Depth Plane Separation Affects Both Lightness Contrast and Assimilation

Abstract

Lightness contrast and assimilation are two opposite phenomena: contrast occurs when a gray target perceptually acquires a complementary color than the bordering, inducing, surfaces; assimilation is when a gray target perceptually acquires the same color component as the inducers. Previous research has shown that both phenomena are affected by the manipulation of depth between the inducers and target. However, different results have been reported; it is not clear whether contrast persists when inducers are non-coplanar with the target. Previous studies differ for the spatial configuration of the stimuli and the technique adopted to manipulate depth. The aim of this research was to measure the effects of manipulating the depth between inducers and target in comparable conditions. Results show that contrast persists, but largely reduces, after depth manipulation while assimilation reverses to contrast. Furthermore, interesting asymmetries between white and black inducers emerged with white inducers favoring contrast and black inducers favoring assimilation. These results provide further evidence that high-level processes of visual processing are involved in both phenomena, with important consequences for lightness theories.

Keywords: anchoring theory; belongingness; layer theories; lightness assimilation; lightness contrast.

Figure 1

Lightness contrast (top row) and lightness assimilation (bottom row). Top row: the gray surrounded by white (A) is perceived to be darker than an equivalent gray surrounded by black (B). Bottom row: the gray area with small white elements on top (C) is perceived to be lighter than an equivalent gray with black elements (D).

Figure 2

Sketch of the experimental stimuli.

Figure 3

Sketch of the coplanar and non-coplanar conditions (see text for details).

Figure 4

Example matching chart.

Figure 5

Mean difference between baseline and perceived luminance of the target (log luminance) for each condition. Zero represents the baseline value (measured luminance of the target). Positive values indicate that the match was perceived to be lighter than the target and negative values indicate that the match was perceived to be darker. Error bars represent standard errors. (A) shows the results for the Coplanar conditions whilst (B) indicates the results for the Non-Coplanar conditions.