Luminance detectors in the olivary pretectal nucleus and their relationship to the pupillary light reflex in the rat. II. Studies using sinusoidal light.

The luminance detectors in the olivary pretectal nucleus, which are likely candidates mediating the pupillary light reflex, responded to all frequencies of sinusoidally modulated light up to 12-25 Hz. At low frequencies (0.05-4.0 Hz) the luminance detectors responded with modulated firing to different stimulation rates. The modulation depth of the cell response increased with the increase in stimulation frequency up to 20 Hz, then rapidly fell. There was a delay between the peak intensity of the stimulus and the peak firing of cell response of about 30-40 ms. The amplitudes of the consensual pupil responses to the same sinusoidal stimulus, on the other hand, decreased with an increase in frequency and no discernible response was recorded above 2 Hz. The pupil responses were little affected by sympathectomy. The differences in the frequency response characteristics of luminance detectors and the pupil were attributed to the sluggish dynamic properties of the pupil muscles. This was demonstrated using an electronic model of the iris muscle which modified the responses of the luminance detectors giving output waveforms broadly resembling pupil responses to square and sinusoidally modulated lights.