Pupillary escape intensified by large pupillary size.

Pupil responses to light are greatly influenced by initial pupil size. Small pupils, operating under photopic conditions, show tonic responses to step increases of light and high gains; thus the pupil is a good regulator of light. Large pupils, operating under mesopic or scotopic conditions show phasic responses, "pupillary escape", and smaller gains; the pupil only transiently influences retinal flux. By using accommodation level to set the size of the pupil, the mechanism of the "pupil size effect" is shown to be dependent on retinal light level only so far as retinal activity sets pupil size.