Fig. 1.

a Illustration of the visual motion stimulus used for the center-surround paradigm in the experiment and model. b Theoretical psychometric function of a subject for discriminating the direction of motion of the center target from the vertical (upward) reference direction. It represents the proportion of “rightward” answers as a function of the center motion direction, and helps to visualize the perceived vertical reference direction (midpoint) as well as the discrimination threshold for reliably (above p=0.84) seeing a deviation from the perceived reference. Theoretical models need to predict the perceived value of the stimulus. The discrimination threshold is the error on the perceived value