The retina is a tiny piece of neural tissue, weighing less than 100 mg in humans. The retina’s importance is out of proportion to its size for two reasons. First, the retina has long served as a model for understanding complex parts of the nervous system, and it has attracted a great deal of attention from neuroscientists of all types, including bioengineers. In fact, the quantitative and systems approaches of engineering have been central to the understanding of retinal function for more than four decades. Many of the retina’s properties hold up well in vitro, and it is accessible to microelectrodes both in vivo and in vitro. It has a modest number of principal cell types, and the total number of output neurons (ganglion cells) in each eye is about 1 million in humans, and much less in nonprimate mammalian species, numbers that are almost manageable by comparison with the outputs of other parts of the central nervous system. The retina can be studied while it responds to its natural input, patterns of light, which can be controlled easily. For deeper neural structures, one often has to make the choice between studying responses to electrical stimulation, which is unnatural, or responses to natural inputs from other locations in the nervous system that may be difficult to control or completely characterize. The retina is also simpler than many areas of the brain because there is almost no feedback from the brain to the retina. In short, no other region of comparable complexity provides the advantages for study that the retina does.
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