Magnocellular (M-), but not parvocellular (P-), neurons of the macaque lateral geniculate nucleus (LGN) differ distinctively in their responses to counterphase-modulated and drifting gratings. Relative to stimulation with drifting gratings, counterphase modulation reduces the responses of M- cells in a band around 25 Hz, producing a "notch" in the temporal modulation transfer function (tMTF). The notch is prominent in nearly every M- cell with little variation in the temporal frequency at which it is deepest. The machinery responsible for the notch lies mostly outside the classical linear center. Directly driving the notching mechanism with annular gratings evokes no linear response but elicits a second harmonic (F2) modulation of the discharge accompanied by a drop in the mean discharge (F0). Analysis of the S- potential, which reveals inputs from ganglion cells, shows that 1) tMTFs of the afferent retinal ganglion cells are not notched and 2) during stimulation with annular gratings, the second harmonic component is present, but the drop in the F0 is largely absent from the responses of parasol ganglion cells. These results suggest that the notch is caused by the combined action of the linear response and the second harmonic response, both inherited from retina, and a suppression that originates after the retina. Our results reveal a distinctive signal transformation in the LGN and they show that nearly every M- cell exhibits a spatial nonlinearity like that observed in Y cells of the cat.
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