PUPIL RESPONSE TRIGGERED BY THE ONSET OF COHERENT MOTION

Background: Recent studies have shown that transient constrictions of the pupil can be elicited by visual stimuli that do not cause an incre ment in light flux level on the retina. Such stimuli include achromati c gratings and isoluminant chromatic patterns. Method: We investigated pupillary responses to the onset of coherent movement generated in a pattern of dots in random motion. Measurements were carried out in nor mal observers and in a subject with hemianopia caused by damaged prima ry visual cortex. Results: The experimental findings show that the ons et of coherent motion triggers systematic constrictions of the pupil t hat cannot be accounted for in terms of a pupil light reflex response. We labelled these constrictions Pupil motion responses (PMRs). Result s show that PMRs have large response latencies and on average are of s mall response amplitudes. The dependence of PMRs on changes in motion parameters such as stimulus speed and direction of motion has been inv estigated. Conclusions: The existence of PMRs to the onset of the cohe rent motion in human vision has been demonstrated. These new findings are discussed in relation to the psychophysical and physiological data on motion perception and the possible pathways involved in the contro l of the pupil response.