Difficulty and Distance 1

The Effect of Task Difficulty on the Perception of Distance

John M. Franchak University of Virginia

Difficulty and Distance 2 Abstract Past research has indicated that the perception of distance is determined by optical information about the stimulus and the physiological state of the observer. Perceived distance is affected by the amount of effort needed to perform an intended action (Proffitt et al., 2003). The current study shows that the difficulty of an action not due to physical effort also has an effect on the way an observer perceives the associated distance. Participants threw beanbags to a target under easy or difficult circumstances and then walked to the target while blindfolded. Greater throwing difficulty induced greater blindwalking distances, indicating that increasing the difficulty of interacting with a target also increases the perceived distance to the target.

Difficulty and Distance 3 The Effect of Task Difficulty on the Perception of Distance The most fundamental perceptual stimulus is the ground plane on which we stand. Its two main components, slant and extent, are the subjects of a great deal of research. This research has primarily investigated how the optical characteristics of the stimulus relate to what is perceived. However, these studies place little weight on the role of the visual system in planning and executing actions in the world. Past research has treated the observer as a passive entity, but in reality the observer is free to act. The ecological approach to perception, as proposed by Gibson (1979), stresses the importance of the presence of the body in the environment and its potential to engage in a variety of actions. Recent research has shown that the physiological state of the observer has an effect on the way that he or she perceives slant and extent. The current study explores how non-visual factors other than the observer’s physiological state affect perception. Specifically, we investigate the role of task difficulty in the perception of distance. The difficulty of a task depends on the physical effort involved in performing the task as well as the degree of accuracy and coordination that the task requires. Most research on perceptual biases has involved the physical aspect of difficulty, which will be referred to as effort. The current study focuses on the need to be accurate and finely coordinate actions in a given task, which will be referred to as the task’s difficulty. Perceptual Biases The classic view of perception likens our visual system to a video camera – the image of a camera is an exact optical representation of how the world looks. In this view, perception is thought to be veridical. However, researchers continue to uncover perceptual biases that challenge the veridicality of the visual system. Notably, there is a general bias to perceive hills as steeper than they really are. A 5° hill tends to be estimated as 20°, and the slants of 10° hills

Difficulty and Distance 4 are usually judged as 30° (Proffitt, Bhalla, Gossweiler, & Midgett, 1995). This bias exists when hills are estimated verbally or through the use of a perceptual matching task. The bias is absent when the observer estimates the slant of the hill using a motor method, such as adjusting his or her hand to match the degree of slant. Perceptual biases affect conscious visual perception, but they do not affect the aspect of visual perception that is responsible for the guidance of action. This explains why we see a hill as steeper than it is, but we do not have difficulty walking on the hill despite our distorted perception of its incline. Distance perception is also affected by systematic biases. Distances less than 2m tend to be overestimated, while distances over 2m and less than 15m (including those used in the current study) tend to be underestimated (Loomis, Da Silva, Philbeck, & Fukusima, 1996). Common measures of perceived distance include verbal estimates of distance as well as visually directed actions. The visually directed action employed in the current study is blindwalking – walking the distance to a previously viewed target while blindfolded. Verbal reports and visually directed actions are both measurements of the same internal representation of perceived distance, although the two types of measurement may give different output values (Philbeck & Loomis, 1997). The existence of these perceptual biases supports the argument that vision is not a veridical representation of the world. Rather, it seems that perception is malleable. Our physiological state is one factor that contributes to this malleability – effort increases estimates of slant and extent. Effort Affects Slant and Distance Perception Bhalla and Proffitt (1999) showed that the observer’s physiological state affects verbal and perceptual matching estimates of the steepness of hills in a natural setting. Participants who

Difficulty and Distance 5 wore a heavy backpack estimated hills to be steeper than unencumbered participants. Hills appeared steeper to individuals who had low physical fitness, declining health, or who were fatigued from jogging. Collectively, these results indicate that increasing the physical effort needed to climb a hill also increases one’s conscious perception of the hill’s incline. Proffitt, Stefanucci, Banton, and Epstein (2003) found that effort plays an analogous role in the perception of egocentric distance. An observer wearing a heavy backpack sees distances as farther than an observer who is unencumbered. Both of these studies manipulate walking effort, but effort manipulations for other types of actions also can affect perception. Witt, Proffitt, and Epstein (2004) had participants throw a ball towards a cone in a field, and then estimated the distance to the cone using three measures: verbal, perceptual matching, and blindwalking. According to verbal and perceptual matching estimates, the cone appeared farther to participants who threw a heavier ball compared to participants who threw a lighter ball. However, there was no difference in blindwalking between participants throwing heavy balls or light balls. Why would verbal and perceptual matching estimates show an effect based on throwing effort when blindwalking does not? Witt et al. explain that it is the effort attributed only to the intended action of an observer that affects the perception of distance. So, effortful throwing makes distances appear farther only if an individual intends to continue throwing, and increased walking effort makes distances appear farther only if the individual intends to walk. The specificity of these perceptual effects with regard to intended actions highlights the plastic relationship between the visual and motor systems. All of the above effects are demonstrations of visual motor adaptations. The amount of energy we expend walking relates to the speed at which we perceive the world moving past us (optic flow). However, this

