Figure 1.1.

Overview of the visual system as related to visual prostheses. In most retinal dystrophies, the first order photoreceptor neurons (rods and cones) are lost. Thus, second order neurons (bipolar cells) are the earliest viable target, typically for subretinal and suprachoroidal devices. Epiretinal devices typically target retinal ganglion cells (RGCs), the third order neurons that form the output of the retina. Likewise, optic nerve devices target these neurons either within the eye at the nerve head or outside of the eye. The fourth order neurons, relay cells of the thalamic lateral geniculate nucleus (LGN) are targeted with penetrating electrode arrays. Finally, cortical implants target the fifth and higher order neurons found in the primary visual cortices. (Modified from Krey, H.F., & Brauer, H. (1998). Chibret Augenatlas: Eine Repetition für Ärtze mit Zeigetafeln für Patienten. Munich: Chibret Med Serv.)