Perception, 2007, volume 36, pages 1752 ^ 1768
doi:10.1068/p5617
Seeing beyond the target: Environmental context affects
distance perception
Jessica K Witt, Jeanine K Stefanucciô, Cedar R Riener ½, Dennis R Proffitt#
Department of Psychological Sciences, Purdue University, 703 Third Street, West Lafayette,
IN 47907, USA; ô Department of Psychology, College of William and Mary in Virginia, Williamsburg,
VA 23187, USA; ½ Department of Psychology, Mills College, Oakland, CA 94613, USA;
# Department of Psychology, University of Virginia, Charlottesville, VA 22903, USA;
e-mail: jkwitt@purdue.edu
Received 25 April 2006, in revised form (lost in the post) 14 November 2006, resubmitted 29 October 2007;
published online 5 December 2007
Abstract. It is commonly assumed that perceived distance in full-cue, ecologically valid environments
is redundantly specified and approximately veridical. However, recent research has called this assumption into question by demonstrating that distance perception varies in different types of environments
even under full-cue viewing conditions. We report five experiments that demonstrate an effect
of environmental context on perceived distance. We measured perceived distance in two types of
environments (indoors and outdoors) with two types of measures (perceptual matching and
blindwalking). We found effects of environmental context for both egocentric and exocentric
distances. Across conditions, within individual experiments, all viewer-to-target depth-related
variables were kept constant. The differences in perceived distance must therefore be explained
by variations in the space beyond the target.
1 Introduction
An enduring problem in perceptual research is how our visual system determines the
distance between ourselves and a target object? Many researchers have attempted to
solve this problem by relating optical and ocular-motor variables to apparent distance.
It is generally conceded that under full-cue viewing conditions when the optical variables informative to depth are robust, perception of the same physical distance should
be consistent. Despite this assumption, there have been several reports of inconsistency
in perceived distance under full-cue viewing conditions (see below). These studies
demonstrated differences in perceived distance across various environmental contexts
despite a full range of available optical depth cues. The environments in which these
inconsistencies were noted differed mostly in the space between the perceiver and the
target (subsequently called the viewer-to-target space or VTT space). In the following
studies, we assessed perceived distance to targets in several contexts that differed only
in the space beyond the target (which we will call vista space (1) ). The distinction
between VTT space and vista space is not based on a general divergence in the visual
properties of the scene, but rather on the task of perceiving the distance between
oneself and a target along the ground. Further, our contexts differed beyond the target
only in cues that were not informative to the actual depth of the target. For example,
environmental contexts may differ beyond the target in the height of the horizon,
which is informative to the actual depth of the target, or in color, which is not informative to the depth of the target. We found that differences in the vista space context
uninformative for depth influenced perceived distance.
(1) Cutting and Vishton (1995) define egocentric regions of space relative to the robustness of various
depth cues. Vista space was defined as the space 30 m beyond the perceiver. We are simply using
the term to describe the space beyond the target. In our studies, what we call vista space was
sometimes as close as 11 m.
Environmental context
1753
Several recent studies show that environmental context can affect distance perception
even when all depth-informative cues (2) are constant. For example, Lappin et al (2006)
documented a surprising case in which a visual bisection task differed depending
on the place in which it was assessed. Observers judged the midpoint of a distance in
three types of full-cue environments: a hallway, a lobby, and an open field. Despite
equally robust optical and ocular-motor cues in each environment, these judgments
differed systematically. Halfway points to targets were placed farther in the hallway
and, to a lesser extent, in the lobby compared to the field.
In Lappin et al's studies, the environment differed in both VTT space and vista
space. Much is known about factors in VTT space and the manner in which they
influence perceived distance. These factors include cues, such as occlusion, height in
the visual field, relative size, binocular disparity, and motion, which convey geometrical
depth information (see Cutting and Vishton 1995; Proffitt and Caudek 2002). However,
new evidence indicates that other cues within VTT space also affect perceived distance
despite carrying little to no geometrical information about depth. For example, perceived
distance is affected by gaps in the ground plane (Sinai et al 1998), barriers blocking
the target (He et al 2004), and the texture of the terrain (Lappin et al 2006; Sinai et al
1998). All of these experiments were conducted in full-cue environments with depthinformative cues being held constant, yet aspects of the environment in VTT space still
influence perceived distance.
In contrast, little is known about cues in vista space. Geometrically informative
depth cues that have been explored include the horizon and relative size scaling.
However, these depth-informative cues were held constant in Lappin et al's study and
therefore cannot account for their findings of differences in perceived distance. In their
study, depth-related cues were constant in both VTT space and vista space, but nondepth-informative cues changed both within VTT space and in vista space. Therefore,
it cannot be determined whether the differences in perceived distance were due to
differences in VTT space, as in the previously mentioned examples of He et al (2004)
and Sinai et al (1998), or were also due to differences in vista space.
