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A New Look at Spatial Perception: The Effect of Sensory Adaptation During Blind-walking
Introduction
Attending less to how clothes feel against one’s skin, getting used to sitting in an
uncomfortable chair, and becoming less aware of loud city noises after moving from a small
town are all appropriate examples of sensory adaptation. Sensory adaptation is a change in
responsiveness of the sensory system over time to a repeated stimulus. Evidently, sensory
adaptation is a regular occurrence in the lives of all humans, and although its prominence is
unquestionable, it is often overlooked in experiments on spatial perception. Spatial perception
is an important cognitive ability that has been preserved through natural selection due to its
role as a survival mechanism. Scientists have been conducting experiments to further
understand spatial perception for years; however, in order to fully understand this skill, the
possible influence of sensory adaptation must be taken into consideration. Thus, with
understanding the phenomenon of sensory adaptation, scientists move closer to
comprehending spatial perception, the human mind, and why certain cognitive skills have
remained over time.
A vast amount of research regarding spatial perception has been accomplished to date.
Although multiple important discoveries have been made, most studies have shown
drawbacks and problems, especially in experimental design. For example, Elliot manipulated
walking speed, previous practice and walking delay to see if these factors influenced
locomotor behaviour (Elliot, 1987). The focus of his study was to disprove Thomson’s
findings, therefore Elliot didn’t make any significant or unique contributions to the field of his
own (Elliot, 1987). His experiment also presented a few design flaws that could have biased
his results. For example, subjects being tested listened to white noise to prevent them from
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using auditory cues in guiding their locomotion (Elliot, 1987). Instead of having subjects
listen to this artificial sound, testing could have been conducted in a quiet, more natural
setting, like an empty field in the afternoon. Furthermore, after practice trials and a short rest,
each subject participated in 90 experimental trials (Elliot, 1987). Although the distances
walked were not very far (4, 8 and 12m), it is likely that subjects became fatigued or bored of
doing the numerous tedious trials, thus skewing the experimental results. Lastly, it is worth
noting that Elliot failed to mention the practical applications of his research.
Proffitt is another researcher whose studies show that perceived egocentric distance
increases when subjects are wearing a backpack or have completed adaptation that diminishes
the optic flow accompanied by walking effort (Proffitt, Stefanucci, Banton, Epstein, 2003). In
the second of the three experiments presented in the journal, experimenters had subjects in
both the flow and no optic flow condition walk on a treadmill at 3 mph for 3 minutes (Proffitt
et al., 2003). Instructing subjects to walk on a treadmill while being presented with a visual
stimulus is an unnatural circumstance, as treadmills allow people to walk at a consistent pace
more easily. In addition, in the third experiment, subjects wore foam earplugs during practice
to block out ambient noise (Proffitt et al., 2003). This is also unrealistic since background
noise is a common occurrence that is usually dealt with or ignored if found distracting.
Finally, although Proffitt describes his three experiments with great detail, he fails to
thoroughly discuss the practical importance of his work and in what ways his discoveries will
make a unique contribution to the field.
While vast research has been done to further understand spatial perception, previous
literature has presented obvious faults that should not be overlooked. Numerous experiments
on spatial perception, including those previously discussed by Elliot and Profitt, fail to
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consider the possible occurrence of sensory adaptation and the effect it has on making nonvisual distance estimations. With this experiment, we hope to not only add to what is already
known about adaptation, but to also provide exceptional results to this psychological domain.
Unlike Proffitt’s experiments, in which subjects made verbal distance judgments while
wearing a heavy backpack or after walking on a treadmill (Proffitt et al., 2003), this
experiment involves blindly walking towards a target to demonstrate spatial perception.
Subjects will complete the experiment over two days, in two different conditions:
experimental and control. In the experimental condition, subjects will walk around
blindfolded for 10 minutes before 12 blind walking test trials. In the control condition,
subjects walk around normally for 10 minutes before completing the same blind-walking
trials. The 12 trials in each condition will be grouped into 3 blocks of 4 trials each. The
independent variables of this study include the changing distances of the test trials, the
number of blocks, and whether the subject was blindfolded or not. The dependent variables
are the distance estimations made by the subjects during each trial. We predict that if
adaptation exists, subjects will over-estimate distances more in the experimental condition
than during the control condition. This is a between-subjects effect. The logic behind this
prediction is that once adapted, subjects will become progressively less sensitive to the
distance walked during each trial, resulting in a tendency to overshoot. Another prediction is
that subjects will over-estimate more in the latter block of trials than in the first block,
demonstrating a within-subjects effect.
