Distance Perception and Sensory Adaptation: the Effects of Repeated Trials of the Blind

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Distance Perception and Sensory Adaptation: The Effects of Repeated Trials of the Blind Walking Task

Introduction

Spatial perception is critical for our everyday navigation through our external environment and is an essential part of our survival. Our spatial perception defines our ability to perceive and react to our spatial world and is one of our most fundamental functions. Spatial perception can be measured by a variety of physical characteristics such as our perception of location and distance. Historically, it was measured through subjects giving a verbal response of their estimation of a distance or location, but the problem with this method was that individual’s knowledge of units of measurement varied, and this could result in unreliable findings based on the subject’s individual knowledge. Revolutionary to the field was James Thomson (1983), who was among the first to introduce the use of blind walking tasks as an appropriate measure of a human being’s spatial perception of distance. One of Thomson’s goals was to demonstrate through his studies that even without visual information available, subjects can accurately assess distance, demonstrated through blindly walking to a previously viewed target (Thomson, 1983). Through analysing the subjects motor output, the blind walking task has replaced the more unreliable verbal report of spatial distance, and has been used innumerable times in a variety of literature regarding spatial perception. Following Thomson’s work, Digby Elliot carried out his own experiments using the same blind walking principle as Thomson had suggested. He found that Thomson’s results were not replicable in his studies, and criticised Thomson’s method of pooling within-subjects and between-subjects variability (Elliot, 1987). Thomson countered Elliot’s claims with criticisms of his own, stating that Elliot’s experimental subjects were walking much slower than Thomson’s and that and that there were significant differences in the practice subjects received prior testing (Elliot, 1987). In an attempt to offset Thomson’s criticisms, Elliot (1987) carried out an another experiment and his findings suggested that the variability in practice prior to testing and the speed of walking did not produce significant enough differences to substantiate Thomson’s claims. Adding to the literature, Profitt, Steranucci, Banton and Epstein (2003) carried out a variety of experiments analysing spatial perception, with the blind walking task deeply embedded within their research. One of the most significant findingsin their work was that the absence of optic flow provoked an aftereffect that caused people to walk forward even when trying to remain stationary (Profitt et al., 2003). These results are significant to this particular study because they implicate the possibility of sensory adaptation occurring during the repeated use of blind walking tasks.

Upon examination and review of the literature surrounding spatial perception, it is clear that there are some limitations in the work previously completed. Although repeating an experiment to determine a mean can provide a better representation of the results, it seems evident that the repeated trials of the blind walking task can lead to the contamination of distance estimation results. The assessment of the literature has led to questions about whether there is a possible sensory adaptation occurring in the repeated use of the blind walking task, and about whether there is a possibility that this adaptation is skewing the results of accuracies in distance perception. Although Thomson, Elliot and Profitt et al. all bring forth substantial research, in all the studies completed they did not successfully account for the possibility of sensory adaptation in repeating these blind walking tasks, even when sensory adaptation was apparent in their results. These studies were able to contribute significant information regarding various degrees of human spatial perception; but this shortcoming of their research should be a significant consideration to the validity of the results.

That it mind, the purpose of this study was to examine whether sensory adaptation occurs in repeated trials of the blind walking task by having subjects complete the blind walking task in 2 conditions (visual and non-visual) and analysing the effects of both conditions on the accuracy of distance perception. Through this experiment, the aim was to study the effects of prior exposure of free walking in either a visual or non-visual condition and to also monitor how the earlier trials compare to the later trial blocks. In this way, both a between-subjects and within-subjects analysis of the data was completed, and overall, a 2 (visual or non-visual condition) x 2 (early block and later block of trials) analysis of distance perception was carried out. To analyse the effects of both the prior condition and earlier vs. later trial blocks (independent variables), subjects walked blindfolded to a visual target seen for 3 seconds, and their accuracy in overall distance perception was measured (dependent variable). Expected results were that over repeated trials sensory adaptation would in fact occur, and subsequently affect the subject’s performance on distance estimation in the blindfolded state. It was expected that subjects would overestimate by walking a longer distance in both the non-visual condition and on later trial blocks suggesting that sensory adaptation (experiencing a lower sensitivity to sensory information and therefore overshooting the distance) has occurred. If findings indicate that sensory adaptation has taken place over repeated trials, this could implicate a potential complication is using the blind walking task repeatedly as a reliable analysis of sensory perception. Additionally, as sensory adaptation and perception is explored more throughout the literature, this enables the scientific community to better understand the inner workings of the human’s sensory perception and skills, and can be applied to aid the understanding and rehabilitation of those who have lost their sight, or are suffering from other perceptual dysfunctions.