Difficulty and Distance 6 relationship between effort and perception is continually adapted. Proffitt et al. (2003) showed that changing the calibration between walking effort and optic flow affects perceived distance. Optic flow refers to the visual experience of the motion of the world moving by as we walk. Participants in the study walked on a treadmill for three minutes at 3 mph while wearing a virtual reality head display. In the flow condition, the virtual world displayed optic flow of 3 mph that matched the participants’ walking speed. In the no-flow condition, the participants walked on the treadmill but received no optic flow in the virtual display. Consequently, participants in the no-flow condition learned that it takes a lot of walking effort to remain stationary, whereas participants in the flow condition saw the visual world change in accordance with the amount of effort they expended. After this adaptation, participants verbally estimated the distances to cones in a hallway. Distances appeared farther to participants who received no optic flow than to participants that had received optic flow. When walking on the treadmill without optic flow, the observer learns what amount of walking energy (a brisk 3 mph pace) corresponds to his or her visual experience (remaining stationary). When the observer intends to walk to a cone after this adaptation, the distance seems farther because the observer has learned that walking takes more effort. This adaptation is comparable to putting on a heavy backpack – it manipulates our physiological potential to act, which in turn influences perceived distance. Witt et al. (2004) replicated this finding using a similar apparatus. However, in their study, all participants received no optic flow while walking on the treadmill and thus became adapted to greater walking effort. After adaptation, participants were taken to a hallway and shown a cone. Half of the participants were told that they were going to throw a ball to the cone, while the other half were told that they were going to walk to the cone. Distances appeared farther to those that intended to walk to the cone compared to those who intended to throw to the

Difficulty and Distance 7 cone. All of the participants in this experiment adapted to increased walking effort as in Proffitt et al. (2003), which showed that adaptation to high walking effort makes distances appear farther. In this experiment, the researchers replicated that same effect, but showed that it only applied if the participants intended to walk to the cone. The distances did not appear farther to the participants that intended to throw to the cone because only walking effort was subject to the adaptation. Rieser, Pick, Ashmead, and Garing (1995) showed similarly that adaptation to high or low throwing effort affects post-adaptation throwing but not post-adaptation walking. Thus, perception is affected by visual motor adaptations that are influential only with respect to the actions that an individual intends to perform. All of these studies strengthen the claim that changes in the physiological state of an observer change the perception of slant and extent. Specifically, increased effort required to perform an intended action will lead to overestimation of slant and extent. These effects are due to feedback between the visual and motor systems. However, optical and physiological information might not be the only governing forces of perception. Perceived Difficulty Affects the Perception of Object Size The physical effort required to traverse a distance or throw a ball contributes to the difficulty of performing either action. The degree of fine motor coordination and accuracy involved in a task also relate to our perception of its difficulty. Recent research has investigated how difficulty affects perception in ways unrelated to physical effort. Wesp, Cichello, Gracia, and Davis (2004) examined the size estimates of objects in the context of purposeful actions. Participants tried to drop darts onto a small, circular target until they hit the target. Half of the participants estimated the size of the target before dropping the darts, and the other half made estimates afterwards. There was a strong negative correlation found between the number of darts

