Gabel, V.P. (ed): Artificial Vision: A Clinical Guide

Excerpt

If a person loses their vision, can it be restored? Until recently, the answer was a definitive “no” and such an option was considered strictly in the realm of mythical, extraordinary events, like some of the ones described in religious texts, or, alternatively, in the realm of science fiction. This is not surprising at all. First, what needs to be considered is the staggering biological complexity of the human retina. Without going into exhaustive detail, it should be mentioned that there are at least 100 million primary photoreceptor cells represented by the rods and the cones, ~ 5000 secondary photoreceptors (intrinsically photosensitive retinal ganglion cells), a million retinal ganglion cells, and an uncertain number (but with estimates in the millions) of bipolar cells, amacrine cells, and Muller cells. However, it’s not just the numbers of cell types in the human retina that poses a challenge. There is an enormous diversity in the appearance (morphology) and functional specialization of the cells. For example, at least 18 different morphological types of retinal ganglion cells have been identified in the human retina and the function of only five types has been well-understood and described. Therefore, it is clear that at the present time, formidable challenges remain in our understanding of retinal function and these challenges are even greater in understanding the morphological and functional details of higher visual centers and pathways given the degree of their accessibility and difficulty in functional probing. …