1. All the cells (158) that we studied in the lateral geniculate nuclei of Macaca nemestrina and Macaca irus could be distinguished as either X-like or Y-like on the basis of their responses to tests developed to classify cat retinal and lateral geniculate nucleus cells. These tests include responses to stationary spots, fast moving wands and moving gratings. 2. Response latencies to electrical stimulation of the optic chiasm were determined for 130 cells; no X-like cell showed a latency shorter than 1-7 ms, no Y-like cell showed a latency longer than 1-6 ms. Primate lateral geniculate nucleus cells with X-like properties thus receive their excitatory input from retinal cells with slowly conducting axons and these most probably include the tonic ganglion cells described by Gouras (1968, 1969); Y-like lateral geniculate nucleus cells are driven by retinal cells with faster conducting axons, most probably including the phasic ganglion cells described by Gouras. 3. Wiesel & Hubel (1966) classified monkey lateral geniculate nucleus cells into four main types based on their receptive-field properties, as revealed by spectrally and spatially distinct stimuli. We find that all Type I and Type II cells show X-like properties; all type IV cells show Y-like properties. Type III consists of a subtype that show X-like properties, here termed Type IIIx, and a subtype that show Y-like properties, here termed Type IIIy. 4. The first cells encountered as the micro-electrode reached the lateral geniculate nucleus were always X-like. In some penetrations only X-like cells were encountered as the electrode moved downward through the lateral geniculate nucleus. In the remaining penetrations, after recording X-like cells through most of the lateral geniculate nucleus, Y-like cells were then encountered. No X-like cells were found below Y-like cells. thus these two classes of cells are anatomically segregated within the primate lateral geniculate nucleus. Electrode marking showed the borger between X-like and Y-like cells to correspond to the border between the paro- and magnocellular layers of the lateral geniculate nucleus. Thus X-like cells (i.e. Types I, II and IIIx) occur in the parvocellular layers, Y-like cells (i.e. Types IIIy and IV)in the magnocellular layers.
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