Pupillary response to chromatic flicker

There is significant evidence for higher-level cortical control of pupillar y responses to visual stimuli, suggesting that factors other than luminance changes may induce a pupillary response. In the present study, the pupilla ry responses to equiluminant flickering stimuli in a range of 3-13 Hz were examined. Flicker stimuli included color-black (luminance-modulated) and co lor-color (hue-modulated) flicker. Equiluminance was determined both by obj ective luminance measures as well as by subjective, perceptual equiluminanc e for each subject. For both objectively and subjectively equiluminant flic ker, significant, sustained pupillary constrictions were recorded. The magn itude of these responses was sensitive to both color and frequency paramete rs; red-blue color-paired flicker consistently produced the strongest const rictions. These responses occurred even when the nicker was of a lower lumi nance, both physically and perceptually, than a preceding nonflickering col or, indicating that chromatic rather than luminance-sensitive mechanisms ar e involved in this response. Interestingly, the color- and frequency-sensit ivity of constriction parallels those of flickers which maximally stimulate photosensitive epileptic patients, raising the possibility that chromatic response may be a factor in photosensitivity.