination learning task]. Zhou et al. (2006) reported significant amount of transfer to the untrained non-amblyopic eye only for those observers who practiced on a grating detection task at one single spatial frequency, but not for those who practiced the task over a range of 9 spatial frequencies (0.5–16 c/deg). In this study, the threshold improvement in the untrained fellow eye, for the same task (identifying contrast-defined letters), averaged 10% (range = −47–36%), and is not statistically different from the null effect. However, there were substantial individual differences in the magnitude of the transfer. Specifically, half of the observers (three strabismic and two anisometropic) showed a sizeable improvement in their untrained fellow eye. The transfer ranged from 57% to 100% of the direct learning effect. Using similar stimuli and paradigm, we previously established that there was an almost complete transfer of the improvements following training from the trained to the untrained eye in normal peripheral vision, suggesting that the improvements occur at a site after the input from the two eyes are combined (Chung et al., 2006). The fact that the two anisometropic observers who showed a sizeable transfer effect had relatively good stereoacuity (25″ and 70″) whereas the third anisometropic observer who did not have stereoacuity showed no transfer of learning to the untrained eye is consistent with this postulation. However, we cannot easily explain the substantial transfer shown by three strabismic amblyopes unless we postulate the continued presence of residual binocular neurons that are unable to support stereopsis (e.g., Levi, Harwerth, & Smith, 1979). Previously, Keck and Price (1982) found substantial interocular transfer of the motion aftereffect in strabismic observers who were stereoblind (both from amblyopic eyes to non-amblyopic eyes, and vice versa). They, too, postulated the presence of binocular neurons that are unable to support stereopsis as an explanation for their findings. We note that the five amblyopic observers who showed interocular transfer were the five with the mildest amblyopia (observers AA, GK, JS, ED and SC), thus it is possible that the relatively good vision in the amblyopic eye allowed the observers to retain some degree of binocular fusion, although not necessarily stereopsis (three failed the clinical stereopsis testing). However, Keck and Price (1982) reported that the magnitude of the interocular transfer of motion aftereffect among their strabismic observers did not correlate with visual acuity.

4.3. Acuity for luminance-defined vs. contrast-defined letters

Acuity for contrast-defined letters is coarse in normal and amblyopic eyes. The comparison of acuities (letter size threshold) for contrast-defined vs. luminance-defined letters in Fig. 2 shows that the acuity ratios remain similar between the non-amblyopic and amblyopic eye of our amblyopic observers, and that they do not depend on the type of amblyopia. These ratios are around a value of 6, i.e. the letter size for identifying contrast-defined letters needs to be about six times larger than that for identifying luminance-defined letters. These ratios are also comparable to those obtained in the fovea and 10° eccentricity in observers with normal vision. These findings imply that, at least for acuity (letter size) threshold, there is no additional deficit in identifying contrast-defined letters for (1) either eye of the amblyopic observers, when compared with observers with normal vision, and (2) the amblyopic eye, compared with the fellow non-amblyopic eye.

Our finding that there is no additional deficit in identifying contrast-defined letters with respect to luminance-defined letters seems to, at first glance, contradict the findings of previous studies that showed an amplified loss in detecting second-order spatial information in the amblyopic eyes, compared with normal eyes (Giaschi, Regan, Kraft, & Hong, 1992; Mansouri et al., 2005; Simmers et al., 2003, 2005; Wong et al., 2001). Wong et al. (2001) further showed that the non-amblyopic eyes had a detection deficit for second-order spatial information compared with control eyes in observers with normal vision. There are (at least) two possible reasons for the discrepancy between our finding and those of previous studies. The first and most important one is related to the difference in tasks, viz., detection vs. identification of a suprathreshold stimulus. Indeed, Wong and Levi (2003) showed that in all but the most severe amblyopes, suprathreshold second-order contrast discrimination was normal or near normal despite detection deficits. A second difference is related to spatial scale. Wong et al. (2001, 2005) reported marked deficits in amblyopes for detection of low spatial frequency second-order stimuli whereas our stimuli (letters close to the acuity limit) most likely tap into high spatial frequency mechanisms.

The acuity ratios for both the non-amblyopic and amblyopic eye of observer JS were ≈10, a value much higher than those for other observers (≈6). The higher ratios imply that JS might experience more deficits in identifying contrast-defined letters than other amblyopic observers. Indeed, she demonstrated a larger spatial interaction (“crowding”) zone for second-order letters, compared with other (strabismic) amblyopic observers (Chung, Li and Levi, in preparation).

5. Conclusion

Following an intensive period of learning to identify contrast-defined letters (a second-order task) in the amblyopic eye of a group of amblyopic observers, we found that the performance for identifying contrast-defined letters improved in the trained eye in eight of the 10 observers. Five observers showed a transfer of improvement to the same task in the untrained fellow eye. However, across all observers, there was very little transfer of improvement to the task of identifying luminance-defined letters (a first-order task) in the same trained eye. The minimal transfer of learning from a second- to a first-order task suggests that unlike Vernier acuity (Levi & Polat, 1996; Levi et al., 1997) or first-order contrast sensitivity measurement (Polat et al., 2004; Zhou et al., 2006), perceptual learning of second-order stimuli may have limited value in the treatment of amblyopia, at least with respect to recovering acuity. However, it remains to be determined whether the improvement might generalize to other tasks that might be important in everyday life, such as seeing structures in natural scenes (Schofield, 2000) and breaking camouflage (Regan, 2000).

Acknowledgments

This study was supported by research grants R01-EY12810 (S.T.L.C.) and R01-EY01728 (D.M.L.) from the National Eye Institute, National Institutes of Health. We thank Dr. Harold Bedell for statistical advice and Dr. Ron Harwerth for helpful comments.

Footnotes

1

We used 1.3× the size threshold for testing because this factor represented the best compromise of allowing us to include as many amblyopic observers in this study as possible, and at the same time, being able to present the stimuli in an efficient manner (larger contrast-defined stimuli took longer to generate and dislplay).

2

The white noise was generated by first creating a noise array of 256 × 256 pixels, with the luminance of each pixel randomly assigned a value from 0 to 1 according to a rectangular distribution. These luminance values were then scaled to a background maximum luminance contrast of 0.25, which corresponded to a rms contrast of 0.07 (see Chung et al., 2006 for details).

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