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. Author manuscript; available in PMC: 2012 Mar 9.
Published in final edited form as: Atten Percept Psychophys. 2010 Jul;72(5):1338–1349. doi: 10.3758/APP.72.5.1338

Duck!: Scaling the height of a horizontal barrier to body height

Jeanine K Stefanucci 1, Michael N Geuss 1
PMCID: PMC3298367  NIHMSID: NIHMS359367  PMID: 20601715

Abstract

Recent research shows that the body is used to scale environmental extents. We question whether the body is used to scale heights as measured by real actions (Experiments 1 and 2), or judgments about action and extent made from a single viewpoint (Experiments 3 and 4). First, participants walked under barriers either naturally or when wearing shoes or a helmet. Participants required a larger margin of safety (ducked at shorter heights) when they were made taller. In follow-up experiments, participants visually matched barrier heights and judged whether they could walk under them when wearing shoes or a helmet. Only the helmet decreased visually matched estimates; action judgments were no different when taller. The final experiment suggested that the change in matched estimates may have been due to lack of experience wearing the helmet. Overall, the results suggest that perceived height is scaled to the body and that when body height is altered, experience may moderate the rescaling of height.

Keywords: height perception, affordances, perceptual rescaling, body size, eye height

J.J. Gibson hypothesized that perception was directly related to action. Specifically, actors can directly perceive what the environment affords them (termed affordances: Gibson, 1979). What an environment affords an individual is determined by the characteristics of the environment (height, width, etc.) and by the capability of the actor. For example, a doorway affords walking under upright if the height of the doorway is sufficiently higher than the upright height of the actor.

Some research has demonstrated that people can perceive whether the width of an aperture affords passage. Warren and Whang (1987) found that participants scaled their decision of whether an aperture afforded passage to the width of their shoulders. Specifically, when the ratio of aperture-to-shoulder width was 1.16 or larger, participants indicated that they could pass through an aperture. Wagman and Taylor (2005) found that participants who were asked to hold or view objects of different sizes when anticipating walking through an aperture adjusted their judgments of passability to take into account the size of the object. Ishak, Adolph, and Lin (2008) also investigated participants’ ability to judge whether they could reach into an aperture to retrieve candy. Their findings showed that participants scaled their decisions to their hand size. Ishak et al. also varied the size of the hand by placing a prosthesis on participants. The participants, in turn, adjusted their reachability judgments accordingly, showing they were sensitive to the new size of their hand.

Though much has been done to assess how people perceive whether they can fit through an aperture, fewer studies have determined how people perceive if they can walk under a barrier. Perceiving whether an aperture affords passing under is reliant upon perceiving the height of the aperture or barrier in relation to the height of the body. We will first review the literature that describes how people normatively estimate heights and then we will discuss the literature that suggests that a certain aspect of the body, eye height, seems to be the most likely reference for deciding whether one can pass under a barrier or not.

Normative Estimation of Heights

Sinai, Ooi, and He (1998) found that participants who stood at the top of heights (~ 6 ft or 2m in size) overestimated the vertical distance to the ground by about 50% (see also Jackson & Cormack, 2007 and Stefanucci & Proffitt, 2009, for even more overestimation of taller heights). They concluded that this overestimation was likely due to a misperception of eye height, but did not include a situation in which participants viewed the height from the ground. Yang, Dixon, and Proffitt (1999) also tested perception of the height of short objects (such as a light pole or a door) and found approximately 6% overestimation. Their results suggested that taller objects (buildings) were overestimated more than shorter objects (doors) even when the visual angle to all of the objects from the viewing position of the observer was held constant. Overall, many studies have demonstrated that heights are overestimated, and larger heights are overestimated more than shorter heights. However, no one has looked at the normative estimation of heights that can be acted upon (for example, walking underneath).

Scaling Perceived Height

It is possible that for shorter heights (or those in the range of standing height of the participants) observers will reference their body size to scale the extent of the height more accurately. Previous research has shown that people scale visually matched estimates of spatial layout to their bodies. For example, Witt, Proffitt, and Epstein (2005) showed that the perception of distance in near or peripersonal space (the area defined as reachable and just beyond, see Cutting & Vishton, 1995) was scaled to the length of the participants’ arms. When participants held a baton, which extended the space within which they could reach targets, targets that were formerly out of reach appeared closer. Linkenauger et al. (2009) have also shown that the perceived distance to an object and the perceived size of an object is related to the arm length and the hand size of the participant. Participants who perceived their arms as longer also perceived targets as closer and the space within which they could reach objects as larger. Likewise for perceived hand size, participants who estimated their hand to be larger also reported that objects appeared smaller. Similarly, Stefanucci and Geuss (2009) have also shown that perceived aperture width, outside near space, is related to the perceived size of the body. They found that wider participants estimated aperture widths to be smaller than narrower participants. In addition, when participants held a large object or held out their arms, they judged apertures to be smaller than participants who did not hold an object or whose arms were by their sides.

More importantly for this paper, van der Meer (1997) showed that both tall (> 190 cm) and short (< 175 cm) adults who walked under barriers of varying heights ducked at a height that was 1.04 times their standing height, whereas children ducked at a height that was 1.11 times their standing height. Recent work has shown that people who viewed a horizontal barrier from a static viewpoint also scaled their action judgments (estimates of whether they could successfully walk under the height without ducking) to their own height (Wagman & Malek, 2008). In more recent work, Wagman and Malek (2009) measured participants’ willingness to walk under barriers of different heights, that they anticipated walking or running under (though they never truly performed these actions). Not surprisingly, they found that participants were more willing to walk under barriers, especially those that were tall. Thus, willingness to pass under a barrier was influenced by the anticipated speed of passage and the height of the barrier relative to participant height. Further research has shown that participants’ judgments about whether they could pass under a lintel when performing a novel action suggests that practice with the novel action (wheeling a wheelchair) can increase the accuracy of the judgments (Stoffregen et al., 2009). Wagman and Malek (2008) also found that participants’ action decisions were slightly altered by the location of their eye height (in contrast to anticipated speed as manipulated in Wagman & Malek, 2009) when viewing a horizontal barrier from a non-moving viewpoint. For example, when participants were sitting on the ground or standing on a stool, their judgments of whether they could walk under a barrier were less cautious than when they were standing on the ground. Though they argued that the reason for the change in action judgments was due to participants needing to complete a series of nested goals before estimating whether they could pass under, we believe that the change in action judgments could have been due to changes in eye height.