Difficulty and Distance 8 needed to hit the target and the estimated size of the target. However, this relationship was found only for the group that made estimates after dropping darts. Wesp et al. concluded that the participants’ perceived difficulty of the task affected the way that they saw the target. If they found the task easy, the target appeared large. If the task was difficult, the target seemed small. Thus, it seems that the perceived size of an object is influenced by information about the difficulty of interacting with it. Witt and Proffitt (2004) found a similar relationship in a correlational study. The researchers attended a number of intramural softball games and surveyed the players about their hitting performance. The players also estimated the size of the softball by choosing the closest matching circle from an array of circles of varying sizes. Batting average correlated positively with the perceived size of the ball. This study indicates that efficacy affects the perception of objects used in purposeful actions. Baseball players who had performed well actually saw a larger softball than players who had hit poorly during the game. This finding matches the anecdotal reports of ballplayers who said that they felt like they were swinging at grapefruits during outstanding performances. Both studies show that success at performing a task affects the perception of the size of objects involved in the task. Both results are intuitive – the target of our action, either the circle on which we drop darts or the ball at which we swing the bat, appears larger to us when we perform well. The physical dimensions of the task are malleable with respect to how easy we believe the task is and how well we are performing. The current study searches for a similar bias in the perceived distance associated with a task of varying difficulty rather than in the perceived size of the target.

Difficulty and Distance 9 Can Task Difficulty Alter Perceived Egocentric Distance? Previous research has shown that increasing the energy needed to perform an intended action leads to overestimation of the distance and slant associated with the action (Bhalla & Proffitt, 1999; Proffitt et al., 2003; Witt et al., 2004). There is an invariant relationship between physical effort and distance; more effort is expended in walking or throwing to farther distances. Similarly, there is an invariant relationship between difficulty and distance. It is more difficult to interact with objects that are farther away, especially if the interaction demands motor accuracy. Difficulty seems to be the cognitive analog to physical effort, and perceived difficulty affects the size estimates of objects used in purposeful actions (Wesp et al., 2004; Witt & Proffitt., 2004). What remains to be known is if the difficulty of a task can affect egocentric distance perception. The current study attempts to show that increasing the difficulty of a task unrelated to physical effort also increases the perception of the associated distance. Confirmation of this hypothesis would suggest that cognitive factors as well as physiological factors play a role in the perception of the ground plane. Methods Participants Forty University of Virginia students (20 male and 20 female) participated. All participants had normal or corrected-to-normal vision. Participants were either paid or recruited in order to fulfill a requirement of an introductory psychology course. Participants were naïve to the purpose of the experiment. Five participants did not complete the study and their data were not included in the analyses. One of these participants refused to be blindfolded and the other 4 participants were disrupted by people walking through the experiment location.

Difficulty and Distance 10 Apparatus & Stimuli The experiment was conducted in the basement hallway of the psychology building of the University of Virginia. Two small sports cones placed across the width of the hallway created a baseline where participants stood during the experiment. Tape markers along the side of the hallway marked 4m, 5.5m, 7m, and 8.5m in both directions from the baseline and were not visible to the participants. At each of these distances, two small marks were made on the floor 1m apart and centered in the hallway. These marks allowed four small orange cones to be placed on the floor creating rectangular targets 1m wide and 1.5m long at three distances (4m, 5.5m, and 7m, see Figure 1). During each trial, only four cones were placed on the floor to create a single target. These three targets served as the perceptual stimuli in this experiment. Three small beanbags were used in this experiment. Each beanbag was 10cm by 10cm and weighed about 125 grams. Design Participants were assigned in alternating order to either the blind-throwing or sightedthrowing condition. Ten males and 10 females participated in each condition and all participants were naïve to condition. Each participant completed one practice block followed by three test blocks, one for each of the three target distances. In the practice block, all participants completed the block according to the procedure for the sighted-throwing condition. The order of the presentation of test blocks was counterbalanced. The direction the participant faced in the hallway was also counterbalanced for each target distance. Successive test blocks alternated between the two directions to minimize the use of comparisons between trials that might aid in distance judgment.

Difficulty and Distance 11 Procedure For participants in both conditions, each block in the experiment consisted of two phases – throwing and blindwalking. In the throwing phase of each block, the participant threw a beanbag to the target six times. The goal of each throw was to slide the beanbag along the floor and have it stop within the target. The throw was considered a “hit” if (a) the beanbag stopped within the outer boundary created by the four cones, (b) the beanbag did not touch any of the cones or the walls, and (c) the beanbag came in contact with the ground before entering the target. All other throws were considered misses. After each throw, the experimenter told the participant if the throw was a hit or a miss. The number of hits the participant managed for each target was recorded as a measure of task difficulty. After the sixth throw, the participants completed the blindwalking phase of the block. The participants were given a blindfold and then received instructions. Participants were told to walk to the center of the target and then tell the experimenter when they were finished walking. After hearing the instructions, the participants put on the blindfold and tried to walk to the target. Once the blindfold was on, the experimenter removed the target cones from the floor and kept silent. Participants were removed from the study if people in the hallway made noise that might have provided location cues during the blindfolded phase. When the participant indicated that he or she was done walking, the experimenter marked the distance for measurement. Still blindfolded, the participants were led back to the baseline so that they received no feedback about the distance walked. Participants in the sighted-throwing condition completed the procedure exactly as described. Participants in the blind-throwing condition completed the throwing phase differently in two ways. First, participants in this condition threw with their non-dominant hands. Second,

