THE LATERAL INHIBITION OF PERCEIVED CONTRAST IS INDIFFERENT TO ON-CENTER OFF-CENTER SEGREGATION, BUT SPECIFIC TO ORIENTATION

When a central test patch C, composed of an isotropic spatial texture, is surrounded by a texture field S, the perceived contrast of C depends substantially on the contrast of the surround S. When C is surrounded by a high contrast texture with a similar spatial frequency content, it appears to have less contrast than when it is surrounded by a uniform field. Here, we employ two novel textures: T+ which is designed to selectively stimulate only the on-center system, and T-, the off center system. When C and S are of type T+ and T-, the reduction of C's apparent contrast does not vary with the combination of T+, T-. This demonstrates that the reduction of C's apparent contrast is mediated by a mechanism whose neural locus is central to the interaction between on-center and off-center visual systems. We further demonstrate orientation specificity: the reduction of grating C's apparent contrast by a surround grating S, of the same spatial frequency is greatest when C and S have equal orientation. Using dynamically phase-shifting sinusoidal gratings of 3.3, 10 and 20 c/deg, we measured reduction of apparent contrast using different contrast-combinations of C and S. Results: (1) S gratings, both parallel and perpendicular to C, cause a reduction in C's apparent contrast relative to a uniform surround. (2) In all of the viewing conditions, the reduction of apparent contrast induced by the parallel surrounds was at least as great as that induced by the perpendicular surrounds. Often it was much greater. (3) Orientation specificity increases with increasing spatial frequency and with decreasing stimulus contrast.