Comparison of spatial summation properties of neurons in macaque V 1 3 and V 2

37 38 In visual cortex, responses to stimulation of the receptive field (RF) are modulated by 39 simultaneous stimulation of the RF surround. The mechanisms for surround modulation 40 remain unidentified. We previously proposed that in V1, near surround modulation is 41 mediated by geniculocortical and horizontal connections, and far surround modulation by 42 inter-areal feedback connections. To understand spatial integration in V2 and its 43 underlying circuitry, we have characterized spatial summation in different V2 layers and 44 stripe compartments, and compared it to that in V1. We used grating stimuli in circular 45 and annular apertures of different sizes to estimate the extent and sensitivity of RF and 46 surround components in V1 and V2. 47 V2 RFs and surrounds were twice as large as in V1. As in V1, V2 RFs doubled in 48 size when measured at low contrast. In both V1 and V2, surrounds were ~5 times the size 49 of the RF, and the far surround could exceed 12.5° in radius, averaging 5.5° in V1 and 50 9.2° in V2. The strength of surround suppression was similar in both areas. Thus, 51 although differing in spatial scale, the interactions among RF components are similar in 52 V1 and V2, suggesting similar underlying mechanisms. As in V1, the extent of V2 53 horizontal connections matches that of the RF center, but is much smaller than the largest 54 far surrounds, which likely derive from inter-areal feedback. 55 In V2, we found no laminar or stripe differences in size and magnitude of 56 surround suppression, suggesting conservation across stripes of the basic circuit for 57 surround modulation. 58 59 60 61