There is significant evidence for higher-level cortical control of pupillary responses to visual stimuli, suggesting that factors other than luminance changes may induce a pupillary response. In the present study, the pupillary responses to equiluminant flickering stimuli in a range of 3-13 Hz were examined. Flicker stimuli included color-black (luminance-modulated) and color-color (hue-modulated) flicker. Equiluminance was determined both by objective luminance measures as well as by subjective, perceptual equiluminance for each subject. For both objectively and subjectively equiluminant flicker, significant, sustained pupillary constrictions were recorded. The magnitude of these responses was sensitive to both color and frequency parameters; red-blue color-paired flicker consistently produced the strongest constrictions. These responses occurred even when the flicker was of a lower luminance, both physically and perceptually, than a preceding nonflickering color, indicating that chromatic rather than luminance-sensitive mechanisms are involved in this response. Interestingly, the color- and frequency-sensitivity of constriction parallels those of flickers which maximally stimulate photosensitive epileptic patients, raising the possibility that chromatic response may be a factor in photosensitivity.
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