Comment on inferred positive phototropic activity in human photoreceptors

A paper entitled ‘Inferred positive phototropic activity in human photoreceptors’ was presented by Enoch & Birch (1981). That paper contained two different experiments. In one experiment, the observer was patched and showed a markedly flattened Stiles-Crawford (S .-C .) function of the first type. (The S.-C . function is a psychophysical measure of the directional sensitivity of the retina which normally shows a peak of sensitivity for light passing through a point near the centre of the entrance pupil of the eye.) In another experiment a unilateral displaced pupil aperture contact lens was worn over the dilated pupil by an observer. The S.-C. peak shifted into the aperture. On the basis of these two separate experiments Enoch & Birch suggested that there was a phototropic effect present in retinal receptors. Here, the authors call attention to the fact that the interpretation of the first of the two experiments, the patching study, was apparently in error. However, the overall conclusion that a phototropic effect is present in human retinal photoreceptors is not altered.