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The magnitudes of spike responses of area 17 (striate cortex, area V1) neurons to stimulation of their classical receptive fields were reduced (suppressed) when the stimuli extended into the silent surround regions. We found that when optimally oriented sine-wave drifting grating patches extended into the distant parts of silent surround regions, over 35% of V1 neurons showed a 'counter-suppression', that is, a reduction in the magnitude of suppression. The magnitudes of both the suppression and the counter-suppression effects were dependent on stimulus contrast, that is, with a decrease of contrast the magnitude of suppression decreased, while the magnitude of counter-suppression increased. Overall, the surround modulation tended to be clearly suppressive at high contrast and less suppressive or even facilitatory at low contrast. The contrast-dependent effects described here appear to represent one of the fundamental properties of neurons in the mammalian visual system. These properties allow improvement of recognition (high contrast) or detection (low contrast) of visual objects under varying conditions. Putative changes of center and surround mechanisms at low contrast are discussed.

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