Vision is a complex, computationally demanding, perceptual task that begins at the retina, a layered sheet of tissue, 200–300 μm thick, that lines the posterior part of the eye. The cornea and lens of the eye focus light rays onto the retina, leading to the activation of photoreceptors located in the back of the retina. The photoreceptors are one of the five major classes of neurons in the retina, the other four being bipolar cells, horizontal cells, amacrine cells, and ganglion cells, the output neurons of the retina. Within each class of neuron, there are a number of different types based on morphological and/or functional criteria, and current estimates are that in all there are more than 60 different cell types in the mammalian retina (Masland 2012).
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