From waveguides to directional antennas – understanding the optics of retinal photoreceptors

The Stiles-Crawford effect (i.e., the psychophysical Stiles-Crawford effect of the first kind) has remained somewhat of an enigma in vision science since its discovery 90 years ago. It is often treated as a curiosity, or even a minor effect, but the reality is that at a fundamental level it is so much more. It represents the last optical process in the eye where photon absorption triggers the visual pigments in the cones and rods resulting in neural signals and, ultimately, a visual sensation via the visual cortex. Naturally, most ophthalmic studies focus on the anterior eye, the cornea and lens, that determines how light is focused onto the posterior eye and retina. The latter is typically considered as a screen onto which an image of the outside world is projected. Nonetheless, we can only hope to comprehend vision and the optics of the eye once the detectors, i.e., the photoreceptors and their visual pigments, are included. This has been central to much of Prof Jay M Enoch’s work. In my own research, I was initially intrigued by a study on the relationship between the psychophysical Stiles-Crawford effect and the directionality in backscattered light in the pupil plane often referred to as the optical Stiles-Crawford effect. This led me to revisit waveguide models of photoreceptors in what I initially thought would be just a single study. Yet, as a pebble in the shoe, it has kept returning for nearly two decades posing additional questions. Now, we may be at the end of the road in terms of understanding the Stiles-Crawford effect of the first kind, but we are still only in the infancy of comprehending how this vital last optical step in our visual system adapts to changes in relation to age and to disease. Only when comprehending its adaptability and function, can we hope to fully separate optical effects from neural factors to obtain an entirely satisfactory and accurate model of vision.