Surround modulation measured with functional MRI in the human visual cortex.

Visual context profoundly influences 1) the responses of mammalian visual neurons and 2) the perceptual sensitivity of human observers to localized visual stimuli. We present data from functional MRI studies demonstrating contextual modulation in the human visual cortex. Subjects viewed a circular grating patch that was continuously present. A surround grating was added in an ON-OFF block design to reveal its effect on the central region. Stimulus-correlated activation was quantified and visualized on a flattened map of the occipital gray matter. Modulation was measured in a region of interest activated by the central grating alone. The observed effects were predominantly suppressive, consistent with the effects typically found in single neurons and perception. Suppression was greatest when the surround and center had the same orientation and was reduced or absent when it was orthogonal. When spatial phase was manipulated, suppression was greatest for in-phase center/surround gratings and much reduced or reversed (facilitation) for opposite-phase stimuli. With eccentric stimulus presentation, suppression was reduced and facilitation became more common. The findings provide a direct demonstration of the existence of powerful and stimulus-specific surround effects in human visual cortex.