(A) Turi et al. used a visual stimulus in which two sets of dots (white and black) moved in opposite directions (thin arrows). (B) To an observer this stimulus appears as a 3D cylinder that rotates in a clockwise direction with the black dots at the front (top), or in a counter-clockwise direction with the white dots at the front (bottom). This perceived direction of rotation switches every few seconds. (C) Turi et al. found that pupil size increased when the black dots appeared to be at the front, and decreased when the white dots appeared to be at the front. The size of this change was correlated with Autism-Spectrum Quotient score.