Many researchers have found that observers can scale the sizes or heights of targets to their eye height (Bertamini, Yang, & Proffitt, 1998; Dixon, Proffitt, Wraga, & Williams, 2000; Sedgwick, 1973; Wraga, 1999). Sedgwick (1973) was the first to propose this idea and outlined the mechanism by which participants could use eye height information when standing on the ground plane to scale the sizes of objects that were also on the ground plane, regardless of their distance to the object. Testing this notion further, Bertamini et al. (1998) found that observers were better at judging the heights of objects when the sizes of the objects were in the range of the eye height of the participant, whether seated or standing. For example, the height of an object was more accurately perceived to be its height when the observer was standing and the object’s size was approximately the standing eye height of participants. Wraga (1999) also showed that manipulating the height at which participants viewed an object could influence their estimates of the size of the object. Specifically, she used a false floor to present objects to participants either at the normal floor level or at 17cm above the normal level (effectively reducing eye height information for participants). When the floor was raised, participants overestimated the height of the objects, as predicted, by approximately 14%. Dixon, Wraga, Proffitt, and Williams (2000) extended these findings by showing that manipulations of the location of an observer’s eye height using virtual reality (VR) resulted in a change in their estimates of the height of objects in the virtual world. When the center of projection of the virtual scene was placed at 0.7 times actual eye height (unbeknownst to participants), participants estimated the objects to be larger than when the center of projection was placed at actual eye height.

Eye height is also used to make action judgments in the environment, such as whether a surface affords sitting or climbing (Mark, 1987) and whether a doorway affords passage (Warren & Whang, 1987). For example, Warren and Whang (1987) employed a raised floor and found that participants in that condition judged narrower apertures as passable compared to participants who viewed the apertures on a normal floor. Mark (1987) strapped 10-cm blocks onto the feet of participants in order to raise their eye height. He found that participants’ judgment of the maximum seat height that was perceived as “sitable” was greater when wearing the blocks and the maximum step height that was perceived as “climbable” was shorter.

Current Studies

Overall, the previous findings suggest that participants scale the heights of objects to their eye height. In addition, few studies have tested the perception of the height of objects for which observers can walk under. Furthermore, many of the affordance judgments obtained in the previous work did not involve real interaction with the height, rather a judgment of “passability” or “sitability” from a static viewpoint only. We will add to this literature by first asking participants to walk under a horizontal barrier to find the ratio of actual height to barrier height at which participants will walk under a barrier without ducking. To our knowledge, this action boundary has only been documented in one previous study (van der Meer, 1997). We will document this boundary in order to compare it to manipulations of physical height and eye height, as well as to gauge when people decide to duck in order to walk under an object.

After identifying this action boundary for a horizontal barrier, we will then change the dimensions of the body to discern whether these bodily changes will influence participants’ ducking behaviors (Experiment 2). We will manipulate the height of participants in two ways to test whether eye height scaling is present. One manipulation will include a change to eye height, by having participants stand on blocks, a similar manipulation to that of Mark (1987). The other manipulation will involve participants donning a helmet, thereby changing the height of their body, but not their eye height. In this manner, we will be able to test for differences in action boundaries when a change to the eye height of the observer is present or not. We expect participants to be more cautious walking under (ducking at lower heights) when made taller, but that caution may be greater when there is no change to eye height because they may be less sure of their physical height.

In Experiments 3 and 4, we will obtain both action judgments and visual estimates of the height of the barrier from participants who view the barrier from a static position. In this manner, we will expand on previous work by relating visual estimates of the vertical extent to affordance judgments of passability. We believe that visually matched estimates of the height that are taken from a static viewing position may inform later interactions with the height, as well as being influenced by changes to the body. However, manipulations of eye height may affect decisions about actions in a different manner than they affect matched estimates of the height. For example, in order to decide whether a barrier affords walking under, one may simulate walking under the barrier to make the decision. However, this simulation may not be necessary to match a different extent to the height of the barrier. Again, we will assess the influence of a change to the body on the matched estimates of height and the static affordance judgments to discern whether manipulations of the body similarly or differentially affect these perceptual estimates. Finally, we suspect that participants will be cautious both in their judgments of action from a single viewpoint (obtained in Experiments 3 and 4) and their actual actions observed in Experiments 1 and 2. Very few studies have directly compared action judgments and real actions (see, e.g., Richardson, Marsh, & Baron, 2007), so we hope to be able to compare these measures for perceiving heights as well.

Experiment 1

The purpose of this experiment was to determine the point at which upright walking transitions into bent walking or crouching when participants have to walk under a barrier. Participants were videotaped walking underneath a horizontal barrier (a PVC pipe) positioned at different heights. The tallest barrier height for which participants had to bend to walk under was recorded. This height and the participant’s height determined the action boundary for each participant. We determined this boundary by scaling the highest passable height to the participant’s height, which created a ducking point relative to physical height. This allowed for a comparison of boundaries across participants of different physical heights (short and tall). We predicted that there would be no difference between tall and short individuals’ judgments if their actions were scaled to their own heights (as observed by van der Meer, 1997).

Method

Participants

Ten (4 female, 6 male) volunteers participated in the study. Volunteers were recruited via e-mails and flyers during a summer session. All participants were naïve to the purposes of the experiment and gave written, informed consent to participate. All participants came to the experiment wearing comfortable shoes for walking.

Apparatus

All judgments were made in a 304.8 cm × 304.8 cm room with a solid colored wall. A horizontal barrier was constructed by suspending a pipe (152.4 cm long, 2.54 cm) from the ceiling. The height of the barrier was adjustable through a series of pulleys. The barrier was placed 152.4 cm in front of the starting position, which was 114.3 cm from a wall opposite the barrier. The barrier was raised and lowered to 13 different test heights (147.3 cm to 208.3 cm in 5.08 cm intervals).

