Dynamic Property of Spatial Summation of V1 Neurons of the Cat

In the primary visual cortex (V1) of the cat and monkey there is an extensive surround region beyond the classical receptive field (CRF). This surrounding field is conventionally referred to as “nonclassical receptive field” (nCRF). In recent years, some investigators reported that the extent of CRF and/or nCRF of V1 neurons is not fixed but may vary with stimulus contrast. We have reexamined the spatial summation property for 101 V1 neurons at a high (20–80%) and a low (3–15%) contrast. By fitting the spatial summation curves of the neurons obtained at different contrast using the difference of Gausians (DOG) model, we were able to estimate quantitatively the contrast-induced variation of the extent of CRF and nCRF for this group of cells. Our results showed that both the excitatory CRF and suppressive nCRF expanded at low stimulus contrast, but the expansion was more marked for the CRF than that for the nCRF. Moreover, the strength of surround suppression decreased or even disappeared under the low contrast condition. Our data suggest that at higher stimulus contrast both the shrinkage of excitatory CRF and the increase of surround suppression may improve spatial resolution for the perceived images.