Live Cells as Optical Fibers in the Vertebrate Retina

The vertebrate eye is equipped with an inverted-type retina; that means, light must pass through all proximal retinal layers before it arrives at the photoreceptor cells which are aligned at the back of the tissue. Though it is often stated that the transparency of the intact vertebrate retina is ‘almost total’ (Enoch and Glisman, 1966), it contains numerous structures which differ in size and refractive index. These differences should lead to significant scattering. Accordingly, it has been pointed out that the situation in the inverted retina ‘is equivalent to placing a thin diffusing screen directly over the film in your camera’ (Goldsmith, 1990). In fact, many current digital cameras posses such a "diffusing screen", in order to prevent artifacts due to the discrete and periodic sampling of the image: the anti-aliasing filter. As the layout of the retina exhibits a similar sampling strategy, one might hypothesize that the cell layers in front of the photoreceptors act as an anti-aliasing filter. However, the fact that aliasing artifacts can be observed in the vertebrate eye (Coletta et al. 1990) argues against such a hypothesis. On the contrary, these reports confirm the high resolution provided by the vertebrate retina, close to its physical limits. We will discuss here whether this apparent discrepancy is resolved by the presence of cellular light guides within the retinal tissue.