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Adam J. Woods, John Philbeck; Comparison of rope-pulling and blindwalking as measures of perceived egocentric distance. Journal of Vision 2008;8(6):753. doi: 10.1167/8.6.753.
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Pulling a length of rope between the hands could potentially be used to indicate the perceived distance of objects. Like blindwalking, this response requires integration of body motion between limbs, but unlike blindwalking, it does not require locomotor ability or updating changes in one's location into a representation of the environment. These differences make rope-pulling an attractive alternative for measuring distance perception (e.g., in brain-injured patients, or when holding vestibular cues constant is of interest). Here, we assessed whether rope-pulling is indeed responsive to perceived distance by observing whether it shows a similar pattern of errors as blindwalking when egocentric distance cues are removed. Participants (n=13) previewed targets in 3 viewing conditions: (1) Full Cue: binocular viewing of ground-level targets in a well-lit room. (2) Floor Occlusion: monocular viewing of ground-level targets under low lighting conditions. A box occluded the nearby ground-plane. (3) Reduced Cue: monocular viewing of glowing eye-level targets in an otherwise dark room. Targets were placed at 2.5, 3.5, 4.9, or 6.9 m. After viewing the target, participants donned a blindfold and indicated target distance by: (1) blindwalking to the previously viewed target location or (2) pulling a rope hand over hand to match the perceived target distance. Responses using the two behaviors were highly correlated. More importantly, a nearly unchanging relationship existed between the two behaviors as distance cues varied. One-way ANOVA on the slopes and intercepts of the best-fitting lines relating walking to rope-pulling showed no significant differences between viewing conditions (slopes: F[2,36] = 0.08, p = 0.9; intercepts: F[2,36] = 0.19, p = 0.8). Thus, we found no indication that rope-pulling and blindwalking are controlled by distinct representations. Given previous evidence that blindwalking is controlled by perception (Philbeck & Loomis, 1997), we conclude that rope-pulling is, as well.
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