Susceptibility to Optical Illusions in Autism Spectrum Disorder Depends on Illusion Characteristics

Background: Chouinard et al (JADD, 2016) examined susceptibility to 13 optical illusions as a function of AQ traits in the general population. They demonstrated how one group of illusions with strong within-object relational properties (consisting of the Shepard’s tabletops and square-diamond illusions) was associated with reduced susceptibility as a function of AQ whilst a different group of illusions with strong between-object relational properties (consisting of the Ebbinghaus and Delboeuf illusions) was not. From these results, the authors speculated that susceptibility to the former but not the latter group of illusions might also be reduced in ASD.

Objectives: Examine whether participants with a clinical diagnosis of ASD show reduced susceptibility to illusions with within-object relational properties, while simultaneously showing typical levels of susceptibility on illusions with between-object relational properties.

Methods:  The participants included 15 children with ASD (11 males, mean age = 12.17 yrs, age range = 7.92 to 15.5 yrs) and 15 age and Performance IQ matched typically developing children (11 males, mean age = 12.21 yrs, age range = 8.45 to 14.7 yrs). The participants completed four trials of each illusion, in pseudorandom order (Shepard’s tabletops, Square-Diamond, Ebbinghaus, Delboeuf 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 to see greater 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 = .21, SD = .05), t (28) = 2.41, p = .023. There were no differences between groups on the other illusions. 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).

Conclusions: We conclude that reduced illusory susceptibility in ASD is confined to certain groups of illusions, particularly those with strong within-object relational properties. We did not find any evidence to suggest that these differences could be driven by eye scan patterns.