CONTRAST DEPENDENCE OF CONTEXTUAL EFFECTS IN PRIMATE VISUAL-CORTEX

The responses of neurons in the visual cortex to stimuli presented wit hin their receptive fields can be markedly modulated by stimuli presen ted in surrounding regions that do not themselves evoke responses(1-7) . This modulation depends on the relative orientation and direction of motion of the centre and surround stimuli, and it has been suggested that local cortical circuits linking cells with similar stimulus selec tivities underlie these phenomena(8-16). However, the functional relev ance and nature of these integrative processes remain unclear. Here we investigate how such integration depends on the relative activity lev els of neurons at different points across the cortex by varying the re lative contrast of stimuli over the receptive field and surrounding re gions. We show that simply altering the balance of the excitation driv ing centre and surround regions can dramatically change the sign and s timulus selectivity of these contextual effects. Thus, the way that si ngle neurons integrate information across the visual field depends not only on the precise form of stimuli at different locations, but also crucially on their relative contrasts. We suggest that these effects r eflect a complex gain-control mechanism that regulates cortical neuron responsiveness, which permits dynamic modification of response proper ties of cortical neurons.