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Functional Organization of Parietal Neuronal Responses to Optic-Flow Stimuli

¹ Brain Sciences Center, Veterans Affairs Medical Center 55417;; ² Department of Neuroscience, University of Minnesota Medical School, Minneapolis, Minnesota 55455; ³ Department of Neurology, University of Minnesota Medical School, Minneapolis, Minnesota 55455; ⁴ Department of Psychiatry, University of Minnesota Medical School, Minneapolis, Minnesota 55455; ⁵ Cognitive Sciences Center, University of Minnesota, Minneapolis, Minnesota 55455

Submitted 4 April 2003; accepted in final form 21 April 2003

We analyzed the dissimilarity matrix of neuronal responses to moving visual stimuli using tree clustering and multidimensional scaling (MDS). Single-cell activity was recorded in area 7a while random dots moving coherently in eight different kinds of motion (right-, left-, up-, and downward, clockwise, counterclockwise, expansion, contraction) were presented to behaving monkeys with eyes fixated. Tree clustering analyses showed that the {rightward, leftward}, {upward, downward}, and {clockwise, counterclockwise]} motions were clustered in three separate branches (i.e., horizontal, vertical, and rotatory motion, respectively). In contrast, expansion was in a lone branch, whereas contraction was also separate but within a larger cluster. The distances among these clusters were then subjected to an MDS analysis to identify the dimensions underlying the tree clustering observed. This analysis revealed two major factors in operation. The first factor separated expansion from all other stimulus motions, which seems to reflect the prominence of expansion during the common activity of locomotion. In contrast, the second factor separated planar motions from motion in depth, which suggests that the latter may hold a special place in visual motion processing.

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