There are important implications in running this experiment. The blind-walking test
has been used in numerous studies; however, if our predictions are correct, researchers will
have to re-evaluate the accuracy of this test. Experiments that use this blind-walking to study
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spatial perception often overlook the possibility of sensory adaptation, which may bias their
data and final results. This experiment tests to see if sensory adaptation exists after repeated
trials of blind-walking tests. In addition, the findings, if significant, may contribute to the
practical applications of research in this field. For example, our understanding of sensory
perception and adaptation will contribute to the advancement of the rehabilitation of nonvisual people. Experts can aid those who have lost their slight in becoming adapted and
familiar to their surroundings.
Methods
Subjects
The subjects were 4 male and 4 female undergraduate students from McMaster
University who participated in this experiment as part of an assignment for a course credit.
The experiment consisted of two conditions, experimental and control. Half of the subjects
were randomly assigned to the control condition on day one and then experimental condition
on day two, while the other half were part of the experimental group day one and control on
day two.
Apparatus and procedure
Testing was conducted outdoors on the oval field on McMaster University campus.
Before taking subjects to the testing site, the experimenters explained the instructions and had
subjects sign a consent form. Also prior to running the experiment, the various distances were
measured and marked. Four starting positions were used (-2, -1, 0, 1, 2) as well as four
different distances (6m, 9m, 12m, 15m). These distances were measured using retractable
measuring tape and marked on the grass with coloured golf tees. Once everything was set up,
the subject being tested was asked to walk around the field for 10 minutes, while being timed
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by a stopwatch. If the subject was in the experimental condition, they walked around
blindfolded; if the subject was in the control condition, they walked around without wearing a
blindfold. After 10 minutes of walking, the first trial was run. Assigning different roles to the
three experimenters enabled the experiment to run smoothly and without fault. Experimenter
1 held the clipboard with the data-sheet containing the distances and start-positions, and
instructed experimenter 2 of the starting points. This was done non-verbally using hand
gestures (i.e for a starting position of 3 meters, experimenter 1 would raise three fingers).
Experimenter 1 also communicated to experimenter 3 the distances to place the target.
Experimenter 2 positioned the subject at each starting point and remained by their side in case
they felt uncomfortable walking without vision. Lastly, experimenter 3 began each trial
standing next to experimenter 1 who indicated on the data-sheet what distance to place the
target for the current trial. Then experimenter 3 would place the visual target, an orange
pylon, at the specified distance and wait for the subject to begin walking. As the subject began
walking towards the target, experimenter 3 removed the pylon.
A single trial in the experimental condition was carried through as follows:
After 10 minutes of walking around blindfolded, the subject was guided to their first
starting position. He or she was brought to this beginning point by experimenter 2, who was
instructed where to go by experimenter 1. As experimenter 2 positioned the subject,
experimenter 3 placed the visual target at the pre-determined distance for the first trial. Next,
the subject was instructed to remove their blindfold for 3 seconds, which was timed by a
stopwatch. After a brief glance at the visual target, the subject put his or her blindfold back on
and began walking towards the pylon, which experimenter 3 removed from the ground shortly
afterwards. Once the subject stopped walking, indicating they had completed their distance
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estimation, experimenter 3 reported to experimenter 1 the distance they had traveled in
meters. Distances up to approximately 25 meters were pre-measured before testing, ensuring
that the experiment would run efficiently. The distance was measured consistently from the
front of the subject’s left shoe. After the distance was determined, experimenter 2 guided the
subject to the starting position of the next trial. This procedure was repeated an additional 11
times, for a total of 12 trials per subject. The exact experiment was repeated the next day,
except this time the subjects switched conditions. Furthermore, subjects were not given any
feedback while testing and the entire experiment was run without verbal communication
among experimenters.
The data collected from this experiment will be analyzed to see if our results support
the predictions made prior to testing. With the aid of computer-based statistical software, it
will be determined if a significant main effect exists for the condition variable, which looks at
the between-subjects effect. If a main effect exists, we can conclude that non-visual distance
estimation depends on whether subjects are in either the control or experimental condition.
We will also be looking for a possible main effect for the block variable. If significant, this
within-subject effect proves that accuracy of distance estimation depends on whether a subject
is in their first or last block of trials. Lastly, the presence of an interaction between block and
condition will also be examined.
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References
Banton, Ton., Epstein, Willian., Proffitt, Dennis R., Stefanucci, Jeanine. (2003). The Role of
Effort in Perceiving Distance. Psychological Sicence, 14 (2, 106-122).
Elliot, Digby. (1987). The Influence of Walking Speed and Prior Practice on Locomotor
Distance Estimation. Journal of Motor Behavior, 19 (4, 476-485).