Abstract
The Müller-Lyer Illusion (ML) is known to bias perception of size estimation. Previous studies have reported mixed findings regarding a possible reduction in susceptibility to the ML that is associated with autistic traits. The present study examined ML bias with three perceptual tasks involving neurotypical participants (n=30) assessed for autistic trait expression by the Autism-Quotient (AQ) and Systemizing-Quotient (SQ) questionnaires. Figures were presented in three different forced-choice conditions: concurrently (Task1), successively with a discrete response slide (Task2), or successively with responses made upon presentation of the second figure (Task3). Participants identified via keypress the figure with the longer central shaft. Questionnaire scores were found to not significantly correlate with any of the outcome measures of selection accuracy nor Response Time (RT). However, task procedure did affect illusion susceptibility where biased figure selections were significantly greater in Task1 relative to Task2 (M=9.33; p<0.001). A main effect for RT found significantly faster selections occurred in Task2 (M=314ms) relative to Task1 (M=579ms; p<0.001) and Task3 (M=478ms; p<0.001), which were significantly different from each other (p<0.001). Our findings show that illusory bias is greatest when viewing both figures concurrently. We argue this susceptibility is a result of superadditivity, where oppositely oriented ML figures produce a greater illusion when presented concurrently. Overall, the present results add to existing evidence that autistic trait expression, as evaluated by the AQ and SQ, does not affect susceptibility to the ML illusion in neurotypical individuals when assessed using forced-choice paradigms.