Participants walking underneath the horizontal barrier were recorded using a JVC Everio GZ-MG330 video camera mounted on a tripod 182.88 cm from the barrier in the back corner of the room (see Figure 1). Participant’s standing height and eye height were recorded with a standard 12ft (3.63 m) tape measure.

Figure 1.

Figure 1

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Overhead view of the experimental set-up for all experiments. A) Represents the horizontal barrier raised and lowered to different heights. B) The location of the participant during static judgments and the starting position during action trials. C) The approximate position of the camera used in Experiment 1 and 2. D) The location of the projection screen in Experiments 3 and 4.

Procedure

Participants were instructed to walk naturally underneath the barrier and bend as needed to avoid collision. Once they passed under the barrier they were instructed to stop, make a 180-degree turn, and return back under. Each participant walked underneath each test height 6 times, resulting in 78 passages underneath the barriers in total. Test heights were presented in random order.

After the participants returned to the wall opposite the barrier, they remained facing away from the barrier. The next aperture height was set while the participants faced the wall. The experimenter moved out of view before the participants turned to face the barrier again. This process was repeated for each test height. When all 78 heights had been completed, the participant’s height was recorded.

Results

Videos were analyzed by two coders who were blind to the absolute height of each trial. The coders recorded any ducking or bending behavior for each trial. Bending behavior was defined a priori as any motion by any part of the body (head, neck, torso, or knees) that resulted in a lowered height of the participant. Inter-rater reliability was 97.69%.

Average Height of Participants

A median split on the standing height of participants was conducted to form the tall and short groups. The average height of the short group (3 female, 2 male) was 165.86 cm (SD = 4.4 cm). The average height of the tall group (1 female, 4 male) was 183.26 cm (SD = 7.1 cm). Females had an average height of 168.51 cm and males an average height of 178.60 cm1. The average height of all participants was 174.56 cm (SD = 10.8 cm). An independent samples t-test confirmed that the tall group was significantly taller than the short group, t(8) = −4.58, p = 0.002.

Action Boundaries

To create an action boundary, we divided the barrier height at which participants ducked 50% of the time by the participant’s height. This scaled the required environmental extent (the barrier height) to each actor’s height. The mean barrier height to participant height (P/H) was 1.03 with a standard error of 0.01. Action boundaries ranged from 0.99 to 1.09. In other words, participants required, on average, a barrier height that was 1.03 times their own height to walk under without bending.

Independent-samples t-tests were run to compare action boundaries between height groups. The analysis revealed no significant differences between height group, t (8) = .29, p = .68. This tentatively suggests that tall and short individuals require relatively the same amount of space above their heads to walk under a horizontal barrier without ducking.

Discussion

In this experiment, participants were asked to walk under different barrier heights and to duck as needed to avoid collision. As expected, we found that taller individuals ducked at higher heights. However, when barrier heights were scaled to participant height, we found no differences in action boundaries across the tall and short groups. These results suggest that participants scale barrier heights to their own height when walking underneath the barrier. The barrier must be at least 1.03 times their height for them to walk under while standing upright (or on average 5.24 cm taller than their height). This ratio almost exactly matched that obtained by van der Meer (1997). A ratio that is greater than one suggests that participants allowed for a margin of safety above their head when passing under an object as they did in Warren and Whang (1987) when passing through an aperture.

Experiment 2

In this experiment we sought to replicate the results of Experiment 1, but we also wanted to discern whether participants would scale the height of a barrier to their body when their body height had been altered. Therefore, we added to participants’ height by either placing a helmet on their head or asking them to stand on blocks of wood that were attached to the feet. As in Experiment 1, participants walked under horizontal barriers of different heights. We hypothesized that when wearing an object, the observer would require a larger margin of safety when passing under the barrier.

Method

Participants

Ten (3 female, 7 male) undergraduates from the College of William & Mary participated in the study for course credit (Mean age = 21.5 years). All participants were naïve to the purpose of the experiment and gave written, informed consent to participate.

Apparatus

All aspects of the barrier and the video recording were the same as in Experiment 1. The barrier was raised and lowered to 10 different test heights (152.4 cm to 198.12 cm in 5.08 cm intervals). For one height manipulation, participants were asked to don an ROTC helmet that increased their height by approximately 2.54 cm. A chin strap secured the helmet on their head for the duration of the experiment, and a net inside suspended it at a consistent height above the head. It is important to note that the helmet did not obscure or block the field of view of participants in any way (as a baseball cap might). For the other height manipulation, participants were asked to strap wooden blocks that were 3.18 cm tall onto their shoes with Velcro. All participants were told to wear tennis shoes when signing up for the experiment to ensure some consistency across this manipulation and to reduce the chances that participants would show up in high heels.

Procedure

Participants were instructed to walk naturally underneath the barrier and to bend as needed to avoid collision. Once they passed under the barrier they were instructed to stop, make a 180-degree turn, and return back under. Participants walked under each of the ten heights twice when wearing the helmet, the blocks, and not wearing anything. The order of the object conditions was such that the no object condition was always second. The helmet and blocks conditions alternated between being either first or third for participants. After all blocks of trials were completed, the participant’s true height was recorded, as well as their height when wearing the helmet and the blocks.

Results

Videos were analyzed by two coders who were blind to the absolute height of each trial. The coders recorded any ducking or bending behavior for each trial across blocks (helmet, nothing, or block). Bending behavior was defined a priori as any motion by any part of the body (head, neck, torso, or knees) that resulted in a lowered height of the participant. Inter-rater reliability was 94%.

Average Height of Participants in Each Condition

The average height of the group when wearing the helmet was 174.85 cm (SD = 2.4 cm). The average height of the group when wearing the blocks was 176.53 cm (SD = 2.6 cm). The average height of all participants when not wearing any object was 172.22 cm (SD = 2.6 cm).