There are several sources for this ambiguity. First, the texture of the terrain in
VTT space was different in each environmental condition. The field was grass, and the
hallway had tiles. The floor of the lobby was carpeted and had a pattern of rectangles
and triangles of different colors. Both the lobby and the hall had landmarks, such as
windows, doors, and stairwells on the sides, which provide cues for linear perspective
and familiar size, and the texture of the floors allowed for scaling. In addition, the
features of the environment in vista space, such as the point at which the perceived
environment was occluded, were also different. Although these features do not contain
information about depth, it cannot be determined whether non-depth informative factors
within VTT space or vista space caused the differences in apparent distance in Lappin
et al's studies.
Previous studies demonstrated that aspects of VTT space alone can affect perceived
distance (He et al 2004; Sinai et al 1998). However, it is not known whether aspects
of vista space that carry no geometrically relevant depth information can also influence perceived distance in isolation of differences in VTT space. We conducted several
studies in both indoor and outdoor environments where VTT space was constant and
only vista space was different. Our findings indicate that differences in vista space, despite
providing no geometrically relevant cues to distance, nonetheless influence perceived
distance.
(2) By
depth-informative cues we mean cues that could be geometrically informative to the distance
to the target. These cues include monocular depth cues, such as motion parallax, accommodation,
linear perspective, and perceived eye height, as well as cues such as convergence and stereopsis.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
2 Experiment 1: Visually matched estimates of distance in a hallway
The purpose of this experiment was to test whether participants' visually matched
estimates of ground distance would be influenced by differences in vista space. The
viewing conditions varied only in vista space, not in VTT space, and only in nondepth-informative factors. Participants viewed targets on either end of a hallway, one
end of which was bounded by a door to a laboratory (see figure 1). They were asked
to adjust an experimenter in the opposite end of the hallway to match the distance to
the target in the viewed end. Targets were positioned so that the hallway extended
much farther past the target in one viewing direction compared to the other direction;
thus there were no differences in VTT space but there were differences in vista space.
(a)
(b)
Figure 1. Hallway used in experiments 1, 3, and 4. (a) The near end of the hallway. (b) The far end
of the hallway.
2.1 Method
2.1.1 Participants. Twenty (sixteen female, four male) University of Virginia students
participated in the experiment for course credit in an introductory psychology course.
All participants were naive to the purpose of the experiment and gave written, informed
consent to participate. All participants had normal or corrected-to-normal vision.
2.1.2 Apparatus. Participants judged distances in a carpeted hallway (see figure 1). The
carpet had a very dense pattern, which could not be used to count off distances.
From end-to-end the hallway was approximately 33 m long and 1.5 m wide. In addition, the hallway opened onto rooms at either end, though the doors of these rooms
were usually closed. Participants stood 11.45 m from one end of the hallway and a
small piece of duct tape on the carpet marked this location. This location was 21.5 m
from the other end of the hallway. A tape measure was used to measure participants'
estimates of distance.
2.1.3 Design. Participants were randomly assigned to the `near' or `far' condition, which
determined the direction of the hallway in which they viewed the target distance. For
each condition, the 6 target distances were 1, 1.5, 2, 2.5, 3, and 4 m, and participants
made one visually matched estimate for each target distance. The order of presentation
of the target distances was randomized and counterbalanced across conditions.
Environmental context
1755
2.1.4 Procedure. Participants were brought to the home position and were told that they
would be judging distances to an experimenter who would be standing in the hallway.
On each trial, one experimenter (Target) stood at the target distance in the direction that
each participant had been assigned. The other experimenter (Match) stood in the opposite direction either 0.5 m or 8 m away from the participant. The starting position of
the Match experimenter was randomized across trials. Both experimenters held orange
cones. Participants verbally told the Match experimenter to move closer or farther until
the distance between the participants and the Match experimenter looked equivalent
to the distance between the participants and the Target experimenter (see figure 2).
Specifically, participants were told to attend to the distance between their feet (located
at the home position) and the experimenters' feet. Participants were allowed to turn
back and forth between the Target and Match experimenters as often as they liked.
The Match experimenter then placed the cone on the floor to mark the distance to her
toes and measured the distance between this cone and the home position. Participants
judged distance to the experimenter; the cone was used only to mark the distance
to measure. Participants' eyes were closed while the distance to the matched cone was
measured, so no feedback was given to them. This process was repeated for each of
the 6 target distances. Participants viewed targets only in one direction of the hallway.
Match experimenter
Target experimenter
Near condition
b
Target experimenter
a
Match experimenter
Far condition
a
b
Figure 2. Perceptual matching task used in experiment 1. One experimenter (Target) stood at
the target distance (distance a). This experimenter stood by the closer end of the hallway for
the `near' condition (top figure) and by the farther end in the `far' condition (bottom figure).
Another experimenter (Match) stood in the opposite direction. Participants verbally instructed
the Match experimenter to move closer or farther until they judged the distance between themselves and the Match experimenter (distance b) to be equal to the distance between themselves
and the Target experimenter (distance a).
