Phasic Pupil Dilation Response to Noxious Stimulation

Abstract The pupil dilation response (PDR) to a painful stimulus is a subtle, event-related increase in pupil diameter that occurs following brief noxious stimulation. It varies in amplitude with increasing stimulus intensity. Two competing hypotheses for the PDR are that (1) it reflects event-related cognitive efforts largely independent of pain, such as the task demand of discriminating intensity and readying a subjective report; and (2) like the pupillary light reflex, it is a simple reflexive response. In this study 21 subjects (11 male, 10 female) experienced painful stimuli matched in subjective intensity at two sites: finger tip and ear lobe, but they produced no subjective reports. If the PDR is a product of discriminating stimulus intensity levels and forming a report, then it should be minimal or nonexistent when subjects have no choices and make no overt judgments. On the other hand, if it is primarily a simple sympathetic reflex, then onset and peak latency should be longer for finger tip than for ear lobe stimulation because the conduction distance is longer. In addition to addressing these issues, we explored sex differences and the influence of age. Clear and similar PDRs appeared in response to stimulation of both sites despite an absence of subjective report requirements. PDR onset, peak amplitude and peak latency did not differ significantly across stimulation sites. Therefore, the PDR is neither an artifact of subjective report nor an uncomplicated sympathetic reflex. It appears to be an emotional arousal response, such as the defense response, the strength of which depends upon the magnitude of the noxious sensory input. Sex differences emerged, with women demonstrating a borderline larger PDR peak amplitude than men (P = .059) and a more rapid half recovery latency (P = .031). Age did not affect the PDR except at onset, with older subjects demonstrating a later onset latency (P = .003). The PDR may reflect preconscious processing associated with attention and perhaps other cognitive aspects of perception.