Abstract
INTRODUCTION: The diameter of our eye-pupil is primarily set by luminance, but subtle modulations also occur under constant luminance reflecting cognitive or perceptual events, such as visual motion. It has been shown that neural circuits underlying motion detection differ depending on motion speed, with very fast motion specifically engaging area “prostriata” (Mikellidou et al., 2017). Here we asked whether pupil responses reflect this differentiation, as this could open new insights into the neural circuitry underlying non-luminance dependent pupil responses. METHODS: We measured pupil size changes in response to fast drifting motion in 11 participants. Stimuli were sinusoidal gratings of 50% contrast moving at moderate or very high speeds (38 or 570 °/s) with identical temporal frequency (10Hz), projected over a large field of view (~60°), as in Mikellidou et al., (2017). Motion directions and speed were randomly interleaved within sessions; each motion stimulus was presented for 5 s, followed by a 5 s blank interval, repeated 6 times. RESULTS: While both stimuli caused pupil constrictions, we found that the very high-speed stimulus elicited stronger responses than the moderate speed one. We verified that differences in the eye-movement pattern could not account for the pupil response difference. Measuring fMRI responses to the same stimuli (in a previous study) we found that the only visual areas showing a preference for the very high-speed motion are the recently characterised area prostriata and the extreme periphery of the adjacent V1. These areas also shared with pupil responses a preference for leftward moving over rightward moving stimuli. DISCUSSION: While the origin of these preferences remains to be clarified, they suggest a novel association between pupil control and cortical visual processing, possibly reflecting a phylogenetically ancient circuit for immediate detection of fast-moving threats.