2.2 Results and discussion
We ran a 2 (viewing direction: near or far)62 (Match experimenter's starting position: close or far)66 (distance) repeated-measures ANOVA with distance estimate as
the dependent variable. There was a main effect of viewing direction (F1, 16 ˆ 34:94,
MSE ˆ 0:11, p 5 0:001, prep ˆ 0:97).(3) Targets viewed in the shorter end of the hallway
looked farther compared to targets placed in the distant end of the hallway (see figure 3). There was no main effect for the starting location of the Match experimenter
(F1, 16 ˆ 0:01, p 4 0:94, prep 5 0:64). There was a main effect of distance (F5, 80 ˆ 408:00,
MSE ˆ 0:05, p 5 0:0001, prep ˆ 0:97), and the interaction between distance and direction was significant (F5, 80 ˆ 4:01, MSE ˆ 0:05, p ˆ 0:003, prep ˆ 0:97). Perceived distance
(3) For
explanation of prep , see Killeen (2005).
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
near end
Matched distance=m
4
far end
3
Figure 3. Perceptually matched distance to targets positioned by the
near end or far end of the hallway
from experiment 1. Targets positioned
by the near end of the hallway looked
farther away than targets positioned
by the far end of the hallway. Error
bars depict standard error of the mean.
2
1
1
2
3
Actual distance=m
4
increased more when the targets were in the direction of the closer end than in the
distant end of the hallway (see table 1). No other interactions were significant.
These results showed that non-depth-informative variables in vista space can influence perceived distance. When participants were asked to equate the distance between
the two experimenters, they adjusted the experimenter in the longer end of the hallway
to be farther when compared to the same distance viewed in the shorter end of the
hallway. All depth cues were constant between both viewing conditions, and the VTT
space was virtually identical in both directions. Thus, the differences in perceived distance
are likely to be due to non-depth informative variables in vista space.
Table 1. Errors in distance estimates across all 5 experiments. Errors were calculated as the
estimated distance divided by the actual distance.
Experiment
Measure
(a) Inside environment
1
matching
3
blindwalk to target
4
blindwalk distance
Direction
near
far
near
far
near
far
Distance=m
1
1.5
2
2.5
3
4
1.07
0.98
1.03
1.05
1.18
1.07
1.14
1.01
1.02
0.92
1.20
1.01
1.11
0.90
0.98
0.94
1.14
0.93
1.09
0.95
0.95
0.94
1.19
1.01
1.05
0.93
1.00
0.89
1.09
0.95
1.07
0.91
1.03
0.95
1.15
0.96
Distance=m
(b) Outside environment
2
matching egocentric
5
matching exocentric
near
far
near
far
2
3
4
5
1.07
1.00
1.05
0.95
1.02
0.94
1.02
0.94
1.11
0.98
0.99
0.85
1.04
0.97
0.95
0.91
3 Experiment 2: Visually matched estimates of distance outdoors
The purpose of this experiment was to test whether the effects of vista space generalized to different environments. Participants viewed targets in an outdoor environment,
and we used a similar visual matching task to that in experiment 1. Similar to the
indoor environment, the boundaries of the viewed environment were closer in one
direction than the other.
Environmental context
1757
3.1 Method
3.1.1 Participants. Twenty (seven female, thirteen male) University of Virginia students
participated in the experiment for course credit in an introductory psychology course
or for $5. All participants were naive to the purpose of the experiment and gave
written, informed consent to participate. All participants had normal or corrected-tonormal vision and had not participated in the previous experiment.
3.1.2 Apparatus. Participants judged distances in an outdoor, grassy field with a wall
in one direction and no clear boundary in the other direction (see figure 4). A building
was on one side and a street on the other. Participants stood on a manhole cover,
18.6 m from a wall located at one end of the outdoor environment. An orange cone
was placed in the middle of the cover, so that participants stayed in the same location
throughout the experiment. Distances were marked in the field with golf tees, which
were not visible to the participants. A tape measure was used to measure participants'
estimates of distance.
(a)
(b)
Figure 4. Grassy field used in experiments 2 and 5. (a) The bounded end of the field. (b) The
unbounded end of the field.
3.1.3 Design. The design was similar to that of experiment 1. Participants were assigned
to either the `near' or `far' viewing direction and performed a visual matching task in
the outdoor environment; however, there were 4 target distances (2, 3, 4, and 5 m), and
targets were marked with a cone rather than another experimenter. All participants
made one visually matched estimate for each target distance. Order of presentation of
the distances was randomized and counterbalanced across viewing conditions.
3.1.4 Procedure. Participants were brought to the starting position and were assigned
in alternating order to a viewing direction for the field. They were told that they would
be judging the ground distances to targets placed in the field in front of them. Participants were asked to close their eyes while the experimenter positioned the target
(a small orange sports cone) at the target distance. After looking at the target, participants were asked to turn around and adjust the experimenter, who was now standing
directly behind them, to be at the same ground distance from them as the target (see
figure 5). For each trial, the experimenter started 1 m from the participant. Participants
were allowed to turn back and forth between the target and the experimenter as often as
they liked. Participants told the experimenter to move closer or farther until the distance
to the experimenter looked equivalent to the distance to the target. The experimenter then
placed an orange cone she was holding on the ground to mark the distance to her toes.
