The discharges of X and Y type neurones were recorded extracellularly from the binocular segment of the A laminae of the dorsal lateral geniculate nucleus of anaesthetized, paralysed cats. X type geniculate cells are referred to as XG cells and Y type geniculate cells as YG cells. They were differentiated on the basis of a test of linear spatial summation and the relatively higher spatial resolution of the XG type. Contrast sensitivities of these cells were measured for a series of spatial frequencies at 5.2 Hz. Sensitivity measurements took account of the variability inherent in the cells’ maintained discharges. Maintained discharges of XG and YG cells were found to be similarly noisy and the level of noise was stable in the time range of seconds to hours. The noise level was greater than in corresponding ganglion cells, suggesting that an extra source of noise is added at the geniculate level. The criterion set routinely to measure ‘threshold’ contrast corresponded to a level of reliability of about two false positives in fifty. YG cells had higher contrast sensitivities at low spatial frequencies and XG at high. YG cells were found also to have higher peak sensitivities. The optimum spatial frequency of XG cells was found to be higher than that of YG cells. YG cells also show less attenuation in contrast sensitivity for gratings of spatial frequencies below their optima. Contrast sensitivities of both XG and YG cells were found to be lower than those of corresponding ganglion cells. The optimum spatial frequencies and spatial resolutions of XG and YG cells decreased as the retinal eccentricities of their receptive fields increased. XG cells were found to have higher spatial resolution in lamina A than lamina A1. No difference was found between on‐ and off‐centre types of either cell class. Although individual YG cells are more sensitive to low spatial frequencies than individual XG cells, the ensemble of XG cells of one centre‐type which overlaps a particular YG cell receptive field of the same centre‐type has a contrast sensitivity at optimum spatial frequency very close to that of the YG cell. This leads one to believe that XG cells could by themselves account for the contrast sensitivity of the cat's visual system.
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