A REDUCTION IN THE NUMBER OF DIRECTIONALLY SELECTIVE NEURONS EXTENDS THE SPATIAL LIMIT FOR GLOBAL MOTION PERCEPTION

Dynamic random-dot targets were used to study neural mechanisms underl ying motion perception. Performance of cats with severely reduced numb ers of cortical directionally selective neurons (reduced DS) was compa red to that of normal animals. We assessed the spatial properties of t he residual motion mechanism by measuring direction discriminations at various dot displacements. At small displacements, reduced DS cats' m otion integration thresholds for opposite direction discrimination wer e nearly normal. At larger displacements, their thresholds surpassed t hose of normal cats and their upper displacement limit (d(max)) was in creased by 0.35 deg. The accuracy of direction discrimination was redu ced at small displacements, but at larger displacements direction diff erence thresholds of reduced DS cats approached or surpassed those of normals. These data were compared to the performance of humans who sho wed an extension of d(max) for peripherally viewed targets. The data s upport the hypothesis that expansion in spatial scale of the motion me chanism may contribute to extension of d(max). Additional support for this hypothesis is provided by a modified direction discriminating lin e-element model. The model also suggests that changes in sampling of m otion mechanisms in the reduced DS system may play a role.