The spatial distribution of the antagonistic surround of MT/V5 neurons

The majority (217/325, 66%) of the neurons in the middle temporal (MT)area/V5 show strong antagonistic surrounds, defined here by a decrease ofat least 50% in the summation curve. We mapped the antagonistic surround in145 such cells, using eight circularly distributed surround stimuluspatches (Surround Asymmetry Test, SAT) and also mapped the surround in 51of these 145 cells using a grid consisting of 25 square patches (SurroundMapping Test, SMT). Both tests showed that the angular surrounddistribution was non-uniform in the majority of these neurons. In half theneurons, the antagonistic surround was asymmetric, and arose from a singleregion on one side of the excitatory receptive field (ERF). In anotherquarter of the sample the surround was bilaterally symmetric, and arosefrom a pair of regions on opposite sides of the ERF. Only the remaining 20%showed a circularly symmetric surround distribution. These three groupsdiffered in their laminar distribution. The SMT showed that, radially, thesurround antagonism reached a maximum, on average, at 1.5 times the ERFradius. Detailed comparisons of the spatial relationships of excitatory andinhibitory regions of the RF components shows that non-homogeneity of thesurround influence appears to be an intrinsic property of the surround.Such a property may underly the extraction of the surface orientation andcurvature from speed patterns.