The Effect of Sensory Adaptation on the Blind Walking Task

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The Effect of Sensory Adaptation on the Blind Walking Task

Introduction

Perception of distance is critical for our survival. We need to be able to estimate how far away an item is in order to accurately grasp it and to avoid hitting objects in our path. This mental representation of distance arises from practise walking towards a target. Through practise, we relate the effort required to walk a distance to visual distance cues (Proffitt, Stefanucci, Banton & Epstein, 2003). Distance is defined by two characteristics: the objects direction or location relative to the viewer, and the physical distance between the viewer and the object. To test distance perception, Thomson created the Blind Walking Test (1983). In this test, the participant is shown a distance, blindfolded, and then asked to walk to where they think the target was. Thomson found that the perceived distance was influenced by the length of time between when the subject saw the target, and when he reached the target. He suggested that if the trial took longer than eight seconds to complete, the subject would forget their mental representation of space and distance. Therefore, the further away the target was from the subject, the more likely inaccurate they would be. In 1987, Elliot tried to duplicate the results of Thomson’s experiment. After the study was completed, Elliot disagreed with Thomson’s hypothesis that the subject’s mental representation faded over time since he could not duplicate the results. He found that perceived distance was not influenced by either the target’s distance from the subject, the time it took the subject to walk to the target or the speed at which the subject walked. The only variable that was found to influence the subject’s accuracy was the number of trials. The more trials a subject completed, the more likely they were to overshoot the distance. In a further study done by Proffitt et al (2003), the researchers did a variation on the blind walking task. They tested whether optic flow and effort would have any significant impact on the subject’s ability to judge distances. The researchers found that distance judgements increased when the subject lacked optic flow or was given a heavy backpack to carry. These findings suggest that distance perception can be influenced by visual input and by the effort required. The drawback of this experiment was that the subjects were asked to verbally report the distance between themselves and the target. This variation caused some possible confounds since this measure was fundamentally subjective. Each subject could have a different mental representation of theunit of measure used to report the distance.

In the current experiment, the confounds of previous studies have been removed. This study attempts to expand on Elliot’s hypothesis that the only significant manipulation in the Blind Walking Task is the number of trials. This experiment endeavours to measure how sensory adaptation will affect the distance walked. There are two conditions that each subject completed. In condition one, the subject is asked to walk around in the field where the experiment was taking place for ten minutes. In the second condition, the subject is asked to walk around blindfolded for ten minutes in the same field. The subject was asked to walk blindfolded so that the possible affect of sensory adaptation to blind walking could be measured. In the first condition, the subject was asked to walk for the same amount of time so that fatigue would not be a confound. Therefore, the independent variables are prior exposure to blind walking and also the number of trials completed. The distance walked, or the error, was measured. The results of each subject in condition one was compared to the same subject’s results in condition two. The data was also analyzed to measure whether there was a significant difference between earlier and later trials.

The hypothesis that is being tested draws on the idea presented in Elliot’s paper (1987). With increased number of trials, the subject should begin to overestimate the distance needed to be walked. One possible explanation for this data would be sensory adaptation which is described as a cell’s reduced response to a stimulus. If the number of trials is increased, than the subject should become adapted to the distance and to walking blindfolded. To compensate for this adaptation, the subject will unconsciously increase the amount of space travelled since the subject will become less sensitive to the motor input of walking. There is also the possibility that the subject will overestimate the distance due to increased confidence with walking blindfolded, or that the subject could be overestimating because they are becoming less confident with their abilities to estimate the distance. This experiment is important because it could serve as evidence that sensory adaptation can occur in locomotion.

Methods

Subjects

Subjects were selected from an undergraduate psychology laboratory course as a part of the course requirements. Subjects were between the ages of 19 and 23. There were eight subjects, with an equal number of males and females. All participants had normal or corrected-to-normal vision and no physical impairments.

Apparatus

A large, relatively flat grassy field on the McMaster campus was selected as the location of the experiment. Distances of one metre were marked off with golf tees using a standard metric tape measure. A large orange spike was used as the target, and was moved to the correct distances throughout the experiment. The subjects were blindfolded to eliminate visual cues.

Procedure

Before the subjects arrived, the experimenters measured 17 metres and placed a golf tee at each metre. The subjects are met at the field and asked to sign a consent form and fill out a subject information form. The experimenter informs the subjects about what they will be required to do during the experiment, and explains that they can withdraw from the study at any point during the experiment if they so choose. Half of the subjects are randomly selected to complete the non-visual condition first. These subjects are blindfolded and, along with the other half of the subjects who aren’t blindfolded, are asked to walk around the field for ten minutes. The experimenter stays with the group to ensure that the subjects who are blindfolded do not trip or walk into anything. This also ensures that the two groups have an equal amount of fatigue. After the ten minutes have elapsed, the experimenter escorts the group to the section of the field where the distances are marked off. All of the subjects are blindfolded for this part of the experiment. The experimenter arranges the subject so that they are at the -2, -1, 0, 1 or 2 metre starting point. The subject is asked to remove the blindfold and is able to look at the target for three seconds, after which time they are asked to place the blindfold back over their eyes. The subject is then asked to walk in a linear line to where they think the target is. One experimenter accompanies the subject on their walk with the measuring tape while another experimenter anchors the measuring tape to the starting point. Once the subject is satisfied with their location, the distance between the starting point and their current position is recorded. The experimenter that accompanied the subject places the end of the measuring tape at the back of the left foot, while the other experimenter records the distance. The distances measured are 6, 9, 12 and 15 metres. Once the distance is measured, the experimenter accompanies the subject back to the starting point. No feedback about how accurate the subject was is given. The subject is lead back to the starting point while blindfolded so that they cannot receive any feedback on their accuracy. This process continues until the block of four distances in complete. Two more blocks are completed, with the assignment of the 4 distances randomized. Once the three blocks are completed, the subject is asked to come back in 24 hours to complete the next half of the experiment. In the second half of the experiment, everything is conducted in the same way with the only difference being that the subjects who were blindfolded for the initial ten minute walk are now allowed to see, whereas the subjects who were initially allowed to see are now blindfolded.

References

Elliot, D. (1987). The influence of walking speed and prior practice on locomotor distance estimation. Journal of Motor Behaviour, 19(4), 476-485.

Proffitt, D.R., Stefanucci, J., Banton, T., & Epstein, W. (2003). The role of effort in perceiving distance. Psychological Science, 14(2), 106-112.

Thomson, J.A. (1983). Is continuous visual monitoring necessary in visually guided locomotion. Journal of Experimental Psychology – Human Perception and Performance, 9(3), 427-443.

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