On the role of X and simple cells in human contrast processing.

We investigated the potential role of retinal X and cortical simple cells in determining human psychophysical detection performance under contrast masking conditions. Since both X and simple cells exhibit a null phase, the phase of a background mask should affect the visibility of a test grating processed by such cells. Sinusoidal test gratings of either 1 or 7 c/deg were presented as a sustained or transient increment against a background mask of the same size and spatial frequency at either 0 or 90 deg phase. For background contrasts from 0.5% up to 40%, psychophysical contrast sensitivity was phase-independent for all conditions. Therefore, either (1) contrast threshold is mediated by cells with non-linear spatial summation properties, such as Y or complex cells, or (2) the masking effect of the background occurs after a phase-insensitive combination or pooling of simple cell responses in the cortex.