Contrast Dependence of Contextual Effects in Macaque Striate Cortex

Abstract

We have studied the effect of varying stimulus contrast on the modulatory effects exerted on V1 receptive fields by surrounding portions of the visual field. We used standard extracellular techniques to record unit activity in striate cortex of paralysed, opiate-anaesthetised macaque monkeys. We measured the orientation and direction tuning of neurons at several contrasts, with and without the presence of a surrounding stimulus that itself evoked minimal responses from the neuron. At both high and low stimulus contrasts, surround stimuli modulated responses to centre stimuli when the orientation and direction of the centre and surround were in the appropriate (though not necessarily matched) relationship. At low contrasts, we observed more profound suppression and facilitation. However, this did not simply reflect release from response saturation. At low contrasts, a greater range of surround orientations could modulate neuronal responses. This sometimes resulted in identical surround stimuli being suppressive when pairs with high-contrast centre stimuli, but facilitative when paired with low-contrast centre stimuli. Further evidence against response saturation at high contrasts was the frequent anisotropy in the suppression, ie suppression was direction-selective at high contrast, but nondirectional at low contrasts. Such contrast-dependent effects were also revealed by measurements of contrast response functions in the presence of the surround. We sometimes observed not only a decrease in the response to high-contrast stimuli and a lower slope of the response-versus-contrast curve but also an increase in responsiveness to low-contrast centre stimuli in the presence of nominally suppressive surround stimuli. Contextual effects in striate cortex thus depend importantly on the relative contrasts of centre and surround stimuli.