Participants closed their eyes while the distance to the matched cone was measured, so no
feedback was given to them. This process was repeated for each of the target distances.
Participants viewed targets only in one direction of the field.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
Match experimenter
target
Near condition
Far condition
b
a
a
b
Match experimenter
target
Figure 5. Perceptual matching task used in experiment 2. A target cone was placed by the
bounded end of the field for participants in the `near' condition and towards the unbounded
end for participants in the `far' condition. An experimenter stood in the opposite direction, and
participants verbally instructed the experimenter to move closer or farther until they judged the
distance between themselves and the Match experimenter (distance b) to be equal to the distance
between themselves and the target cone (distance a).
3.2 Results and discussion
We ran a 2 (viewing direction: near or far)64 (distance) repeated-measures ANOVA
with direction as a between-participants factor and distance estimates as the dependent
variable. There was a main effect of viewing direction (F1, 18 ˆ 8:32, MSE ˆ 0:25,
p ˆ 0:01, prep ˆ 0:95). Targets viewed in the end of the field with the closer boundary
were adjusted to be farther than targets placed at the same distance in the unbounded
end of the field (see figure 6). There was a main effect of distance (F3, 54 ˆ 347:93,
MSE ˆ 0:10, p 5 0:001, prep ˆ 0:99). Unlike in experiment 1, the interaction between
distance and direction was not significant (F3, 54 ˆ 1:19, p ˆ 0:32, prep ˆ 0:63ösee
table 1). This suggests that there may be some difference between the two directions
in the indoor environment that was not present in the outdoor environment. However,
we used a slightly different range of distances in the outdoor environment (2 ^ 5 m)
than in the indoor environment (1 ^ 4 m), and there were some procedural differences
as well, so it is unclear whether the difference in significant interactions is due to
some aspect of the environment or to a feature of the design.
5
near end
Matched distance=m
far end
4
3
2
1
2
3
4
Actual distance=m
5
Figure 6. Perceptually matched
distance to targets positioned by
the near end or far end of the field
from experiment 2. Targets positioned by the near end of the field
looked farther away than targets
positioned by the far end of the
field. Error bars depict standard
error of the mean.
Environmental context
1759
The significant result of direction suggests that the effects of non-depth-informative
aspects of vista space on perceived distance are not specific in the indoor hallway
environment used in experiment 1, and generalize to different environments. Participants had to position the experimenter to be farther in the unbounded end of the
field to make the distance equivalent to a distance viewed in the bounded end of
the field. In other words, targets positioned closer to the bounded end of the field
looked farther than targets positioned towards the unbounded end of the field.
4 Experiment 3: Blindwalking to the location of the target
The purpose of this experiment was to test whether a visually directed action would
also be influenced by the length of the hallway behind the target. Participants walked
to the target without vision. In accord with the findings of experiments 1 and 2,
we hypothesized that participants would blindwalk farther when viewing the target
distance in the short end of the hallway relative to blindwalked estimates of distance
when viewing the target in the long end of the hallway.
4.1 Method
4.1.1 Participants. Twenty (thirteen female, seven male) University of Virginia students
participated in the experiment for course credit in an introductory psychology course.
All participants were naive to the purpose of the experiment and gave written,
informed consent to participate. All participants had normal or corrected-to-normal
vision and had not participated in any of the previous experiments.
4.1.2 Apparatus. Participants judged distances in the hallway described in experiment 1.
However, a small, orange construction cone was used to mark the target distances,
rather than an experimenter. Participants stood 11.45 m from one end of the hallway.
A small piece of duct tape on the carpet marked the location. A tape measure was
used to measure participants' blindwalked estimates of distance.
4.1.3 Design. The design was similar to that of experiment 1 except that we measured
perceived distance in a blindwalking task. Participants viewed 6 target distances (1, 1.5,
2, 2.5, 3, and 4 m). All participants made one blindwalked estimate for each target
distance. Participants were assigned to either the `near' or `far' viewing direction, determined by the length that the hallway extended past the target. Order of presentation
of the distances was randomized and counterbalanced across viewing conditions.
4.1.4 Procedure. Participants were brought to the starting position and asked to face
towards their assigned end of the hallway. The target cone was placed in the hallway
at one of the target distances. Participants closed their eyes while the experimenter
positioned the cone. Participants then looked at the cone for as long as they wished.
When they were ready, participants closed their eyes and walked until they thought
their toes were aligned with the middle of the cone's location (see figure 7). One experimenter moved the cone after the participants closed their eyes, but before they started
walking so that they would not run into the cone and get feedback. Another experimenter followed alongside the participant to prevent collision with the walls. After each
participant stopped where he/she thought the cone had been placed, the experimenters
measured the distance between the starting location and the toes of the participant.
