Pupil diameter tracks statistical structure in the environment

Pupil diameter determines how much light hits the retina, and thus, how much information is available for visual processing. This is regulated by a brainstem reflex pathway. Here, we investigate whether this pathway is under the control of internal models about the environment. If so, this would allow adjusting pupil dynamics to environmental statistics, and hence optimize information transmission. We manipulate environmental temporal statistics by presenting sequences of images that contain internal temporal structure to humans and macaque monkeys. We then measure whether the pupil tracks this structure not only at the rate that immediately arises from variations in luminance, but also at the rate of higher order statistics that are not available from luminance information alone. We find entrainment to environmental statistics in both species during the image sequences. Furthermore, pupil entrainment predicts later performance in an offline task that taps into the same internal models. Thus, the dynamics of the pupil are under control of internal models which adaptively match pupil diameter to the temporal structure of the environment, in line with an active sensing account.