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 responses can be modified by stimuli overlying the surrounding area. For example patch-suppressed cells respond to gratings of a specific orientation within their classical receptive field, but the response diminishes if the grating is expanded to cover the surrounding area. We report here more complex effects in many such cells. When stimulated at their optimal orientation, introducing a surrounding field at a significantly different (for example, orthogonal) orientation enhanced their output by both a disinhibitory mechanism and an active facilitatory mechanism producing 'supra-optimal' responses. Importantly, some cells responded well if the orientations of centre and surround stimuli were swapped. The output reflected the discontinuity because neither stimulus component alone was effective. Under these stimulus conditions simultaneously recorded cells with orthogonally oriented receptive fields showed correlated firing consistent with neuronal binding to the configuration. We propose a mechanism integrating orientation-dependent information over adjacent areas of visual space to represent focal orientation discontinuities such as junctions or corners.