The spatial extent over which neurons in macaque striate cortex pool visual signals

We recorded activity of single units in macaque monkey primary visual cortex (VI) to define the retinotopic extent of the visual inputs that drive or modulate VI neuron responses in parafoveal and peripheral (calcarine) cortex. We used high-contrast drifting grating stimuli to define the extent of the area over which responses summate and the extent of the receptive-field surround. We found responses of most VI cells to summate over I deg, with a suppressive surround typically twice that in diameter, though for some cells (even in parafoveal VI) surrounds exceeded 13 deg in diameter. Surprisingly, we found no significant laminar differences in these dimensions or in the strength of surround suppression. We found that surround suppression in most cells arises from both the ends and sides of the receptive field. Our measures indicate that the strongest modulatory input arises from regions immediately adjacent to the excitatory summation area. These physiological measures suggest that the high-contrast summation field of VI neurons can be accounted for by the sum of lateral geniculate nucleus (LGN) inputs offered to the local cortical column, with monosynaptic lateral connections within area VI adding the larger dimensions of the low-contrast summation field and the near surround. Neither of these inputs suffice to explain the largest surrounds, which most likely derive from feedback from extrastriate visual areas.