Thin edges and the induced contrast asynchrony

Abstract

The Induced Contrast Asynchrony is a visual effect that demonstrates a perceptual separation between luminance and contrast (Shapiro et al, in press). The basic version of the effect consists of two physically identical rectangular patches whose luminance levels are modulated at 2 Hz (.75×1.5 deg, 40 + 15*sin(tw) cd/m2); one patch is surrounded by a dark field (6×6 deg, 20 cd/m2) and the other by a light field (6×6 deg, 60 cd/m2). This produces a configuration in which the luminance of the two patches modulates in phase and the contrast modulates in antiphase. Observers perceive primarily the contrast signal (i.e., the patches modulate in antiphase), but at low frequencies, observers can also attend to the luminance signal. This year we present the effects of the edge widths on the appearance of the asynchronous contrast signal. We measure psychometric functions for the perceived contrast signal versus the width of the introduced lines over a range of patch sizes (width .15 to 1.2 deg). We show that 1) the contrast asynchrony can be produced when the surrounding field is of minimal width, indicating that the contrast signal arises primarily at the patch/surround edge; 2) placing a thin (.05 deg) white or black line adjacent to one side of the modulating rectangular patch suppresses perception of the asynchronous contrast signal; 3) a variety of stimulus configurations (e.g., circular patch with a two-tone surround or gradient bar adjacent to a modulating patch) can produce new versions of the contrast asynchrony; and 4) the combination of thin edges and single-patch modulation can produce a novel form of contrast-defined apparent position shifts. Most of the results can be explained by a model with separate contrast (i.e., rectified difference) and luminance pathways. A sophisticated spatiotemporal analysis is required to account for apparent changes in spatial position. The results offer new insights into static lightness illusions such as White's effect.

R15-EY12946