Behavioral/Systems/Cognitive Early and Late Mechanisms of Surround Suppression in

The response of a neuron in striate cortex to an optimally configured visual stimulus is generally reduced when the stimulus is enlarged to encroachona suppressive region that surrounds its classical receptive field (CRF). To characterize themechanism that gives rise to this suppression, we measured its spatiotemporal tuning, its susceptibility to contrast adaptation, and its capacity for interocular transfer. Responses to an optimally configured grating confined to the CRF were strongly suppressed by annular surrounding gratings drifting at awide rangeof temporal andspatial frequencies (including spatiallyuniformfields) that extended fromwell below towell above the range that drives most cortical neurons. Suppression from gratings capable of driving cortical CRFs was profoundly reduced by contrast adaptation and showed substantial interocular transfer. Suppression from stimuli that lay outside the spatiotemporal passband of most cortical CRFs was relatively stronger when the stimulus on the CRF was of low contrast, was generally insusceptible to contrast adapta-tion, and showed little interocular transfer. Our findings point to the existence of two mechanisms of surround suppression: one that is prominent when high-contrast stimuli drive the CRF, is orientation selective, has relatively sharp spatiotemporal tuning, is binocularly driven, and can be substantially desensitized by adaptation; the other is relatively more prominent when low-contrast stimuli drive the CRF, has very broad spatiotemporal tuning, is monocularly driven, and is insusceptible to adaptation. Its character suggests an origin in the input layers of primary visual cortex, or earlier. Key words: macaque monkey; striate cortex; receptive field; surround suppression; context; cortical feedbac