Spatial pattern as a stimulus to the pupillary system.

Pupillary responses to the contrast reversal of a checkerboard pattern of variable check size and contrast were measured. The results show that the pupillary system is responsive to the contrast reversal. It is argued that this response is the light reflex caused by the local luminance change. The summation characteristics of the pupillary afferent are considered. It has also been found that the pupil size, without any influence from contrast reversal, systematically changes with the changes in the check size. Whether this pupillary change is caused by the variation in accommodative performance with changing spatial factors of visual stimulus is discussed.