An experimenter walked the participants back to the starting location while their
eyes were still closed, so no feedback was given. The process was repeated for each of
the 6 target distances. Participants viewed targets in only one direction of the hallway.
They were not given any practice on the blindfolded walking task before the experimental
trials began.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
target
Near condition
a
Far condition
target
a
Figure 7. Blindwalking task in experiment 3. Participants closed their eyes and walked the distance to the target (distance a). The target was moved out of the way after participants closed
their eyes so that they would not run into the target.
4.2 Results and discussion
We ran a 2 (viewing direction: near or far)66 (distance) repeated-measures ANOVA
with direction as a between-participants factor and blindwalked distance estimates as
the dependent variable. Contrary to our hypothesis, there was no main effect of viewing direction (F1, 18 ˆ 3:11, MSE ˆ 0:20, p ˆ 0:10, prep ˆ 0:82öfigure 8). There was a
main effect for distance (F5, 90 ˆ 250:91, MSE ˆ 0:09, p 5 0:001, prep ˆ 0:99). The interaction between direction and distance was not significant (F5, 90 ˆ 1:21, MSE ˆ 0:09,
p ˆ 0:31, prep ˆ 0:64ösee table 1).
We were surprised by these results. The results of experiments 1 and 2 did not
replicate when participants were asked to blindwalk to the target. Admittedly, there
appears to be a similar trend at the 3 m and the 4 m targets; however, we did not
obtain the significant results of the previous studies. The null findings, while inconclusive, suggest that environmental context may not influence blindwalking measures of
perceived distance. However, our findings could also be the result of two potential
confounds. First, the participants in this experiment walked to the location of the
target. In order to perform the task, participants may have simply acted on the location of the target by spatially updating while blindwalking without relying upon the
near end
Blindwalked distance=m
4
far end
3
Figure 8. Blindwalked distance
to targets positioned by the near
end or far end of the hallway in
experiment 3. Participants did not
walk significantly different in one
direction than in the other direction. Error bars depict standard
error of the mean.
2
1
1
2
3
Actual distance=m
4
Environmental context
1761
perceived distance to the target. Second, both target placement and the judgment of
distance were in the same end of the hallway, whereas in experiments 1 and 2, participants viewed the target in one direction and made their distance judgment in the other
direction. The absence of a comparison between the two ends of the hallway may have
negated the effect. The following experiment was designed to address these confounds.
5 Experiment 4: Blindwalked estimates of perceived distance in the hallway
In the previous experiment, participants did not have to relate the distance in one
direction of the hallway to a distance in the opposite end of the hallway. Also, participants walked to the targets' locations and did not necessarily rely upon their perceived
distances. In this experiment, participants blindwalked the perceived distance to the
target in the opposite direction of the hallway to ensure that they encoded distance
and to reinstate the relative nature of the task from experiments 1 and 2.
5.1 Method
5.1.1 Participants. Twenty (ten female, ten male) University of Virginia students participated in the experiment for course credit in an introductory psychology course. All
participants were naive to the purpose of the experiment and gave written, informed
consent to participate. All participants had normal or corrected-to-normal vision and
had not participated in any of the previous experiments.
5.1.2 Apparatus. The hallway and apparatus used were the same as in experiment 3.
5.1.3 Design. The design was the same as that of experiment 3, except that we measured
perceived distance by asking participants to blindwalk the distance to the target in
the opposite direction of the target (see figure 9). Participants were assigned to either
the `near' or the `far' condition.
5.1.4 Procedure. Participants were brought to the starting position and asked to face
their assigned end of the hallway. The target (an orange cone) was placed in the hallway at one of the target distances. Participants were asked to close their eyes while
the experimenter positioned the cone. Participants then looked at the cone for as long
as they wished. When they were ready, they turned to face the other direction in the
hallway. After realigning the tips of their toes with the tape marking the starting location,
the participants closed their eyes and walked in the new direction the perceived distance
target
Near condition
b
Far condition
a
target
a
b
Figure 9. Blindwalking task in experiment 4. Targets were positioned in the designated end of the
hallway. After looking at each target, participants turned around and walked the same distance
as the distance to the target (distance a) but in the opposite direction.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
to the cone. The experimenter followed alongside the participants to prevent them from
walking into the walls. Participants were told that they should walk until they believed
the distance they had walked was equivalent to the target distance. Measurements
were made from the starting location to the toes of the participants. The experimenter
walked the participants back to the starting location while their eyes were still closed,
so no feedback was given. This process was repeated for each of the 6 target distances.
Participants viewed targets in only one direction of the hallway. They were not given
any practice on the blindfolded walking task before the experimental trials began.
