Egocentric and allocentric localization during induced motion |
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Authors: | Robert B Post Robert B Welch David Whitney |
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Institution: | (1) Department of Psychology, University of California, Davis, CA 95616, USA;(2) NASA Ames Research Center, Moffett Field, Mountain View, CA 94035, USA;(3) Center for Mind and Brain, University of California, Davis, CA 95616, USA |
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Abstract: | This research examined motor measures of the apparent egocentric location and perceptual measures of the apparent allocentric
location of a target that was being seen to undergo induced motion (IM). In Experiments 1 and 3, subjects fixated a stationary
dot (IM target) while a rectangular surround stimulus (inducing stimulus) oscillated horizontally. The inducing stimulus motion
caused the IM target to appear to move in the opposite direction. In Experiment 1, two dots (flashed targets) were flashed
above and below the IM target when the surround had reached its leftmost or rightmost displacement from the subject’s midline.
Subjects pointed open-loop at either the apparent egocentric location of the IM target or at the bottom of the two flashed
targets. On separate trials, subjects made judgments of the Vernier alignment of the IM target with the flashed targets at
the endpoints of the surround’s oscillation. The pointing responses were displaced in the direction of the previously seen
IM for the IM target and to a lesser degree for the bottom flashed target. However, the allocentric Vernier judgments demonstrated
no perceptual displacement of the IM target relative to the flashed targets. Thus, IM results in a dissociation of egocentric
location measures from allocentric location measures. In Experiment 2, pointing and Vernier measures were obtained with stationary
horizontally displaced surrounds and there was no dissociation of egocentric location measures from allocentric location measures.
These results indicate that the Roelofs effect did not produce the pattern of results in Experiment 1. In Experiment 3, pointing
and Vernier measures were obtained when the surround was at the midpoint of an oscillation. In this case, egocentric pointing
responses were displaced in the direction of surround motion (opposite IM) for the IM target and to a greater degree for the
bottom flashed target. However, there was no apparent displacement of the IM target relative to the flashed targets in the
allocentric Vernier judgments. Therefore, in Experiment 3 egocentric location measures were again dissociated from allocentric
location measures. The results of this experiment also demonstrate that IM does not generate an allocentric displacement illusion
analogous to the “flash-lag” effect.
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Keywords: | Localization Induced motion Motion perception |
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