Spatial contrast sensitivities of X and Y type neurones in the cat's dorsal lateral geniculate nucleus.

Abstract

The discharges of X and Y type neurons were recorded extracellularly from the binocular segment of the A laminae of the dorsal lateral geniculate nucleus of anesthetized, paralyzed cats. X type geniculate (XG) cells and Y type geniculate cells (YG) cells 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 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 .apprx. 2 false positives in 50. YG cells had higher contrast sensitivities at low spatial frequencies and XG at high. YG cells also had higher peak sensitivities. The optimum spatial frequency of XG cells was 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 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 had higher spatial resolution in lamina A than lamina A1. No difference was found between on- and off-center 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 center-type which overlaps a particular YG cell receptive field of the same center-type has a contrast sensitivity at optimum spatial frequency very close to that of the YG cell. XG cells could by themselves account for the contrast sensitivity of the cat''s visual system.