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