Transient and sustained components of the pupillary responses evoked by luminance and color.

That the pupil reacts to changes in luminance and color, as well as to spatial features in the retinal image raises questions about whether phasic and tonic and/or color and luminance visual pathways project to the pretectal pupillomotor neurons. The present study compares pupillary responses evoked by heterochromatic and achromatic luminance increments to investigate whether the pupillary responses evoked by color and by luminance are independent of one another. Principal component analysis is used to examine the constituents of the pupil responses. The results support the belief that the visual input to the pupillomotor system is organized into phasic and tonic (but not necessarily independent color and luminance) pathways.