5.2 Results and discussion
We ran a 2 (viewing direction: near or far)66 (distance) repeated-measures ANOVA
with direction as a between-participants factor and blindwalked distance estimates
as the dependent variable. There was a main effect of direction (F1, 18 ˆ 5:54, MSE ˆ
0:91, p ˆ 0:03, prep ˆ 0:91). Targets viewed in the shorter end of the hallway looked
farther than targets viewed in the distant end of the hallway (see figure 10). There was
a main effect for distance (F5, 90 ˆ 208:84, MSE ˆ 0:22, p 5 0:001, prep ˆ 0:99). The
interaction between distance and direction was not significant (F5, 90 ˆ 1:92, p 4 0:10,
prep 5 0:82ösee table 1).
5
Blindwalked distance=m
near end
far end
4
3
Figure 10. Blindwalked distance as
a function of actual distance for
targets positioned by the near
end or the far end of the hallway.
Participants walked farther after
viewing targets in the near end
compared with targets in the far
end. Error bars depict standard
error of the mean.
2
1
1
2
3
Actual distance=m
4
These results support the findings of experiments 1 and 2, this time with an action
measure of perceived distance. An action directed towards the opposite end of the
hallway was affected by the viewing direction. Participants who viewed the target in
the shorter end of the hallway blindwalked farther in the opposite direction than
did participants who viewed the targets in the longer end of the hallway. In other
words, participants perceived targets placed in the near end as being farther away than
targets placed in the far end. The significant effect of viewing direction in this experiment suggests that the ambiguous results in experiment 3 were not due to the
fact that an action measure was used, but rather due to lack of comparison of each
direction. Participants in this experiment were asked to make a relative judgment,
as in experiments 1 and 2, because they had to estimate the distance by comparing it
to the same extent in a different direction (with a different vista space). Also, the
participants in this experiment were not able to walk to the location of the target;
they had to replicate the distance to the target in a different place. This could have
affected their estimates as well, and will be discussed in more detail in the general
discussion.
Environmental context
1763
6 Experiment 5: Visually matched estimates of exocentric distances outdoors
In this experiment, we were interested in whether the results of the previous experiments would generalize to an exocentric measure of perceived distance. Is the effect of
environment in the above experiments confined to the dimension of space in the sagittal plane (along the line of sight) or does it also apply to the frontoparallel plane?
In this experiment, participants performed a visual matching task similar to that of
experiment 2; however, they estimated the extent between two cones oriented in the
frontoparallel plane instead of the distance from themselves to a cone.
6.1 Method
6.1.1 Participants. Thirty (nineteen female, eleven male) University of Virginia students
participated in the experiment for $5. All participants were naive to the purpose of
the experiment and gave written, informed consent to participate. All participants had
normal or corrected-to-normal vision and had not participated in any of the previous
experiments.
6.1.2 Apparatus. Participants judged distances in the same grassy field as in experiment 2. Two orange cones were placed in the field 4 m away in the frontoparallel plane
to indicate the distance to be judged. Distances were marked in the field with golf
tees, which were not visible to the participants. A tape measure was used to measure
participants' estimates of distance.
6.1.3 Design. The design was similar to that of experiment 2, except that participants
viewed an exocentric extent oriented in the frontoparallel plane rather than an egocentric extent. Participants were assigned to either the `near' or the `far' condition. There
were 4 target distances for each condition (2, 3, 4, and 5 m). The order of presentation
of the target distances was randomized and counterbalanced across viewing conditions.
6.1.4 Procedure. Participants were brought to the starting position and asked to face
either the bounded (`near' condition) or unbounded (`far' condition) end of the field.
Participants were asked to close their eyes while the experimenter positioned the targets
(two orange cones) at the appropriate distance. Following observation of the exocentric
extent, participants turned approximately 1358 and verbally adjusted two experimenters
to be the same distance apart from each other as the distance between the target cones
(see figure 11). For each trial, the experimenters started approximately 0.5 m from
each other and 4 m away from the participant. Participants were allowed to turn back
and forth between the targets and the experimenters as often as they liked. They were
encouraged to take their time and to make fine adjustments. When the participants
were satisfied with their estimates, the experimenters dropped orange cones to mark
the distance between the sides of their feet. Participants turned around while the
estimated distance was measured, so no feedback was given to them. This process
was repeated for each of the target distances. Participants viewed targets only in one
direction of the field.
6.2 Results and discussion
We ran a 2 (viewing direction: near or far)64 (distance) repeated-measures ANOVA
with direction as a between-participants factor and distance estimates as the dependent variable. There was a main effect of viewing direction (F1, 28 ˆ 4:64, MSE ˆ 0:58,
p 5 0:05, prep ˆ 0:89). Exocentric distances placed by a nearby boundary looked longer
than exocentric distances placed in the unbounded end of the field (see figure 12).
In order to make the two exocentric extents equivalent, participants adjusted the experimenters to be farther apart when the target distance was located near a boundary.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
There was a main effect of distance (F3, 84 ˆ 310:05, MSE ˆ 0:12, p 5 0:001, prep ˆ 0:99).
