Multiple gain control processes in contrast-contrast phenomena.

Spatial interactions among orientation-tuned gain control processes are presumed to mediate center-surround contrast-contrast phenomena. In this paper, we assess contributions of gain control processes that pool over orientation. We measured the apparent contrast of a luminance-modulated center disk embedded in various modulated surrounds. In all conditions, observers compared the apparent contrast of the test center to an identically modulated disk with no surround. When center and surround are simple, vertical sinusoids and presented in phase, suppression depends upon surround contrast and is marked at high contrasts. When components are presented 180 degrees out of phase, no suppression occurs at any contrast. When a horizontal component is added to the surround, much less suppression occurs. However, strong suppression is reinstated when both center and surround are plaids. Neither of the latter two effects are phase dependent. We suggest that two different sources of gain control are revealed by the simple sinusoidal and the plaid stimuli. One is orientation tuned and phase-dependent. The other pools over all orientations and includes neurons tuned to multiple phases.