Temporal dynamics of a neural solution to the aperture problem in visual area MT of macaque brain

A critical step in the interpretation of the visual world is the integratio n of the various local motion signals generated by moving objects. This pro cess is complicated by the fact that local velocity measurements can differ depending on contour orientation and spatial position. Specifically, any l ocal motion detector can measure only the component of motion perpendicular to a contour that extends beyond its field of view(1,2). This "aperture pr oblem''(3) is particularly relevant to direction-selective neurons early in the visual pathways, where small receptive fields permit only a limited vi ew of a moving object. Here we show that neurons in the middle temporal vis ual area (known as MT or V5) of the macaque brain reveal a dynamic solution to the aperture problem. MT neurons initially respond primarily to the com ponent of motion perpendicular to a contour's orientation, but over a perio d of approximately 60 ms the responses gradually shift to encode the true s timulus direction, regardless of orientation. We also report a behavioural correlate of these neural responses: the initial velocity of pursuit eye mo vements deviates in a direction perpendicular to local contour orientation, suggesting that the earliest neural responses influence the oculomotor res ponse.