Action Boundaries

Because we had homogenous groups (participants were in all conditions and therefore height distribution did not change among the conditions), we did not analyze ducking behavior in addition to determining action boundaries as was done in Experiment 1. Analyzing the action boundary would yield the same results as comparing ducking behavior across the conditions. To create action boundaries for each height condition, we divided the previously used barrier heights by the participant’s height for the nothing condition, the participant’s height when wearing the helmet for the helmet condition, and the participant’s height when wearing the blocks for the blocks condition. This scaled the required environmental extent (the barrier heights) to the actor’s current height when walking underneath the barrier. The mean barrier height to participant height (P/H) in the no object condition was 1.002 with a standard error of 0.006. Action boundaries ranged from 0.99 to 1.02. In other words, participants required on average a barrier height that was 1.002 times their own height to walk under without bending. Their actions were quite accurate. The mean barrier height to participant height when wearing the helmet (P/HH) in the helmet condition was 1.03 with a standard error of 0.011. Action boundaries ranged from 1.006 to 1.06. The mean barrier height to participant’s height when wearing the blocks (P/BH) in the block condition was 1.02 with a standard error of 0.009. Action boundaries ranged from 0.99 to 1.04.

A 2 (Order) × 3 (Object: helmet, blocks, none) × 2 (Direction: forward, return) repeated measures ANOVA was run to compare action boundaries between participant height manipulations. The analysis revealed a significant effect of object, F (2, 9) = 9.54, MSE = .001, p = .002, ηp2 = 0.54. There was no effect of direction, F (1, 8) = 1.18, p = .31, or order, F (1, 8) = 1.12, p = .32.

Post-hoc analyses using Fisher’s LSD revealed that participants were more cautious when wearing a helmet than when wearing nothing. Participants were also more cautious when wearing blocks than when wearing nothing. Action boundaries were no different when wearing the helmet than when wearing the blocks (p = .19; see Figure 2).

Figure 2.

Figure 2

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Action boundaries are represented as a ratio of the barrier height that participants transition from up-right walking to “ducking” by participant height. The control, no change in height, condition required significantly a smaller margin of safety than experimental conditions, helmet and blocks. Error bars represent one standard error.

Discussion

In this experiment, participants were asked to walk under different barrier heights and to duck as needed to avoid collision. They walked under the heights when wearing a helmet, blocks on their feet, or not wearing any object. As expected, participants required taller heights to pass under while standing upright and wearing an object that made them taller. When this barrier height was scaled to the participants’ taller heights, analyses revealed that participants required a larger margin of safety when wearing an object (helmet or blocks) than when not wearing an object. In addition, when not wearing an object, participants were less conservative in their ducking behavior. Though comparisons across experiments were not done, participants in this experiment seemed less conservative than in the previous experiment (action boundary of 1.002 vs. 1.03). We believe that participants in this experiment may have been less conservative because they always performed the no object condition after an object condition. Therefore when participants were not wearing an object, they could have been relieved from being over-cautious (from wearing an object) and acted less cautious than those in Experiment 1, who did not just take off an object. Overall, the results suggest that when a change was made to the height of participants, they allowed for a larger margin of safety when walking under the barrier. In Experiments 3 and 4, we investigated whether participants needed a larger margin of error because they estimated the barrier height as shorter when changes were made to their height.

Experiment 3

Previous work has shown that altering the body (by placing weights on the ankles) can result in changes to participants’ judgments of their jumping ability and the jumping ability of others (Ramenzoni, Riley, Shockley, & Davis, 2008). In this experiment we investigated whether manipulating participants’ heights would influence their judgment of the height of a horizontal barrier as well as their judgment about whether or not they could pass under it. Specifically, we added objects (the wooden blocks used in Experiment 2), to participants’ feet. In contrast to Experiments 1 and 2, participants did not walk under the barrier. Rather, in this experiment participants completed two perceptual judgments from a static viewing point (though participants were allowed to sway and probably gained some information from that, see Mark et al., 1990; Stoffregen et al., 2005). We questioned whether participants who wore the blocks would 1) scale action decisions made from one viewpoint to their new height and 2) estimate the barrier to be shorter with a visual matching task when wearing the blocks. We predicted that participants would indicate requiring a larger margin of safety when wearing the blocks on their feet. Additionally, we expected that when participants wore the blocks they would visually estimate the height of the barrier to be shorter than those who did not wear the blocks.

Method

Participants

Thirty-eight (17 female, 21 male) College of William and Mary students participated in the experiment for credit in an introductory psychology course. All participants were naïve to the purpose of the experiment and gave written, informed consent to participate.

Apparatus

Visually matched estimates were obtained by having participants replicate the extent of the height (from the floor to the barrier) in a horizontal dimension. To do this, participants were asked to adjust two lines that were projected onto a large projection screen (177.8 cm high × 210.82 cm wide) in the experiment room, which stood 76.2 cm from the floor. An NEC NP60 projector was connected to a Dell Inspiron 1521 laptop running Matlab, which was used to display the adjustable lines to participants. Participants made half of their height judgments while wearing wooden blocks attached to their feet (the same as those described in Experiment 2). The blocks added 3.18 cm to the height of the participants. All other apparati were the same as in the previous experiments.

Design

A mixed design was used. All participants were in both conditions (blocks on feet and no blocks on feet) when making the affordance judgments, so the manipulation of height was within-participants for the affordance judgments. Order of condition was randomized and counterbalanced between participants. Participants saw each barrier height twice when wearing the blocks and when not wearing them. Nine heights were presented ranging from 157.48 cm to 198.12 cm at increments of 5.08 cm (or 2 inches). The nine barrier heights were blocked and randomized within each block (such that two blocks of nine heights were presented for each condition: blocks and no blocks). The result was 18 affordance judgments per condition per participant.

However, a between-participants design was used to collect the visually matched estimates of the heights. After making all affordance judgments, participants visually estimated each of the nine heights. For the block of visually matched estimates, participants viewed the heights in the same condition as the previous block of affordance trials (either with the blocks on their feet or without blocks). For example, if participants made affordance judgments wearing the blocks first, then made judgments not wearing the blocks, then they visually matched the heights when not wearing the blocks as well.

Procedure

Participants made two perceptual judgments about the height of the horizontal barrier. Participants stood at the home position 139.7 cm from the horizontal barrier. They judged passage under the barrier and estimated its height from this point. They were instructed to imagine walking underneath the horizontal barrier without bending or ducking in any way to make their affordance judgments.

