Illusion Strength and Associated Eye Movements in Children with Autism Spectrum Disorder While Viewing Shepard and Ebbinghaus Illusion Displays

Background: Previous research examining visual illusion susceptibility in ASD has found inconsistent results. Chouinard et al (JADD, 2016) demonstrated that illusions with strong within-object relational properties (e.g. the Shepard’s tabletops) were associated with reduced susceptibility as a function of AQ, whilst illusions with strong between-object relational properties (e.g. Ebbinghaus) were not. Chouinard et al. (Front Hum Neuro, 2017) also demonstrated that eye-movements moderate illusion strength. From these results, the authors speculated that individuals with ASD may exhibit reduced susceptibility to specific types of illusions, and that differential scan patterns may underlie these differences.

Objectives: To investigate whether participants with ASD show reduced susceptibility to the Shepard illusion, while simultaneously showing typical levels of susceptibility on the Ebbinghaus illusion, and whether individual differences in scan patterns relate to illusion strength.

Methods: The participants included 18 individuals with ASD (12 males, mean age = 11.4 yrs, age range = 6.5 to 15.5 yrs) and 18 typically developing participants (12 males, mean age = 11.4 yrs, age range = 6.0 to 14.7 yrs), matched on age and Raven’s Progressive Matrices raw scores. The participants completed four trials of each illusion, in pseudorandom order (Shepard’s tabletops and Ebbinghaus illusions). Presentation was computerized, with participants adjusting one stimulus to match another. Eye-tracking was used to measure scan patterns while participants completed the task. To allow meaningful comparisons between illusions, we computed normalised indices of susceptibility for each illusion as: ((Perceived Size of Stimulus B - Perceived Size of Stimulus A) / (Perceived Size in Stimulus A + Perceived Size of Stimulus B)); B denoting the stimulus one would expect participants to make larger judgements in perceived size. Participants also completed control tasks to measure basic abilities in visual acuity and discrimination.

Results: The children with ASD (M = .14, SD = .10) were less susceptible to the Shepard’s tabletops illusion than the typically developing children (M = .20, SD = .05), t (28) = 2.41, p = .043. There were no differences between groups on the Ebbinghaus. There were no differences between groups on any eye-tracking measures (saccade time, saccade distance, average saccade velocity, average saccade count, average pupil size, average block duration, time spent fixating). A medium strength correlation (r = .62, p = .024) was found between time spent fixating on the standard and illusory susceptibility to the Shepard, but only among participants with ASD.

Conclusions: We conclude that reduced illusory susceptibility in ASD is confined to certain types of illusions, particularly those with strong within-object relational properties. We suggest, based on the absence of evidence for differences in eye-tracking measures, that differences in illusion strength in ASD depend on perceptual mechanisms required to experience visual illusions.