The interaction between distance and direction was not significant (F3, 84 ˆ 2:09,
p ˆ 0:11, prep 5 0:81ösee table 1).
Thus, the effects of non-depth-informative aspects of vista space on perceived
distance generalize to exocentric distances as well. The direction of the effect is similar
to the effects in the previous experiments such that distances by a close boundary
are expanded relative to distances near a far boundary. Environmental context seems
to influence multiple aspects of perceived spatial layout including egocentric and
exocentric distance.
a
b
Near condition
a
b
Far condition
Figure 11. Perceptual matching task used in experiment 5. Two cones were placed in the frontoparallel plane by the bounded end of the field for participants in the `near' condition and
towards the unbounded end for participants in the `far' condition. Two experimenters stood in
the opposite direction, and participants verbally instructed the experimenter to move closer or
farther until they judged the distance between the two experimenters (distance b) to be equal to
the distance between the two cones (distance a).
5
near end
Matched distance=m
far end
4
3
2
1
2
3
4
Actual distance=m
5
Figure 12. Perceptually matched
distance to targets positioned by
the near end or far end of the
field from experiment 5. Targets
positioned by the near end of the
field looked farther apart from
each other than targets positioned
by the far end of the field. Error
bars depict standard error of the
mean.
Environmental context
1765
7 General discussion
We demonstrated that despite the presence of sufficient depth cues, perceived distance
is influenced by environmental context in full-cue, real-world environments. Within
each environment, the space between the perceiver and the target was homogenous,
and all depth-related information was held constant. The aspect of the environment
that differentially influenced perceived distance was therefore beyond the target, or in
vista space. We found evidence for the influence of vista space on perceived distance
both indoors (experiment 1) and outdoors (experiment 2), as well as with an action
measure (experiment 4) and with an allocentric extent (experiment 5). The evidence
from experiments 3 and 4 urges caution in further generalizing this effect, and suggests
a possible dissociation between a perceived extent and a perceived location.
In experiments 1 and 2, we demonstrated an effect of environmental context in
indoor and outdoor environments on perceived distance using a visual matching task.
Participants adjusted a cone in one direction to match the distance of the target cone
in the opposite direction. In both environments, we observed a striking asymmetry.
When the target was placed in the short end of the hallway, or in the direction of
the near wall outdoors, the adjustment cone was positioned farther away in the other
direction. The opposite was also true. When the target cone was placed in the far end
of the hallway, or the long end of the field, the adjustment cone in the other direction
was positioned closer.
In experiment 4, we demonstrated that the effect generalized to an action measure
of perceived distance. Participants viewed a target, turned around, closed their eyes,
and attempted to walk an equivalent distance but in the opposite direction. Participants walked farther when the target was placed in the short end of the hallway than
when the target was placed in the longer end of the hallway. In other words, when
trying to walk an equivalent distance in the opposite direction, participants walked
farther in the long direction than in the short direction.
The results of experiment 3, however, were inconclusive. When participants viewed
the target and blindwalked directly to its location, there was no significant effect of
direction walked. While there was a trend for a difference at the longer distances
(see figure 8), this difference was decidedly less than in the other experiments. There
are several possible explanations for the different results found in experiment 3. First,
in contrast to the other experiments, participants in experiment 3 looked only in one
direction of the hallway. In each of the other experiments, participants viewed the target
in one direction but also viewed the other direction while estimating perceived distance.
In other words, in every other experiment, each estimate was a relative one, whereas
in experiment 3, distance estimates were not relative because participants looked only in
one direction and then acted in that direction. Second, some recent investigators have
suggested that there is a distinction between distance and location (Kudoh 2005; Philbeck
et al 1997). In the case of experiment 3, it is possible that perceived location was being
assessed, and that perceived location was not influenced by environmental context to
the same degree as perceived distance in the rest of the experiments. Although responses
to a location could be based on distance and a direction, the optical information that
specifies location need not include information about distance. For example, location
could be derived from direction, eye-height, and angular elevation (see Sedgwick 1986).
So even if environmental context influences estimates of perceived distance (as in experiment 4), environmental context may not factor into estimates of perceived location (as
in experiment 3) since the informational bases for judgments of location and distance
might not completely overlap. That responses to perceived distance versus location may
be subject to different biases is an intriguing possibility, which we intend to investigate in
future studies. A final possibility is that there was an effect, at least for the farther target
distances; however, the current experiment had insufficient power to detect this difference.
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J K Witt, J K Stefanucci, C R Riener, D R Proffitt
We also presented evidence that environmental context affects perceived distance
between two targets in the frontoparallel plane. In experiment 5, participants matched
the extent between two external objects (aligned in the frontoparallel plane) in one
direction to the extent of two objects (also aligned in the frontoparallel plane) in the
other direction. As with egocentric distance, when the target cones were in the long
end of the field, the adjustment cones were placed closer together, and vice versa.