Difficulty and Distance 12 instead of throwing with their eyes open, the participants closed their eyes before throwing and counted 2 full seconds before throwing the beanbag. The participants were instructed to open their eyes as soon as the beanbag left their hands. This enabled participants in both conditions to receive almost the same amount of throwing feedback. After completing the throwing phase, participants in the blind-throwing condition opened their eyes and then viewed the target for the same amount of time as the participants in the sighted-throwing condition. Results A 2 (sex) x 2 (condition) x 3 (target distance) repeated measures analysis of variance was performed for the number of hits made at each target, with target distance as the within-subjects factor and sex and condition as between-subjects factors. As expected, the analysis showed an effect of condition on throwing accuracy, F(1,38) = 30.005, p < .001. There was no significant effect of sex (p = .796), and the Sex x Condition interaction was also nonsignificant (p = .642). Furthermore, independent samples t-tests indicated that participants in the sighted-throwing condition hit the target significantly more than the participants in the blind-throwing condition at each target distance (p < .005, two-tailed, see Figure 2). A 2 (sex) x 2 (condition) x 3 (target distance) repeated measures analysis of variance was performed for blindwalking distance. Target distance was the within-subjects factor and sex and condition were the between-subjects factors. As predicted, the analysis yielded an effect of condition on blindwalking distance, F(1,38) = 6.681, p = .014. There was no significant effect of sex (p = .131), nor was there a significant interaction of Sex x Condition (p = .244). Additionally, independent samples t-tests showed that participants in the sighted-throwing condition walked significantly less far than participants in the blind-throwing condition at each of the target distances (p < .05, one-tailed, see Figure 3). As shown in Figure 4, the distances

Difficulty and Distance 13 traversed by participants in both conditions fell short of the actual distance, consistent with the instances of distance compression in many other studies (Loomis, Da Silva, Fujita, & Fukusima, 1992; Proffitt et al., 2003; Witt et al., 2004). At 4.75m, there was a negative correlation found relating number of target hits and blindwalking distance (r = -.413, p = .008). No significant correlations were found relating target hits and blindwalking distance at 6.25m or 7.75m. Discussion The present research supports the proposal that task difficulty affects perceived distance. The only difference between the participants in the two groups was the way that they threw the beanbags. This manipulation proved to have a significant effect on the participants’ success at hitting the target. Throwing without vision seems like a challenging task, and the experience of performing poorly at the task allows the participant to verify that he or she is engaged in a difficult action. Participants who were successful at hitting the target and perceived the task as easy walked a shorter distance than participants who perceived the task as difficult. The difference in blindwalking distances is due to a difference in perception of the target location. The apparent ease or difficulty of throwing the beanbag to the target affected the participants’ perception of the target locations. This influence operates in accordance with the invariant relationship between a target’s distance and the ease of interacting with the target – apparent difficulty extends the perceived distance while apparent ease compresses the distance to the target. Participants in both conditions have the same optical information specifying the location of the target. If only optical information was used to determine the target’s location, then there would have been no difference between the conditions. However, it appears that the participants

Difficulty and Distance 14 incorporated information about their interactions with the target. We learn from experience that throwing a ball accurately to a target is easier if the target is closer and harder if the target is far away. Participants in the sighted-throwing condition hit the target easily and perceived the task to be simple, which in turn influenced them to see the target as close. Participants in the blindthrowing condition struggled to hit the target and perceived the task to be challenging. Since distance makes actions more difficult, they perceived the target as far away. The difference in perceived target distance between the two groups is seen in the differences in their blindwalking distances – sighted-throwing participants walked a shorter distance on average than blindthrowing participants. An alternative account might argue that the perceptual difference between the groups is not due to the difficulty of the task but rather to a difference in experience with the stimulus between the two groups. Participants in the blind-throwing condition do not see the target quite as much as participants in the sighted-throwing condition, and this variation might confound the results. However, the only times that the blind-throwing group does not see the target are the 2 second periods before each of the six throws. They see the target before throwing and they see the beanbag traveling to the target after the throw. It seems implausible that a 12 second difference in viewing a target would affect the perception of it, especially since all participants viewed the target for the same amount of time directly before blindwalking. Even so, the present study seems to be in conflict with previous research on throwing and distance perception. Perceptual effects involving visual motor adaptation are specific to the intended action. Both Rieser et al. (1995) and Witt et al. (2004) found that manipulations of throwing effort did not affect blindwalking distance estimates. Only when blindwalking effort was manipulated and observers intended to walk did perception change. However, in the current

