Disappearance elicited by contrast decrements.

Abstract

The observation that physically present visual stimuli can sometimes disappear from consciousness has intrigued vision scientists for centuries. Two situations are known to cause such disappearance: stationary peripheral images and images (centrally or peripherally viewed) masked by abrupt contrast increments of stimuli in adjacent retinal areas. Both of these situations require near-image stabilization on the retina. Here, we describe a third way to remove stimuli from conscious awareness. It involves contrast decrements (CDs) of nearly stabilized images in the periphery. Unlike the Troxler effect, with sufficient CD, complete disappearance can be achieved almost instantaneously without significant adaptation periods. Unlike traditional masking effects, CD disappearance does not result in an after-image at or near the locus of stimulation. We report the results of four experiments in which some of the characteristics of this newly discovered phenomenon were examined. The results indicate that CDs produced by changes in the luminance of the target (see Experiment 2) or by changes in background luminance (see Experiment 3) result in an immediate loss of sensitivity to stimuli that would take much longer to fade with Troxler-like adaptation (see Experiment 1). However, the duration of such loss of sensitivity (approximately 2 sec) is comparable for the two paradigms. The frequency of disappearance increased with greater contour eccentricity, but disappearance duration remained fairly constant.