Participants gave 4 judgments of passage for each of the 9 heights. Two judgments were made in each condition (wearing blocks or not). Participants were instructed to respond (yes/no) if they thought they could walk under upright without ducking. In the block condition, participants were asked to strap the blocks to their shoes with Velcro. In this condition, participants were instructed to make all judgments assuming they would walk under the barrier with the blocks attached to their feet.

Visually matched estimates of height were always made at the end of the last block of affordance judgments. Again, the condition for the last block of affordance judgments (either blocks or no blocks) was the condition for the matched estimates. To complete the visual matching task, participants matched the height of the barrier to the distance between two vertical bars projected onto a solid projector screen. At the start of each trial, the vertical bars were projected at a random distance from each other. The participant than (via the experimenter hitting the up or down arrows on the laptop keyboard) moved the bars closer together or farther apart until the horizontal distance between the bars was the best representation of the vertical height of the barrier. The experimenter sat out of participants’ view and adjusted the distance between the bars per the participant’s instructions. The participant was encouraged to look back and forth between the barrier height and the horizontal distance between the bars as many times as needed until he or she was satisfied that it best represented the vertical extent between the floor and the barrier. Visually matched estimates of height were made for all nine heights in random order. The participant was instructed to turn around between each presentation of height and never saw the experimenter near the projection screen or the barrier.

Results

Affordance Judgments

Transition points were computed for both blocks of trials for both conditions (blocks and no blocks) for each participant. In both cases the transition barrier height was computed by averaging the lowest height the participant indicated he or she could pass under without ducking and the highest height that the participant indicated he or she could not pass under. The transition point for the head condition was computed by dividing the transition barrier height by the participant’s height (top of the head to the floor; P/H). The transition point for the block condition was computed by dividing the transition barrier height by the participant’s height when wearing the blocks (top of the head to the bottom of the blocks; P/BH). The result was 4 transition points per participant, 2 for each block within each condition.

A 2 (Order: blocks first, head first) × 2 (Condition: blocks, no blocks) × 2 (blocks of trials) repeated-measures analysis of variance (ANOVA) comparing transition points, with order as the only between-participants factor, revealed a significant main effect of condition, F (1, 41) = 4.82, MSE = .001, p =.03, ηp2 = 0.11. When participants wore the blocks on their feet, they indicated that they needed a larger margin of safety (M = 1.018, SE = .007) to pass under than when they wore no object (M = 1.01, SE = .005; see Figure 3A). There was no significant effect of trial block, F (1, 27) = .01, p =.92, or order, F (1, 41) = 1.03, p = .32.

Figure 3.

Figure 3

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Affordance judgments displayed as the average ratio of barrier height that marked the transition from indicating passability to not divided by participant height. A) Affordance data shown for the control and block conditions from Experiment 3. B) Affordance data shown for the control and helmet conditions from Experiment 4. Error bars represent one standard error.

Height Estimates

A 2 (Condition: blocks, no blocks) × 9 (Barrier Height) repeated-measures ANOVA comparing estimates of height, with condition as between-participants and barrier height as within-participants, did not reveal a significant main effect of condition, F (1, 36) = .18, p = .68. Participants wearing the blocks estimated the height of the barrier to be no different (M = 214.61, SE = 4.62) than participants who were not wearing the blocks (M = 217.37, SE = 4.62; see Figure 4). As expected, there was a significant main effect of barrier height, F (8, 288) = 68.76, MSE = 228.98, p < .0001, ηp2 = .73.

Figure 4.

Figure 4

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Ratio of estimate of height over actual barrier height were computer for each barrier height. When wearing the helmet (Experiment 4) participants estimated the barrier as shorter than when not wearing anything. Wearing blocks under ones feet did not significantly influence estimations of barrier height. In all conditions, participants overestimated the height of the barrier. Estimations of height during the control conditions in Experiment 3 and 4 were averaged for display purposes only.

Discussion

The results indicated that changing the height of participants by adding blocks under their feet influenced participants’ judgments about their action capabilities. Participants’ affordance judgments indicated that they would be more cautious walking under the barrier when wearing the blocks than when wearing nothing. These results are consistent with the findings of Experiment 2 where participants were more cautious walking under the barrier when wearing the same blocks. However, we did not see any differences in size estimates between participants who wore the blocks and those who did not wear the blocks. This finding could be the result of a definite change in the eye height of participants, which they used to rescale perceived height. Also, given that the height of the blocks was shorter than the increments that differentiated the barrier heights, the change may have been too small to alter size estimates. Our finding may also be due to participants’ experience with changes to their height under their feet. However, at this point we cannot make any definitive conclusions about the impact of experience on size judgments. Overall, participants did overestimate the height with the visually matched estimates, as we would expect from previous research (Stefanucci & Proffitt, 2009).

Experiment 4

In this experiment we investigated whether manipulating participants’ heights without altering eye height would also influence their judgments of the height of horizontal barriers. Specifically, we added an object, (the army helmet used in Experiment 2), to participants’ heads instead of adding an object to their feet as in Experiment 3. We wondered if participants would adequately scale decisions about actions to their own height when they wore an object that changed their physical height, but not their eye height. Participants, especially females, likely have more experience with changes to height that occur when wearing shoes of different heights and sizes. However, manipulations of height that occur above the eyes may be harder to take into account given less practice with these manipulations. In this study, participants made two perceptual judgments about the barrier’s height, the same judgments as in Experiment 3: a judgment about passage and then a visually matched estimate of the barrier’s height. We predicted that participants would report feeling less able to pass under the barriers when wearing a helmet than not wearing a helmet. Additionally, we expected that when participants wore the helmet they would visually estimate the height of the barrier to be shorter than those not wearing the helmet.

Method

Participants

Twenty-nine (18 female, 11 male) College of William and Mary students participated in the experiment for credit in an introductory psychology course. All participants were naïve to the purpose of the experiment and gave written, informed consent to participate.