Thus, environmental context may influence perceived space in general (both egocentric
depth and allocentric width). However, an alternative explanation is that environmental
context influenced only egocentric distance and affected the measure of allocentric
distance indirectly (see figure 13). If both external objects were perceived to be farther
away, then the matched distance between them would also increase. Thus, experiment 5
could just be an indirect instantiation of the effect of environmental context on perceived
egocentric distance. Additional research is required to disentangle these alternatives.
actual distance
perceived distance
Participant
Figure 13. Birds-eye view of set-up in experiment 5 and hypothetical
(mis)perceptions of the egocentric distance to the targets. If the participant sees the targets to be closer than they actually are, then the
exocentric distance between the targets will also look shorter.
Previous research has also shown effects of environmental context on perceived
distances (eg Lappin et al 2006; Sinai et al 1998). However, in these other experiments,
context was manipulated by changing visual information in the space between the
perceiver and the target. In our experiments, visual information within VTT space was
held constant. Likewise, as with previous studies, all optical depth cues informative of
distance were held constant. In each experiment, observers had an equally rich and
informative full-cue viewing situation regardless of the direction that they were facing.
What then can explain our curious pattern of results?
While we do not know what the effective variables were, our intention was to
vary one aspect of vista space systematically across the two viewing conditions. In
both the indoor and outdoor environments, the range of ground surface viewable
beyond the target varied considerably depending on which direction the observer was
facing. In other words, the endpoint of the environment (defined as the point at which
the viewable ground surface ends) was much closer in one direction than in the other.
In both environments, when the target was near the closer endpoint, the adjustment
was positioned farther away in the other direction.
Given that the distance to the endpoint of the environment may be an effective
variable, what are we to make of the particular direction of this effect? We speculate
that this may be a real-world instance of a class of geometric illusions. For example,
in the Titchener or Ebbinghaus illusion, a circle surrounded by smaller circles looks
Environmental context
1767
larger than a circle surrounded by bigger circles. Analogously, we found that an extent
in the context of a longer extent (farther endpoint) seems shorter than an extent in
the context of a shorter endpoint. In each case, the perceived dimension of the object
(the size of the center circle in the Titchener circles, or distance to the target cone in
our studies) is influenced by surrounding information that is irrelevant and uninformative to the actual dimension. What is especially surprising about our findings is that
the illusion exists despite being in a full-cue environment.
One possible reason for these effects is that the distance to the environment's
boundary may have influenced the observer's visually perceived eye level (VPEL),
which could result in a misperception of the distance to the target. VPEL can be used
to determine the distance to an object: if the perceived eye level is lower than the
true eye level, objects appear closer (Ooi et al 2001). In addition, VPEL generally
coincides with the location of the horizon in vista space. Our observers may have
misperceived their eye level such that the horizon was perceived to be lower in one
direction relative to the other direction, so targets in that direction appeared closer.
Sometimes, observers rely more on the terrestrial horizon as a cue to distance than
VPEL (Sedgwick 1986). The terrestrial horizon is defined as the visible horizon formed
at the far boundary of a surface. When an observer is in a large grassy field, with a
large extended ground surface, the terrestrial horizon is more likely to approximate
the true horizon. In our experiments, the observers may have relied on the terrestrial
horizon to estimate distance to the target, which may have been misperceived when
the boundary of the surface was closer.
In expanding the possible influences on distance perception, these findings can
be of use for interpreting past findings as well as planning future investigations of
distance perception. For example, while past research has shown that perceived distance is compressed when measured with verbal estimates or visual matching tasks,
the degree of compression has varied considerably (Amorin et al 1998; Loomis et al
1992; Norman et al 1996). Our findings suggest that some portion of this variability
may be explained by the type of environment in which the distance was assessed.
However, our findings may generalize only to a short range of distances since we
only tested distances up to 5 m. Qualitatively different cues are informative for distances at this range than at much farther ranges. For instance, disparity and motion
parallax from small head and body movements are informative for distances within
the range that we tested, whereas aerial perspective is informative for much farther
distances (see Cutting and Vishton 1995 for review). Thus, we cannot generalize from
our findings to perception of all distances.
The result that such distal information influences perception may seem to conflict
with recent results that demonstrate that having restricted field of view does not
influence distance perception (Creem-Regehr et al 2005; Knapp and Loomis 2004;
Wu et al 2004). However, these previous studies manipulated not the environment itself
but those aspects of the environment that were visible. In addition, these researchers
restricted view in the periphery and not the center, which is one place where our
environment visibly differed.
In summary, we found that in full-cue environments, where optical information
between the viewer and the target was held constant, perceived distance was influenced
by non-depth-informative information beyond the target. Thus, full-cue viewing does
not guarantee equivalence in perception. Despite the irrelevance of cues in vista space
to the actual distance to a target, estimates of perceived distance are influenced by
this context.
Acknowledgments. This research was supported by NSF ITR/Carnegie Mellon Grant 0121629, and
ONR Grant N000140110060, to the fourth author.
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