Dynamic pupillary response controlled by the pupil size effect.

Pupillary escape has been described as an initial contraction followed by a slow redilatation, occurring in response to a step stimulus of low-intensity light. When the initial pupil size is small, the response to the same step stimulus is pupillary capture, a steady and sustained contraction. In this experiment a comparison was made between three modes of controlling pupil size and thereby of regulating the pupillary response: contralateral light background level, ipsilateral light background level, and accommodative level with which there is no change in retinal adaptation. All three level setting modes showed similar results in illustrating the pupil size effect. In addition, an inhibitory effect was found with both ipsilateral and contralateral light backgrounds that is independent of Weber's Law in the contralateral case. Our results lead to the formulation of a binocular model, featuring an internal parameter control whereby a signal dependent on the static pupil size regulates the gains of the parallel phasic and tonic pathways, the former responsive to transient changes of light, and the latter to background levels of light and accommodative levels. Our findings also raise interesting questions concerning the loci of these complex interactions in the simple neuroanatomy of the pupillary pathways.