Methods

Subjects

Two undergraduate students (one male and one female) with ages ranging from 20 to 21 years of age participated in this study. There were two conditions involved in this study, and the participants were assigned to both groups in random order, with a day apart between the two conditions. This research was approved by the McMaster University Research Ethics Board.

Apparatus/Stimulus Materials

Both conditions took place on the Oval Field, a large field on the McMaster campus. The ground in the testing area was flat, dry, and free of tactile or visual landmarks and had very few visual landmarks present in the surrounding area. During the pre-experimental free walking period, a stop watch was used to ensure both subjects received the same amount of pre-experiment walking practice and used additionally to time the 3 second target viewing. A retractable measuring tape was used to determine the various distances, and distances used for testing were 6, 9, 12, and 15m. These distances were marked ahead of time to ensure accurate measurements in subsequent trials and were discretely marked using coloured golf tees to ensure they were not visible to the subjects during experimentation. The visual target used for this study was an orange stake that was placed in the ground at the appropriate distance during the 3 second viewing interval, and then removed once subjects began the task to allow for an overshoot of the distance. In both conditions, subjects were required to be without vision during the task and therefore wore a material blindfold throughout the blind walking experiment.

Procedure

Before beginning the experiment, the testing site was set up so that the subjects would not have the opportunity to view any of the measurement or distance details. The distances were measured on the grass using the retractable measuring tape, and then marked with the coloured golf tees. Distances up to approximately 25 meters were marked to allow for various starting positions to be used. The various starting positions of (0, -1, -2, 1, and 2) were also marked with golf tees so they would be clear to the experimenters while testing was going on. After the testing site was completely set up, the subjects arrived at different specified times to ensure they could not see the other subject completing the task. When subjects arrived, they were given a written instruction form and then verbally briefed on the task to be completed. After reading this form, the subjects were given information forms to fill out and a consent form to sign. After the subjects were briefed on the experiment, they were sent to complete the pre-experimental free walking task, and were blindfolded while walking if they were in the non-visual condition for that day. The subjects were required to walk around for 10 minutes regardless of their condition and were then brought to the testing site to complete the blind walking task. For completion of the blind walking task, experimenters were designated to1 of 3 roles, so that they could silently communicate between each other and carry out the experiment without external noise. Experimenter 1 was the recorder, and was responsible for holding the clipboard and recording results. During the experiment, experimenter 1 would give silent directions through hand signals to experimenter two and three so that the experiment was carried out accurately. Experimenter 2 was responsible for directing the blindfolded subject to the specified starting position, and leading them back after subsequent trials. Experimenter 3 was responsible for placing the orange stake in the ground and the specified distance (distances were randomly assigned during the 12 trials in each condition) and then removing it once the subject had viewed the target. A single trial was carried out as follows; the subject was lead to the specified starting position by experimenter 2, while experimenter 3 placed the orange stake and the specified distance. The subject was then instructed to lift the blindfold to see the visual target, and allowed three seconds to estimate the distance. After viewing the target, the subject placed the blindfold back on, and then walked to the target. While the subject was walking, experimenter 3 ensured the stake was removed to allow the subject to overshoot the distance. Once the subject had stopped walking, experimenter 3 assessed their travelled distance, always measuring from the front of their shoe, and then relayed that data to experimenter 1. After recording the distance, experimenter 1 signalled to experimenter 2 the new starting position and experimenter 2 lead the subject to their starting place. This procedure was repeated for a total of 12 trials for each subject. The process was repeated on the second day with the subjects in the opposite condition, for a total of 48 trials over two days. The data collected from the experiment was then analysed using 2-way ANOVA to determine if there was a main effectof the visual vs. non-visual condition, which was the between-subjects analysis, and also if there was a main effect of earlier vs. later trial blocks, the within-subjects analysis. Additionally we examined the data to establish if there was an interaction occurring between the independent variables tested.

References

Elliot, Digby. (1987). The Influence of Walking Speed and Prior Practice on Locomotor

Distance Estimation. Journal of Motor Behaviour, 19(4), 476-485.

Proffit, Dennis R., Stefanucci, Jeanine., Banton, Tom., & Epstein, William. (2003). The Role of

Effort in Perceiving Distance. Psychological Science, 14(2), 106-112.

Thomson, James A. (1983). Is Continuous Visual Monitoring Necessary in Visually Guided

Locomotion? Journal of Experimental Psychology: Human Perception and Performance

1983, 9(3), 427-443.