Inhibition, disinhibition, and summation among orientation detectors in human vision.

A vertical test line viewed together with an oblique inducing line appears to be tilted in the opposite direction (orientation contrast); a similar effect results from adapting to an oblique line before presenting the test line (tilt after-effect). The error in perceived orientation caused by a clockwise tilted inducing line may be partially cancelled by a second clockwise tilted inducing line; likewise, adapting to a configuration of two clockwise orientations gives weaker tilt after-effects than adapting to the more effective of the two orientations alone. The angular functions of these "disinhibitory" effects mirror the angular functions of the principal effects. However, combining tilt after-effect and simultaneous contrast in a disinhibition design by having the subject first adapt to a clockwise orientation and then presenting the test line together with a clockwise inducing line results in partial summation rather than disinhibition. The results are consistent with Carpenter's and Blakemore's (1973) hypothesis that orientation contrast and the tilt after-effect are manifestations of cortical inhibition in a network of inhibitory, recurrent lateral connections between cortical orientation detectors. After-effects must be due to prolonged inhibition, probably caused by a sensitivity change in previously inhibited detectors.