VISUAL CORTICAL MECHANISMS DETECTING FOCAL ORIENTATION DISCONTINUITIES

NEURONS in the primary visual cortex (V1) respond in well defined ways to stimuli within their classical receptive field, but these response s can be modified by stimuli overlying the surrounding area(1-7). For example patch-suppressed cells respond to gratings of a specific orien tation within their classical receptive field, but the response dimini shes if the grating is expanded to cover the surrounding area(1-7). We report here more complex effects in many such cells. When stimulated at their optimal orientation, introducing a surrounding field at a sig nificantly different (for example, orthogonal) orientation enhanced th eir output by both a disinhibitory mechanism and an active facilitator y mechanism producing 'supra-optimal' responses. Importantly, some cel ls responded well if the orientations of centre and surround stimuli w ere swapped. The output reflected the discontinuity because neither st imulus component alone was effective. Under these stimulus conditions simultaneously recorded cells with orthogonally oriented receptive fie lds showed correlated firing consistent with neuronal binding to the c onfiguration. We propose a mechanism integrating orientation-dependent information over adjacent areas of visual space to represent focal or ientation discontinuities such as junctions or corners.