Apparatus

Visually matched estimates were obtained in the same manner as in Experiment 3, by having participants replicate the extent of the height (from the floor to the barrier) in a horizontal dimension on a projection screen. Participants made half of their height judgments while wearing an army helmet. The helmet was borrowed from the local ROTC chapter and added 2.54 cm to the height of the participants (the same helmet as used in Experiment 2). All other apparati were the same as in previous experiments.

Design

The design was the same as that of Experiment 3. A mixed design was used; all participants were in both conditions (helmet and no helmet) when making affordance judgments. After participants completed two blocks of trials of affordance judgments (one for each condition), they were asked to complete the visually matched estimates of height for whichever condition they performed second. Thus, the visually matched estimates were obtained between-participants.

Procedure

The procedure was exactly the same as the procedure in Experiment 3. Participants gave two perceptual judgments about the height of a horizontal barrier: affordance judgments and visually matched estimates of height. Participants stood at the home position 139.7 cm from the horizontal barrier. They judged passage under the barrier and estimated its height from this point. They were instructed to imagine walking underneath the horizontal barrier without bending or ducking in any way to make their affordance judgments. In the helmet condition, participants were asked to don the army helmet. The straps of the helmet were pulled snugly underneath the chin to keep the helmet stable. In the helmet condition, participants were instructed to make all judgments while keeping in mind the height of helmet in addition to their own height.

Results

Affordance Judgments

Transition points were computed for both blocks of trials and for both conditions (helmet and no helmet) for each participant. In both conditions the barrier height was computed by averaging the lowest height the participant indicated he or she could pass under without ducking and the highest height that the participant indicated he or she could not pass under. The transition point for the head condition was computed by dividing the barrier height by the participant’s height (top of the head to the floor; P/H). The transition point for the helmet condition was computed by dividing the barrier’s height by the participant’s height when wearing the helmet (top of the hat to the floor; B/HH). The result was 4 transition points per participant, 2 for each block within each condition.

A 2 (Order: helmet first, head first) × 2 (Condition: helmet, no helmet) × 2 (blocks of trials) repeated-measures analysis of variance (ANOVA) comparing transition points, with all factors within-participants, revealed a significant main effect of condition, F (1, 28) = 7.63, MSE < .000, p =.01, ηp2 = 0.21. When participants wore the helmet, they indicated that they needed a larger margin of safety (M = 1.02, SE = .006) to pass under than when they did not wear a helmet (M = 1.01, SE = .005; see Figure 3B). There was no significant effect of trial block, F (1, 28) = .55, p =.46, or order, F (1, 28) = 1.20, p = .28.

Height Estimates

A 2 (Condition: helmet, no helmet) × 9 (Barrier Height) repeated-measures ANOVA comparing estimates of height, with condition as between-participants and barrier height as within-participants, revealed a significant main effect of condition, F (1, 27) = 4.83, MSE = 6262.05, p = .037, ηp2 = 0.15. Participants wearing the helmet estimated the height of the barrier to be shorter (M = 192.71, SE = 7.05) than participants who wore nothing (M = 214.25, SE = 6.81; see Figure 4). There was also a significant main effect of barrier height, F (8, 216) = 60.78, MSE = 303.8, p < .0001, ηp2 = .69.

Discussion

Participants responded that they required a larger amount of space above the helmet to pass under the barrier than they required above their head to pass under the barrier. The results found that altering participants’ height by adding an object to the top of the head influenced how participants scale their action judgments. These results are consistent with how participants acted in Experiment 2 when wearing the same helmet.

In addition, participants who wore the helmet visually estimated the barrier to be shorter than participants who did not wear the helmet. This result suggests that when body height is altered by not manipulating eye height, estimates of the extent may change, in addition to a change in action judgments. We suspect that the effect on perceptually matched estimates is due, in part, to participants’ lack of experience with altering their height by adding objects to their heads. Specifically, seeing the height as smaller when wearing the helmet may have served as an initial warning mechanism to participants whose height is not normally altered in this manner. To test this claim, we examined the influence of altering one’s height on the perception of barrier heights with a population of experienced “hat wearers,” specifically members of the local ROTC chapter.

Experiment 4b

Many studies have found that experience performing an action, such as passing through an aperture, may not be necessary in order for judgments of action capability to be rescaled when dimensions of the body have been altered (Higuchi et al., 2004; Savelsbergh, Dekker, Vermeer, & Hopkins, 1998). However, judgments of whether or not one can pass under a lintel when in a wheelchair are more accurate with only 2 minutes of practice wheeling the chair (Stoffregen et al., 2009). The results of Experiment 3 suggest that changes to eye height, specifically raising eye height with an addition to the shoes, may not rescale visually matched judgments of height. This null result could have been due to participants’ knowledge about the change to their eye height or to their experience with wearing shoes of different heights. The purpose of this experiment was to determine whether experience with changes to body height that occur on the head could alter visually matched estimates of height and judgments of passage. Specifically, do the effects of seeing the barrier as shorter and acting as if it is shorter when wearing the helmet dissipate with experience? To test the effect of experience, participants were recruited from the local ROTC chapter. Participants wore the same army helmet as used in Experiments 2 and 4, an average of 7 hours per week. We thought that people who were enrolled in ROTC might show differences in their judgments of passage and their perceptual estimates given their experience wearing the helmet.

Method

Participants

Eleven (4 female, 7 male) College of William and Mary ROTC students were paid $5 for their participation in the experiment. All participants were naïve to the purpose of the experiment and gave written, informed consent to participate.

Procedure

All other apparati, design, and procedures were identical to those used in Experiment 4.

Results

We were unable to collect one participant’s estimates of height due to a technical error with the laptop. Unfortunately, no more than 11 participants were willing to participate from the ROTC chapter. Therefore, we believe these results to be preliminary. We also conducted more qualitative analyses in addition to the quantitative given the limited number of participants and likely lower power for detecting effects.

Affordance Judgments

Transition points were computed for both helmet and head conditions using the same methodology as in Experiment 4. Once again, the result was 4 transition points per participant, 2 for each block within each condition.

