Susceptibility to the Shepard illusion in participants with autism: reduced top-down influences within perception?

Previous research [Ropar & Mitchell, 2002] has shown that autistic individuals are somewhat immune to biases induced by top-down processes, particularly the influence of previous knowledge on perception. In order to test this hypothesis within perception, 18 participants with autism who had measured intelligence in the normal range were compared against 18 matched controls in their susceptibility to the Shepard illusion. The illusion consists in misperceiving the shape of a parallelogram in the presence of depth cues. It is attributed [Mitchell, Ropar, Ackroyd, & Rajendran, 2005] to the effect of top-down constraints within perception. The task involved adjusting a stimulus to the dimensions of a template on a computer screen. Both groups were susceptible to the illusion and the illusion effect was stronger when three-dimensional perspective cues were prominent. Notably, participants with autism were less susceptible to the illusion than typically developing individuals. The findings raise the possibility that in some instances top-down influences are attenuated in individuals with autism.