Spatial summation in the receptive fields of MT neurons

Receptive fields (RFs) of cells in the middle temporal area (MT or V5) of m onkeys will often encompass multiple objects under normal image viewing. We therefore have studied how multiple moving stimuli interact when presented within and near the RF of single MT cells. We used moving Gabor function s timuli, <1 degrees in spatial extent and similar to 100 msec in duration, p resented on a grid of possible locations over the RF of the cell. Responses to these stimuli were typically robust, and their small spatial and tempor al extent allowed detailed mapping of RFs and of interactions between stimu li. The responses to pairs of such stimuli were compared against the respon ses to the same stimuli presented singly. The responses were substantially less than the sum of the responses to the component stimuli and were well d escribed by a power-law summation model with divisive inhibition. Such divi sive inhibition is a key component of recently proposed "normalization" mod els of cortical physiology and is presumed to arise from lateral interconne ctions within a region. One open question is whether the normalization occu rs only once in primary visual cortex or multiple times in different cortic al areas. We addressed this question by exploring the spatial extent over w hich one stimulus would divide the response to another and found effective normalization from stimuli quite far removed from the RF center. This suppo rts models under which normalization occurs both in MT and in earlier stage s.