Stimulus Orientation and Contrast Constancy

Abstract

The human visual system requires less contrast to detect patterns oriented vertically or horizontally than those oriented obliquely. We investigated whether this orientational anisotropy persists at suprathreshold contrasts. Using a contrast-matching technique, we found that it disappears at contrasts just above threshold, given the stimulus conditions employed. These results suggest that a neural mechanism, at suprathreshold, adjusts the gain of orientational subsystems to compensate for the lower sensitivity of the visual system to oblique patterns. A similar suprathreshold effect, referred to as contrast constancy, has been observed in studies employing grating patterns varying in spatial and temporal frequency. We argue, based on previous electrophysiological findings, that this neural compensation results from antagonism between excitatory and inhibitory processes in the visual cortex

Key Words:

• Contrast marching
• orientation
• oblique effect
• human vision