Abstract
The pupillary light reflex (PLR) is controlled by rods, cones (L, M and S) and intrinsically-photosensitive ganglion cells (melanopsin). The mechanics of pupil action result in minimal sensitivity to sinusoidal flicker above 5 Hz, limiting the usefulness of pupillometry for characterizing the temporal response properties of the visual system. Because of non-linearities in neural processing, however, we were able to use the PLR to amplitude-modulated flicker to measure the temporal transfer function of early photopigment-selective filters. Two observers viewed large-field (27.5°, central 5° blocked, 1125 cd/m2 mean light level) amplitude-modulated flicker with a pharmacologically dilated left eye while the consensual PLR was measured in the right eye. Using a spectral light synthesizer, flicker stimuli were generated to selectively modulate specific combinations of photopigments (L+M, S or melanopsin). We used amplitude-modulated stimuli at four carrier frequencies (5, 10, 20 and 40 Hz) and one envelope frequency (0.5 Hz). The carrier modulation contrast was 45% for all modulation directions; the envelope contrast was 100%. For each modulation direction and both subjects, we found a robust pupil response for 5-20 Hz carriers to a distortion product at the envelope frequency and at the envelope's first harmonic. This indicates that the 5 Hz cutoff in the conventional PLR occurs after a non-linearity. For both the S cones and melanopsin, the early filter revealed by the envelope-frequency response is low-pass and drops steadily to zero between 5 and 40 Hz. The similarity between S and melanopsin responses suggests that signals from these two photoreceptors share a common filter. The pattern for L+M was distinct, with a striking dip in response at 10 Hz—perceptually associated with a transition of stimulus appearance from chromatic to achromatic—and a robust response at 40 Hz.
