The filled-space illusion and visual-field polar anisotropy

Perceptual illusions provide important clues about visual processing mechanisms. The filled-space illusion demonstrates how contextual information within a visual stimulus can distort perceived length, whereas polar anisotropy reflects systematic differences in perceptual performance across the visual field. This study investigated the relationship between the filled-space illusion and visual-field anisotropy by quantifying their combined effects on perceived length. In psychophysical experiments, stimuli consisted of two spatial intervals. The tilt angle of the test interval was fixed at one of the five orientations relative to the horizon, while the orientation of the reference interval varied from 0° to 360° relative to the test interval. In the first block of experimental trials, both intervals were empty, allowing us to assess visual-field polar anisotropy. In the second and third blocks, radially or tangentially oriented line segments were attached to the terminator of the reference interval to examine the combined effects of visual-field anisotropy and the filled-space illusion. The experimental data were analyzed using computational procedures based on a previously developed model of the hypothetical visual mechanism underlying the filled-space illusion. The results showed that the filled-space illusion component increased as the stimulus expanded and reached its maximum at the straight internal angle. The model calculations were consistent with the experimental data, suggesting that polar anisotropy may be related to vertical scale stretching and that its contribution may be distinguishable from the filled-space illusion component, although alternative explanations cannot be excluded.