An electrophysiological assessment of X and Y cells as pattern and flicker detectors in the dorsal lateral geniculate nucleus of the cat.

We tested the hypothesis that geniculate X cells are the neural substrate of psychophysically identified pattern channels and that geniculate Y cells are the neural substrate of psychophysically identified flicker channels. The hypothesis was tested by measuring the relative sensitivity of isolated X and Y cells in the dorsal lateral geniculate nucleus of the cat to counterphase and on-off grating presentations. The fundamental and second harmonic responses of X and Y cells to sinusoidal counterphase and on-off temporal modulation were measured at a number of spatial frequencies using two contrasts, 0.1 and 0.4. The fundamental responses of both X and Y cells to sinusoidal counterphase were greater relative to on-off responses. The second harmonic responses of Y cells to counterphase were larger at high spatial frequencies. Contrast sensitivity also was measured. At all spatial frequencies, both X and Y cells were slightly more sensitive to counterphase than to on-off presentations. Since flicker sensitivity in humans is twice as high for counterphase as for on-off presentations across all spatial frequencies, whereas pattern sensitivity is equal for the two presentations, we conclude that X and Y cells do not subserve uniquely pattern and flicker sensitivity, respectively. This conclusion is based on the result that differences between X and Y cells to counterphase and on-off presentations were inconsistent with the differences observed for pattern and flicker sensitivity. We suggest then that a spatial/temporal dichotomy does not seem to accurately characterize the functional roles of X and Y cells.