Abstract

Masking studies employing cosine gratings have been used previously to measure both the spatial frequency and orientation tuning of spatial mechanisms in human vision. Here we report a powerful nonlinearity whereby a grating orthogonal to the test greatly enhances the masking produced by gratings at other orientations. Thus, a horizontal cosine grating added to a 45° grating, each at 25% contrast, causes a threshold elevation in a vertical test grating approximately three times greater than that produced by doubling the contrast of the 45° grating alone. Similar increases in masking were also observed when a grating orthogonal to the test was added to a mask component at either 22.5° or 67.5°. This orthogonal enhancement of masking is reduced or absent when mask and test differ in spatial frequency by a factor of 3 or more. Finally, orthogonal masking enhancement disappears when the test is flashed 250 ms after the onset of the mask. These results may reflect the operation of a nonlinear feedback mechanism that pools responses of orientation selective units and functions as a contrast gain control.

© 1989 Optical Society of America