Difficulty and Distance 15 study, the participants receive no information indicating that blindwalking to the target will be more or less difficult or effortful than normal. The manipulation deals only with throwing, yet the perceptual effect applies to walking. How can this conflict be resolved? The most satisfying explanation is that the current study falls into another category of perceptual effect than the work of Rieser et al. (1995), Proffitt et al. (2003), and Witt et al. (2004). The latter three effects operate through visual motor adaptations. They relate the physiological state of the observer to the way the world is perceived. Changing the effort of acting also changes the way actions appear to the perceptual system. As this relationship is altered for a specific action, estimates of distance are distorted when the observer intends to perform that particular action. Distance estimates will be distorted for any distances which the observer intends to perform that action on – the effect is specific regarding the intended action but not regarding the visual stimulus. Conversely, the current experiment does not affect the calibration between the motor and visual systems. Participants in both conditions had not experienced any manipulation that might change their physiological potential to act. We find that the distance estimates do not require specificity of the intended action. Perception is not recalibrated based on information about the motor action, such as for visual motor adaptation, but rather perception in this instance is somehow affected by information about interacting with the specific stimulus. This type of effect would not predict that observers would exhibit the same bias when blindwalking to a target that they had not acted on. Instead, we predict that distance estimates will be distorted only for the specific stimuli regardless of the intended action. The results of Wesp et al. (2005) and Witt and Proffitt (2004) also fall into this category. Observers’ distorted perceptions of size should be constrained to the target, not to all similarly shaped spherical or circular objects. This

Difficulty and Distance 16 hypothesis is testable, and needs to be examined in order to determine if there is truly a distinction between effort and difficulty research. The results of the current study and the recent findings in size perception give us reason to investigate a category of perceptual bias that does not involve the observer’s physiological state. However, variations of the current study must be conducted in order to determine if the proposed distinction from visual motor adaptation research is warranted. We predict that the current effect pertains only to the specific stimulus that was acted on. Experimental support of this prediction will help determine the relationship between the research on difficulty and the research on physical effort. It is not clear in this study if perceived task difficulty or efficacy is responsible for the effect on distance estimation. Efficacy and perceived difficulty are certainly related. Efficacy refers to our success at a task, which may influence us to perceive the task as more or less difficult. However, we may perceive a task to be difficult or easy regardless of how well we perform, especially if we do not know how to measure our performance. The research of Witt and Proffitt (2004) and Wesp et al. (2005) shows a correlation between perception and performance, which implies that efficacy is responsible for the perceptual effects. In the current study, such a correlation is only present for the shortest of the three target distances. Yet, there is a difference in perception between the conditions at the other two distances despite the lack of correlation with performance. It might be that the efficacy in the size estimation studies affects perceived difficulty rather than directly changing perception since efficacy was not needed to drive the effect in the current study. A more refined approach is needed to determine the difference, if any, between the effect of efficacy and of perceived difficulty on distance and size perception.

Difficulty and Distance 17 A second ambiguity in the current study is whether or not the participants are encoding the distance to the target or the target’s location. Perceived distance and perceived location are subtly different. Perceived distance is the distance between an observer and a point, whereas the perceived location of a point in space is specified by the direction to the point and its distance. However, the distance between an observer and the point does not need to be perceived if the observer is acting based on a perceived location. The location of a cone can be determined by the right triangle formed by the observer’s eye height and the angle of the observer’s gaze to the cone (see Figure 5). Since the cone’s location can be specified in this way, the observer does not need to perceive the distance in order to blindwalk to the cone. However, in order for the observer to face the opposite direction and match the distance to the cone by blindwalking, it is necessary for the observer to encode perceived distance because the location information is not available. Since participants walk directly to the target during the blindwalking phase, it is not clear if they are encoding the location or distance of the target in the current study. It is important to examine this distinction because perceived distance and perceived location have somewhat different properties. Specifically, perceived location has been shown to be an invariant in the control of action. Philbeck, Loomis, and Beall (1997) showed that different actions directed towards a fixed target converge on a single perceived location, and that this perceived location can be affected by the availability of cues to determine the distance of the target. Wraga, Creem, and Proffitt (2000) demonstrated that the Müller-Lyer illusion affects perceived distance but not perceived location. Verbal reports of the target are affected by the illusion because they represent perceived location, yet blindwalking directly to the target is not affected because it employs perceived location.

