Shaping the pupil's response to light in the hooded rat

The contribution of iris muscle steady state and dynamic response characteristics to the shaping of the pupil response to light in the hooded rat were studied using electrical stimulation of the parasympathetic fibers in the III nerve. The waveforms of pupillary contractions to single or brief trains of electrical impulses applied to the III nerve were virtually identical to those elicited with short duration light flashes. Individual contractions could be resolved at stimulation rates of 2 Hz and below, and the size of the contractions increased with the decrease in frequency. The pupil responded to long trains of stimuli above 2 Hz with smooth tonic contractions. Steady state contraction amplitude was linearly related to log stimulation frequency. The mean time constant of pupil constriction to stimulus trains was 1.41 s (SD +/- 0.71 s) and the shortest mean latency was 292 ms (SD +/- 30 ms). The fastest mean latency of pupil constriction to the brightest light flash used was 295 ms. In contrast, the time constant of pupillary dilation was 7 s (SD +/- 1.4 s) and the shortest latency was 485 ms (SD +/- 74 ms). Therefore, the sluggish dynamic properties of the iris musculature are responsible for the asymmetries in pupil contraction, dilation, and latencies as well as low flicker fusion frequency and constriction amplitude characteristics of pupil responses to light.