Retinal ganglion cells, the integrative-output neurons of the retina, can be sorted into functional classes. In the cat, two ganglion cell classes are labelled X and Y. These are distinguished by the different retinal subnetworks that provide their input. X cells are driven by a single linear receptive field center mechanism. Y cells receive center and surround signals and additional signals from nonlinear subunits in their receptive fields. Both X and Y cells are highly sensitive to contrast. X cells project almost exclusively to the A or A1 layers of the lateral geniculate nucleus (LGN). Y cell axons terminate in the A or A1 layers and also the more ventral C layers, and also the superior colliculus. In the monkey, P cells connect the retina to the parvocellular layers of the LGN, have small receptive fields, are wavelength-selective, and are insensitive to contrast. M cells are ganglion cells that send axons to the magnocellular layers of the LGN, are not wavelength-selective, have somewhat larger receptive fields than P cells, and are very sensitive to contrast. Comparisons between cat and monkey ganglion cell classes reveal several important similarities between M cells and X cells.
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