Abstract
Even when physical luminance is constant, pupil size varies with the perceived luminance and size of the visual image, and also with how attention is directed (to bright or dark features). Here we asked whether pupil size is also driven by the perceived numerosity of an array of bright or dark visual elements (with luminance held constant). We recorded pupil size while 14 adult participants viewed clouds of white or black elements presented in central vision against a gray background. The total number of pixels in the elements (and hence luminance), as well as the area covered by them (convex hull) were kept constant. Either 18 or 24 dots were displayed in a given trial. Perceived numerosity was further manipulated by connecting dot pairs with lines to create 9 or 12 dumbbell-like shapes; in the isolated-dots condition the same lines were displayed in random positions. Participants simply observed the stimuli without performing any task. Pupil size was significantly modulated by both physical and perceived numerosity. The stimulus-evoked pupil constriction or dilation was smallest for 18 connected dots, intermediate for 18 unconnected and 24 connected dots (which have the same perceived numerosity) and largest for 24 unconnected dots. The results suggest that pupil diameter is spontaneously regulated by the numerosity of an array of bright or dark elements, so more items generate a stronger response, with luminance kept constant. In addition, perceived rather than physical numerosity drives these pupillary responses. There is much evidence to suggest that numerosity is a spontaneously encoded basic visual feature. Our results further show that even without an explicit task, perceived numerosity of an array of elements drives measurable automatic responses not subject to voluntary control: pupil size changes.