Retinal Bioengineering

The retina is a small piece of the central nervous system responsible for the first steps in vision, so understanding how it works has great importance for daily life. In addition, features of the retina make it attractive as a model neural system. The only input to the retina is light, which can be easily manipulated, and recordings have been made for many decades from individual output cells of the retina, the retinal ganglion cells (RGCs), allowing application of linear (and to some extent nonlinear) systems analysis methods that define the transfer functions of the retina. The small, non-spiking photoreceptors and retinal interneurons make recordings from these earlier stages difficult in mammals, but this has been partially surmounted by the ability to record massed activity of some types of retinal neurons, including photoreceptors and bipolar cells, in the electroretinogram (ERG) in humans as well as animals. ERG analyses have led to models of signal processing prior to the RGCs. Engineering methods in combination with physiology have thus elucidated the basic features of the retinal network that allow the convergence of signals from many millions of photoreceptors to yield the center-surround organization and response properties of the primary types of RGCs in cats and primates. However, some of the approximately 20 types of RGCs that send parallel signals to the brain are still poorly understood. Recent work has used isolated retinas and multielectrode arrays to record from many retinal ganglion cells simultaneously. Specific contributions of interneurons to the retinal circuits have also been addressed with new methods, some of which are reviewed here. Another aspect of retinal bioengineering concerns the retinal microenvironment. Diffusion models and spatially precise intraretinal measurements of oxygen and pH provide information about retinal metabolism that is useful in understanding dysfunction of the retina in some diseases.