Dynamic Spatial Summation Characteristics Of Cat Primary Visual Cortex

In area V1of the cat and monkey, there is a surround region beyond the classicalreceptive field (CRF) which alone is unresponsive to visual stimuli, but may modulatethe cell's response. This feld is referred to as the "extra-receptive"(ERF). It has beenreported that the extent of the CRF and/or the ERF of V1neurons is not fixed, but varieswith stimulus contrast.Using extracellular recording methods, we measured the contrast response functionof the inhibitory extra-receptive field of41cells in the cat primary visual cortex whenstimulated the receptive alone and simultaneously stimulated the receptive field andextra-receptive field. The result showed that a large contrast stimulus with the preferredorientation that covered the inhibitory extra-receptive field and the receptive fieldresulted in the increment in dynamic range of contrast response function and thedecrement in contrast gain and response gain. However, the discontinuity between thecenter and surround in stimulation orientation largely removed the inhibitory effects ofthe extra-receptive field of some cells and resulted in the increment of the dynamicrange of the contrast response function and increment of contrast gain and responsegain.We re-examined the spatial summation of115V1neurons at high and low contrast.By fitting the spatial summation curves obtained at different contrasts with a differenceof Gausians (DOG) model, we were able to quantitatively estimate the effect of contraston the spatial extent of the CRF and ERF, as well as the strength of surroundsuppression. Our results showed that both the CRF and ERF become expanded at lowcontrast, but that the expansion is more marked for the CRF than for the ERF. Althoughthe effect of contrast on surround suppression was varied, the overall suppressionincreased significantly at high contrast. Moreover, the contrast-dependent change in theextent of CRF is independent of the change in suppression strength of the ERF.We distinguished parvalbumin-expressing inhibitory interneurons (narrow-spikingcells) from pyramidal neurons (broad-spiking cells) based on their briefer actionpotential duration, and then examined spatial summation of two categories at high and low contrast. The neurons with narrow action potentials are less-dependent on stimuluscontrast, that the extent of center and surround increased by about10percent and thestrength of surround suppression was basically invariant when contrast drop from highto low. Whereas, most broad-spiking neurons are highly dependent on stimulus contrast,that comparing to at high contrast, the extent of center and surround have an average1.94-fold and1.33-fold increase at low contrast, respectively. The overall suppressionalso increased significantly at high contrast. This difference in contrast effect on spatialsummation of the two types may be attributable to difference in morphology.