Inferred positive phototropic activity in human photoreceptors

The Stiles-Crawford (S.-C.) function, a measure of the directional sensitivity of the retina, was used to infer the alignment characteristics of the sampled retinal elements. One assumes that the peak of the photopic S.-C. function reflects the central alignment tendency of retinal elements sampled, and that the shape of the function reflects, among other factors, distributive qualities. Here two tests were performed to determine whether the function sampled reflected positive phototropic activity. The natural eye pupil was dilated and artificial pupils were substituted having specified eccentricity from the centre of the natural pupil. This was achieved with a displaced iris contact lens. After a series of complex experiments, it was finally shown that the peaks of the S.-C. function shifted towards the displaced aperture of the contact lens. As a second test, individuals were occluded uniocularly with a black patch for periods of time up to 10 days. This caused remarkable flattening of the measured S.-C. function. That flattening occurred in determinations of both photopic and scotopic S.-C. functions. Comparable effects were not seen in the second eye or if a diffuser was substituted for the black patch. Change and recovery in both experiments occurred within 3-5 days. On the basis of these experiments it is inferred that there is an active mechanism behaving in a positive phototropic manner present in the human retina.