Difficulty and Distance 18 It is possible to determine experimentally if the effect of the current study affects perceived distance and/or perceived location. Since participants are directed to walk to the center of the target, it is not certain that they can encode a location, since their action is directed to an empty space on the floor. By putting a cone in the center of the target, we can be sure that participants perceive the location of the target. We can also look at perceived distance more distinctly by asking participants to walk in the opposite direction from the target when estimating the distance. We might see that difficulty affects both perceived distance and perceived location, but that perceived location is not affected as greatly because it does not necessarily depend on the perception of distance. Clarification of which factors contribute to task difficulty’s effect on distance perception as well as a clearer investigation of its distinction from visual motor adaptation will help establish where this research fits into the domain of perceptual psychology. Although these ambiguities may make it difficult to fully understand the implications of these results, they do not alter the effect that the current study has demonstrated. The current study strengthens the view that perception is not an isolated module that is never affected by the functioning of our body’s other systems. In this study we see that how an observer regards the difficulty of throwing a ball to a target affects the way that he or she perceives the distance of the target. Perception is changed by our current concept of the success of our actions. Past research has clearly shown that the physiological state of the observer alters distance and slant perception. Our current direction has given cause to consider how information about the perceived difficulty or efficacy of a task plays a role in conscious perception. Continually, we find that action is linked intricately with perception – perception guides our actions in the world. In turn, feedback about our interaction with the world alters our perception

Difficulty and Distance 19 of its dimensions. By acknowledging this relationship between perception and action, we can better understand the workings of the visual system.

Difficulty and Distance 20 References Bhalla, M. & Proffitt, D. R. (1999). Visual-motor recalibration in geographical slant perception. Journal of Experimental Psychology: Human Perception and Performance, 25, 1076-1096. Gibson, J. J. (1979). The ecological approach to visual perception. Boston: HoughtonMifflin. Loomis, J. M., Da Silva, J. A., Fujita, N., & Fukusima, S. (1992). Visual space perception and visually directed action. Journal of Experimental Psychology: Human Perception and Performance, 18, 906-921. Loomis, J. M., Da Silva, J. A., Philbeck, J. W., & Fukusima, S. (1996). Visual perception of location and distance. Current Directions in Psychological Science, 5, 72-77. Philbeck, J. W., & Loomis, J. M. (1997). Comparison of two indicators of perceived egocentric distance under full-cue and reduced-cue conditions. Journal of Experimental Psychology: Human Perception and Performance, 23, 72-85. Philbeck, J. W., Loomis, J. M., & Beall, A. C. (1997). Visually perceived location is an invariant in the control of action. Perception & Psychophysics, 59, 601-612. Proffitt, D. R., Stefanucci, J., Banton, T., & Epstein, W. (2003). The role of effort in perceiving distance. Psychological Science, 14, 106-112. Rieser, J. J., Pick, H. L., Ashmead, D. H., & Garing, A. E. (1995). Calibration of human locomototion and models of perceptual-motor organization. Journal of Experimental Psychology: Human Perception and Performance, 21, 480-497.

Difficulty and Distance 21 Wesp, R., Cichello, P., Gracia, E. B., & Davis, K. (2004). Observing and engaging in purposeful actions with objects influences estimates of their size. Perception & Psychophysics, 66, 1261-1267. Witt, J. K., & Proffitt, D. R. (2004). Baseball and Perceived Size. Talk presented at Psychonomics. Minneapolis, Mn. Witt, J. K., Proffitt, D. R., & Epstein, W. (2004). Perceiving distance: a role of effort and intent. Perception, 33, 577-590. Wraga, M., Creem, S. H., & Proffitt, D. R. (2000). Perception-action dissociations of a walkable Müller-Lyer configuration. Psychological Science, 11, 239-243.

Difficulty and Distance 22

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Figure 4: Blindwalking distance as a function of throwing difficulty in comparison to actual distance.

Difficulty and Distance 26

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Figure 5: The perceived location of the cone can be determined by triangulation using the observer’s eye height, x, and the angle of the observer’s gaze towards the cone, ?. With this information, the observer does not need to directly perceive the distance to the cone in order to blindwalk to it.