Investigation of summation mechanisms in the pupillomotor system

To ascertain whether the pupillary response amplitude shows spatial summation of responses with increasing size of retinal stimulation, and to examine the pupillary responses for evidence of surround inhibition, analogous to that found in the receptive fields of the retinal ganglion cells.

By means of infrared-video-pupillography, the pupil reaction to stimuli of increasing size (1-15°) was measured in 30 normal subjects. Four different retinal locations (0°, 20° and 40° eccentricity on the upper temporal retina and 20° eccentricity on the lower nasal retina) were examined at four different stimulus luminances (17, 47, 87 and 140 cd/m(2)).

When the average log amplitude of the pupil light reaction from the 30 subjects is plotted as a function of the log area of the stimulus, a bi-linear response is observed, which is most pronounced for the two higher luminances. The intersection points of the two linear responses are 2.01° in the fovea, 2.80° at 20° upper temporal retina, 2.85° at 20° lower nasal retina and 4.86° at 40° upper temporal retina.

This study suggests that pupillomotor summation areas consist of both summation and inhibitory zones. They show larger diameters than receptive fields of retinal ganglion cells and do not appear to reflect pupillary summation areas of the pretectal olivary nucleus luminance neurons.