Space and time maps of cone photoreceptor signals in macaque lateral geniculate nucleus

We studied neurons in the central visual field representation of the lateral geniculate nucleus (LGN) in macaque monkeys by mapping their receptive fields in space and time. The mapping was performed by reverse correlation of a spike train of a neuron with pseudorandom, binary level stimuli (m-sequence grids). Black and white m-sequence grids were used to map the receptive field for luminance. The locations of receptive field center and surround were determined from this luminance map. To map the contribution of each cone class to the receptive field, we designed red-green or blue-yellow m-sequence grids to isolate the influence of that cone (long, middle, or short wavelength-sensitive: L, M, or S). Magnocellular neurons generally received synergistic input from L and M cones in both the center and the surround. A minority had cone-antagonistic (M-L) input to the surround. Red-green opponent parvocellular neurons received opponent cone input (L+M- or M+L-) that overlapped in space, as sampled by our stimulus grid, but that had somewhat different extents. For example, an L+ center parvocellular neuron would be L+/M- in both center and surround, but the L+ signal would be stronger in the center and the M- signal stronger in the surround. Accordingly, the luminance receptive field would be spatially antagonistic: on-center/off-surround. The space-time maps also characterized LGN dynamics. For example, magnocellular responses were transient, red-green parvocellular responses were more sustained, and blue-on responses were the most sustained for both luminance and cone-isolating stimuli. For all cell types the surround response peaked 8-10 msec later than the center response.