A 2 (Order: helmet first, head first) × 2 (Condition: helmet, no helmet) × 2 (blocks of trials) repeated-measures analysis of variance (ANOVA) comparing transition points, with order as the only between-participants factor, revealed a significant main effect of condition, F (1, 9) = 6.44, MSE < .000, p =.03, ηp2 = 0.42. When participants wore the helmet, they indicated that they needed a larger margin of safety (M = 1.02, SE = .01) to pass under than when they did not wear a helmet (M = 1.00, SE = .007). There was no significant effect of trial block, F (1, 9) = .20, p =.67, or order, F (1, 9) = .06, p =.82. Qualitatively, closer inspection of the data showed that 9 of the 11 participants were more cautious when wearing the helmet as compared to no helmet, which is obviously driving the significance obtained in the analysis.

Height Estimates

A 2 (Condition: helmet, no helmet) × 9 (Barrier Height) repeated-measures ANOVA comparing estimates of height, with condition as between-participants and barrier height as within-participants, revealed no main effect of condition, F (1, 8) = .651, p = .44, ηp2 = 0.07. Participants’ estimates of barrier height for those who wore the helmet (M = 216.51, SE = 10.14) were not different than estimates made by those who were not wearing the helmet (M = 228.08, SE = 10.14). As expected, there was a significant main effect of barrier height, F (8, 64) = 24.76, MSE = 262.25, p < .0001, ηp2 = .76. Qualitatively, closer inspection of these data showed that estimates for 3 of the 5 participants in the helmet condition trended in the opposite-than-predicted direction, such that they judged the barrier to be taller when wearing the helmet. Similarly, 2 of the 5 participants in the head condition had estimates trending in the opposite-than-predicted effect, specifically all of their estimates were shorter than average.

Discussion

People with experience wearing the ROTC helmet anticipated needing a larger margin of safety for passage under a barrier when wearing the helmet as compared to not wearing the helmet. This result is consistent with the affordance judgments of Experiment 4 with those who, we assume, had limited experience wearing helmets and is also consistent with the real actions performed by those in Experiment 2. It seems that even with experience, participants allow for a margin of safety in making action judgments from a non-moving viewpoint when wearing a helmet than when not.

However, the limited data in this experiment also suggests that the experienced ROTC participants may not estimate the height to be shorter, as the inexperienced helmet participants did in Experiment 4. These findings suggest that the perception of barrier heights as shorter when wearing the helmet may dissipate with practice, though more research is needed to verify this with a larger sample. We do believe that the small effect size obtained for the visually matched estimates (7%) suggests that the effect, if it does exist in experienced helmet-wearers, may be smaller than that observed in inexperienced participants. Future studies could train novice helmet-wearers on scaling heights while wearing the helmet to determine if there is a true pre-test, post-test difference when training is more controlled.

General Discussion

Two interesting and novel findings come out of this work. First, we identified the height at which participants will begin to duck when walking under a horizontal barrier (an action boundary for ducking) and verified that this boundary was extremely similar to that observed by van der Meer (1997). Moreover, this action boundary can be altered if participants’ eye height or physical height is manipulated. In addition, not only are real actions affected by these changes to eye height and physical height, but action judgments from a single viewpoint (here termed affordance judgments) are rescaled with changes to the height of the body. The second, potentially more interesting finding of these experiments is that they show that visually matched estimates of the height of the barrier, unlike affordance judgments, may not always be rescaled with changes to height. Specifically, the matched estimates were rescaled when physical height was changed without altering eye height (helmet), but not when eye height was changed (blocks). These experiments do support previous findings that heights are overestimated when measured with visually matched estimates of height (Stefanucci & Proffitt, 2009); however, these studies show that this overestimation can occur even with extents that are in the range of the standing height of the participant. We will discuss these findings and their implications more below. Overall, this series of experiments showed that people scale the height of a horizontal barrier to their own height, but that the rescaling may differ based on whether real actions, action judgments, or visually matched judgments were used to detect changes in perception.

The results from Experiment 1 demonstrated that when participants walked underneath a barrier they required the height of the barrier to be 1.03 times their own height in order to walk under without ducking. This result adds to the findings of Warren and Whang (1987) who documented that when participants walk through a doorway they require the width of the doorway to be 1.3 times the width of their shoulders in order to walk through without rotating their shoulders. The authors claimed that 1.3 times the shoulder width of the observer allowed for a margin of safety that would be needed in order to avoid collision. The findings of Experiment 1 in this paper suggest that participants may need a smaller margin of safety when walking under a barrier (1.03) than when walking through (1.3) an aperture (as also observed by van der Meer, 1997). Though reasons for this disparity were not tested, we believe that this difference in margin of safety could be due to greater amounts of sway (side-to-side) that occur when walking as compared to “bounce” (vertical sway) produced when walking. If bounce is not as great as sway, then the margin of safety needed to walk under an object without ducking could be reduced in comparison to walking through an aperture. Another possibility is that rotating the shoulders to avoid collision can occur in only one dimension, rotating the shoulders around the vertical axis, whereas ducking can occur by moving the head or bending the knees. Therefore, a larger margin of safety may be needed when only one dimension can be rotated or altered to avoid collision.

The other important set of findings, which mainly come out of Experiments 3, 4a, and 4b, suggest that when viewed from a non-moving viewpoint, perceived barrier height may be rescaled due to manipulations of the height of the body. However, the measure of perceived height that was used seemed to contribute to whether an effect was observed. This work employed two perceptual measures of height, affordance judgments and visually matched estimates of height. In all cases, a change in height resulted in a change in action judgments, which we expected given Wagman and Malek’s (2008) findings that eye height could influence affordance judgments and Ramenzoni et al.’s (2008) findings that wearing ankle weights can rescale participants’ judgments of jumping ability. The affordance judgments also mirrored changes in real actions observed in Experiment 2, such that when participants were taller they reported being less likely to pass under from a non-moving viewpoint and they were more likely to duck when actually passing under. This supports other findings in which real action-scaled ratios have been similar to action judgment ratios (see Richardson et al., 2007). However, when participants’ heights were changed by wearing blocks under their shoes, their visually matched estimates of the height were not different from those who did not wear blocks. This finding is interesting in that it represents a situation in which visually matched estimates were not rescaled due to changes in eye height as expected due to the previous research on eye height scaling of size (Bertamini, Yang, & Proffitt, 1998; Dixon, Proffitt, Wraga, & Williams, 2000; Sedgwick, 1973; Wraga, 1999). Some important differences in our experimental design could explain this discrepancy. First, in the previous research on eye height scaling of size, changes were often made to the environment in order to manipulate perceived eye height, usually outside of the awareness of the participants (e.g., with VR or with a false floor). Our participants were clearly aware of a change in height because it occurred on the person. Given that they could also see the extent of the change (the blocks) in some conditions, this could have informed their estimates. Further work is needed to determine whether awareness of the change could influence visually matched estimates. Also, in the previous work on eye height scaling of size, vision was often restricted and participants were only able to use visual cues, such as the level of the horizon, to estimate the height or size of objects. Our experiments were conducted in a full cue environment, so in the case of the blocks, other information could have supplemented the obvious change in eye height to result in similar scaling of heights across conditions (wearing the blocks or not). However, when eye height was not changed, but physical height was (as in the case of the helmet), participants not only adjusted their action decisions to be more cautious but also reported seeing the barrier as shorter.

