Luminance and darkness detectors in the olivary and posterior pretectal nuclei and their relationship to the pupillary light reflex in the rat. I. Studies with steady luminance levels.

In order to identify the pretectal nucleus which contains pupillomotor cells in the rat, cells were sought which were sensitive to changes in luminance level at the eye. Two types were found: Luminance detectors which showed a graded increase in firing with increase in luminance, and darkness detectors which showed a graded increase in firing rate with graded dimming of luminance intensity. All luminance detectors were located in the olivary pretectal nucleus, whereas darkness detectors were located in the posterior pretectal nucleus. Consensual pupil responses were recorded in conscious normal and sympathectomised rats using an infra-red sensitive T.V. pupillometer. Pupil diameter varied 2mm in an approximately linear fashion over six log units range in luminance intensity. Sympathectomy produced a general constriction of the pupil, but the overall response to light was unaffected. The changes in pupil size occurred over the same range of luminance that the firing rates of both luminance and darkness detectors changed. The olivary pretectal nucleus may therefore be involved in pupilloconstruction in the light, and the posterior pretectal nucleus, with pupillodilation in the dark.