Luminance Differences in the Combined Manifestation of Two Geometric Visual Illusions

This study further explored whether the length misperception induced by cross-shaped distractors, formed by two pairs of oppositely oriented Müller-Lyer wings, can be explained by the combined effects of the Müller-Lyer (MLI) and filled-space (FSI) geometric illusions of extent. During the psychophysical experiments, one pair of wings randomly altered luminance, while the other pair’s luminance stayed constant. Two distractor orientations were used: one with the constant-luminance wings on the right side of the cross, and the other with them on the left side. To independently evaluate the MLI under different luminance settings, in the first series of experiments two distracting crosses of identical orientation were attached to the lateral stimulus terminators. In the next four series, a single distracting cross with different orientations was attached to one lateral stimulus terminator, combining constant and background luminance in various ways. Applying particular algebraic operations (summation or subtraction) to the data from the experiments enabled us to distinguish the individual effects of two illusion components. These effects are consistent with an analytical interpretation derived from earlier quantitative models of the visual mechanisms underlying the MLI and FSI. A deeper theoretical examination of the model resulted in a proposed revised scheme for the potential merging of MLI and FSI mechanisms, linked to identical neural summation areas at the same stage of visual processing. It was demonstrated that the theoretical calculations accurately fit the experimental curves for all stimulus variations. This provides strong evidence that the joint effects of the MLI and FSI are among the primary factors influencing the characteristics of the perceptual phenomenon studied.