There are a myriad of reasons as to why there could be a discrepancy in visually matched estimates across experiments other than the issue of awareness. An obvious limitation is that changes to the body using the blocks and the helmet were not conducted within-participants. Therefore, differences across the participants could have led to our null result. However, there is no reason to believe the populations of participants were drastically different, given that the affordance judgments were similar across experiments. Also, it is possible that changes to eye height are more quickly integrated into the visual system, which results in an immediate change to perceptual judgments. Participants wearing the hat may have needed more time to adjust to their new physical height and for visually matched estimates to be rescaled. In other words, it is possible that the change in visually matched estimates may be due to a lack of experience with changes to physical height that are not accompanied by a change in eye height. This explanation is partially supported by Stoffregen et al.’s (2009) finding that experience wheeling a wheelchair made later judgments about passage under a lintel more accurate, even when the practice did not involve wheeling under barriers.

The results of experiments 4a and 4b suggest that experience may influence the rescaling of perceived height, though these conclusions are tentative given the small sample obtained for Experiment 4b. Participants in Experiment 4a, presumably with less experience wearing helmets, reported seeing the aperture as shorter when wearing the helmet than experienced participants in Experiment 4b. Furthermore, this finding may reflect a warning system to exaggerate to the inexperienced participant that he or she is taller and needs to be more cautious, especially when he or she has not acted in the environment. Though the number of ROTC members who were able to participate was small, the findings, along with those of Experiment 3, provide tentative support for the notion that experience with changes to the body may influence visually matched estimates of height. This tentative moderating effect of experience may be especially important when the changes that are made to the body do not influence eye height or are not easily incorporated through some other mechanism.

Future research could investigate the possible moderating influence of experience on size estimates. We believe that fewer people experience changes to their height that involve adding objects to the head, rather than to the feet. Obviously, both occur in daily life (wearing high heels or wearing a tall hat), but the occurrence of changes to the feet may be more common. To further test the notion that experience with changes to bodily height moderates height perception, it would be interesting and useful to test other populations of people who are more experienced with changes to their height in future studies. For example, people who wear stilts would likely estimate a height differently than someone who has never worn stilts. Pilots may be more adept at estimating height given their level of experience with judging heights. Also, participants could be trained with changes to height and their visually matched estimates observed over the course of the training to reveal the point at which experience alters matching estimates. As shown by Stoffregen et al. (2009), the training may not need to be extensive. Other lines of research have also shown that practice learning a skill can alter actions and perception (Bruggeman, Pick, & Rieser, 2005; Repp & Knoblich, 2007). Bruggeman, Pick and Rieser (2005) showed that participants recalibrated their throwing behavior over time when asked to throw to a target while on a rotating carousel. Repp and Knoblich (2007) found that trained pianists were more likely to hear a tone pair as rising when their actions (playing the piano keys) were consistent with that perception.

Another important question to address would be whether the level of change of bodily height directly contributed to the extent of rescaling observed in these experiments. We made only small changes to the body and observed fairly small, but significant changes in action judgments and sometimes in visually matched estimates of height. This rescaling of height perception was not surprising, given we have shown that changes in the body’s width can rescale the perception of the width of apertures (Stefanucci & Geuss, 2009). But, it would be useful to identify how large the change in bodily height needs to be to result in a rescaling of perceived height. Furthermore, it would be interesting to assess whether changes to bodily height influence perception in a linear fashion, or if there is a general threshold at which a change to the body initiates a warning mechanism and alters perceived barrier height. In our previous work on changes to body width resulting in a rescaling of perceived aperture size, we found that once participants’ bodies were made wider than their shoulders, participants perceived the size of the aperture to be smaller (Stefanucci & Geuss, 2009).

Similarly, it is unclear whether small changes of the nature in these experiments would affect the scaling of large heights, like a building, which have mostly been the focus of our previous work (Stefanucci & Proffitt, 2009; Stefanucci & Storbeck, 2009; Teachman et al., 2008). We would not predict a rescaling of tall heights when changes to the body are small, but if someone were standing on stilts or holding a long pole upright, then rescaling might arise. Further work is needed to explore the conditions under which rescaling may occur. The findings of Ramenzoni et al. (2008) suggest that simply adding ankle weights to participants was sufficient to change participants’ judgments about jumping ability, for heights that were higher than those used in this experiment.

Overall, our work suggests that the body, particularly eye height, is used to scale the extent of heights that one anticipates walking under. The findings add to a growing body of literature that has found that the body affects perceptual estimates of passage through an aperture. Here, we find that the body may also influence some (but not all) perceptual estimates of passage under an aperture and estimates of the height of the aperture. Furthermore, these estimates may be influenced by the level of experience that observers have with changes to their eye height.

Acknowledgments

The authors wish to thank Andrea Bidanset and Laura Shepherd for their help in collecting the data. This research was supported in part by NSF grant IIS-0914488, for which the first author is a co-PI, and NIH RO1MH075781-01A2 grant, for which the first author serves as a consultant.

Footnotes

1

We decided not to perform analyses of gender, because gender analyses would have resulted in similar findings, given membership in a height group covaried with gender.

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