COLOR DYNAMICS OF THE PUPIL

In 1960, we observed what appeared to be paradoxical contractions of the pupil in response to flashes of darkness. This led to a description of the essential dynamic asymmetry of the pupil reflex and a delineation of a prominent, light intensitysensitive component of pupillary behavior which we called the rate-sensitive contraction response.'-3 This response is contrasted with the less prominent, slower dilation in response to decreasing light. In our subsequent investigation of the effects of changes of color, rather than light intensity alone, upon pupillary behavior, it was to be expected that such a rate-sensitive pupillary contraction would occur for each change in color, since an increase in stimulation occurs a t different color receptors of the retina for these changes. Such increases in stimulation would not be canceled by a decrease of stimulus in other portions of the retina, in accordance with the principle of unidirectional rate sensitivity. These predictions are confirmed by experiments reported here. It was also predicted that pupillary oscillations could be obtained solely by changing color in response to a change in pupil diameter, without a change in intensity. Such oscillations were, in fact, readily obtained and are reported here. During the course of these experiments, however, a paradoxical pupillary contraction was newly observed in response to a step removal of a color component of the light stimulus. We call it "paradoxical contraction" because a reduction of the light intensity within the same color results in pupillary dilation. This new paradoxical contraction has the general characteristics of a rate-sensitive pupillary contraction, although of smaller amplitude than the contraction of the pupil produced by a flash of light. The phenomenon may be explained by a theory of mutual inhibition of colors. Removal of a color component removes its inhibiting effect on the remaining colors and, in effect, enhances them. The increase constitutes the stimulus for the rate-sensitive contraction. Results of experiments showing the relationship between inhibition and excitation are described in this paper. The newly observed pardoxical contraction may also be of value in observing some gross quantitative relations between inhibition and excitation in the nervous system.