The Shepard Illusion Is Reduced in Children With an Autism Spectrum Disorder Because of Perceptual Rather Than Attentional Mechanisms

Abstract

Earlier studies demonstrate reduced illusion strength in the Shepard illusion in adults and adolescents with an autism spectrum disorder (ASD) and in typically developing (TD) adults with high levels of autistic traits. We measured the strength of the Shepard illusion in ASD and TD children and tested if ten different eye-tracking measurements could predict group differences in illusion strength. The ASD children demonstrated reduced illusion strength relative to the TD group. Despite this, there were no mean differences on any of the eye-tracking measurements between groups. Even though none of the eye-tracking measurements revealed mean differences between the two groups, the degree to which spatial attention was directed toward the standard stimulus, as indexed by the number of saccades within and toward this stimulus, predicted the strength of the illusion in the overall sample. Furthermore, this active scanning of the standard stimulus was found to enhance illusion strength more strongly in the ASD than the TD group. Together, we conclude that scan patterns and the degree to which participants are able to shift between different locations in a visual scene did not account for group differences in illusion strength. Thus, the reduced strength of the Shepard illusion in ASD does not appear to be driven by how attention shifts or is spatially allocated. Rather, differences may relate instead to perceptual mechanisms that integrate visual information. Strategies that may aid ASD individuals to see this illusion more strongly could have them make even more eye movements within and between the stimuli presented in the illusion display.

Keywords: ASD; Shepard illusion; autism spectrum disorder; eye-tracking; visual perception.

FIGURE 1

Illusion and control matching tasks. The figure displays the illusion and control matching tasks that were examined. The illusion task consisted of the Shepard illusion (A). The right side of the top panel (A) demonstrates the full visual display with the buttons at the bottom that the participants used to adjust the comparison stimulus (in this case, the stimulus on the right) to match the standard (in this case, the stimulus on the left). The different tasks (A–E) had similar buttons and differed by the stimuli presented in the black area and the dimensions that had to be matched. The control tasks consisted of size (B), shape (C), alignment (D), and luminance (E) matching tasks.

FIGURE 2

Illusion strength and correlations with saccadic eye movements to the standard stimulus. The top panel (A) displays each individual’s susceptibility index score (green circles, ASD group; blue circles, TD group) for the Shepard illusion. The lines represent the means for each group. As shown in the graph, we found a significant reduction in the ASD relative to the TD group. The other plots demonstrate the strength of the Shepard illusion as a function of the percentage of saccades that were classified as starting from one location to a different location within the standard stimulus (B) and classified as starting from one location on the comparison stimulus to a location on the standard stimulus (C). In both instances, the overall correlations (in black) were significant after correcting for multiple comparisons using the Bonferroni method (p < 0.05). Additional linear regression analyses revealed that these correlations were primarily driven by the ASD group. Specifically, in both cases, the slopes between the two groups were different from each other (p < 0.05), positively correlated in the ASD group (p < 0.05; in green), and not correlated at all in the TD group (p ≥ 0.378; in blue).

FIGURE 3

Proportion of saccade types made and time spent looking at AOIs. Participants tended to make more saccades within than between the stimuli as denoted by the asterisks (^∗) in panel A. Looking times were longer for the comparison compared to the standard stimulus, as shown in panel B. Asterisks (^∗) denote significant differences after correcting for multiple comparisons using the Bonferroni method (p < 0.05). S-to-C refers to standard-to-comparison saccades and C-to-S refers to comparison-to-standard saccades.