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Behavioural and neuroimaging evidence of speed-tuned directional anisotropies in global motion perception Zwicker, Amy Elizabeth


Research has suggested that directional anisotropies, or asymmetries, exist for global motion perception, yet there are inconsistencies among these results. The purpose of this study was to examine directional anisotropies in motion perception, to determine what motion parameters affect anisotropies, and to investigate cortical activation that may contribute to anisotropies. In Experiment 1, coherence thresholds were obtained from 40 subjects for direction discrimination of moving random dot patterns. Lower thresholds were found for centripetal and horizontal motion relative to centrifugal and vertical, respectively, when the speed of motion was 8 deg/s. For motion that was 1 deg/s, lower thresholds were found for upward motion relative to downward. These anisotropies were retested in 4 subjects in Experiment 2 with a detection paradigm; the fast motion anisotropies were confirmed but the slow motion anisotropy was not. Six participants completed an fMRI experiment in which the BOLD response to slow and fast directions of motion was measured. V5/MT+ was functionally defined; within this ROI greater activity was found for fast vertical motion relative to horizontal and fast centripetal motion relative to centrifugal. There were no differences in cortical activity for directions of slow motion. Both the behavioural and fMRI studies suggest that there are global motion pathways tuned to different speeds of motion. Within these speedtuned pathways, there is support for unique directional anisotropies in global motion processing.

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