Previously, we discovered that the broadband cells in the two magnocellular (large cell) layers of the monkey lateral geniculate nucleus (LGN) are much more sensitive to lminance contrast than are the color-sensitive cells in the four parvocellular (small cell) layers. We now report that this large difference in contrast sensitivity is due not to LGN circuitry but to differences in sensitivity of the retinal ganglion cells that provide excitatory synaptic input to the LGN neurons. This means that the parallel analysis of color and luminance in the visual scene begins in the retina, probably at a retinal site distal to the ganglion cells.phosphor), with a mean luminance of 100 cd/M2. Contrast is an important characteristic of the visual world. It is the physical quantity that specifies the relative variation in luminance of a visual stimulus compared to its average level. The visibility and brightness of objects depend mainly on the contrast with their background. Previously, we have related the sensitivity for contrast to the functional significance ofthe organization of the monkey's lateral geniculate nucleus (LGN) into cellular layers: four dorsal layers of small cells (parvocellular) and two ventral layers of large cells (magnocellular). The contrast gain of cells in the magnocellular layers is about 10 times higher than that of the cells in the parvocellular layers (1, 2). This observation has been confirmed by several other investigators (3-8). Here we report that the monkey retina contains two types of ganglion cells, one with high and the other with low luminance-contrast sensitivity. The high contrast sensitivity type projects to the magnocellular layers, and the low-sensitivity type projects to the parvocellular layers. Thus, it is the retina, and not the pattern of connectivity within the LGN, that is responsible for the large difference in contrast gain between parvocellular and magnocellular geniculate neurons. (10)(11)(12). It has been proven that the S potentials originate from action potentials fired by ganglion cells (11)(12)(13). For most recordings in the monkey's LGN, the S potentials appear to be from a single retinal ganglion cell: they are of approximately fixed amplitude with a fixed latency to electrical stimulation of the optic chiasm, and they do not overlap in time (cf. ref. 12). All of the data reported here were obtained from such unitary recordings. By triggering two comparators, one at the level of the S potential and the other at the level of the LGN spike, we discriminated the two events and studied the responses of the ganglion cell and its LGN target separately. Fig. 1 illustrates the electrical record of a typical pair: unitary S potential and its LGN target cell. METHODResponse-Contrast Functions. We compared the dependence of response amplitude on stimulus contrast for magnocellular-projecting and parvocellular-projecting retinal ganglion cells. Fig. 2 shows the average response-contrast functions obtained from 36 S potentials recorded in the magno-and parvocellular...

We studied the receptive field organization and contrast sensitivity of ganglion cells located within the central 80 (radius of 40) deg of the macaque retina. Ganglion cell activity was monitored as synaptic (S) potentials recorded extracellularly in the lateral geniculate nuclei of anesthetized and paralyzed monkeys. Receptive field center and surround regions of magnocellularly-projecting (M) and parvocellularly-projecting (P) cells increase in area with distance from the fovea, with the center radii of M cells being about twice those of neighboring P cells. Peak sensitivities of center and surround regions are inversely proportional to the regions' areas, so that integrated contrast sensitivities (contrast gains) are constant across the visual field, with the gain of M cells being, on average, six times that of P cells. For both M and P cells, the average ratio of surround/center gain is 0.55. Constant gain of P cells across the visual field is achieved by increasing sensitivity to stimuli falling on the peripheral retina to an extent that counteracts the aberrations introduced by the eye's optics.

SUMMARY1. Cells of the lateral geniculate nucleus (l.g.n.) in macaque monkeys were sorted into two functional groups on the basis of spatial summation of visually evoked neural signals.2. Cells were called X cells if their responses to contrast reversal of fine sine gratings were at the f-ndamental temporal modulation frequency with null positons one quarter of a cycle away from positions for peak response. Cells were called Y cells if their responses to such stimuli were at twice the modulation frequency and were approximately independent of spatial phase.3. Ninety-nine percent of the cells in the four dorsal parvocellular layers of the l.g.n. were X cells; about seventy-five percent of the cells in the two ventral magnocellular layers were also X cells. The remainder were Y cells.4. We confirmed previous findings that magnocellular cells had a shorter latency of response to electrical stimulation of the optic chiasm.5. Magnocellular cells had much higher contrast sensitivities than did parvocellular cells.6. Therefore, two distinct classes of X cells exist in the macaque l.g.n.: parvocellular X cells and magnocellular X cells. The great difference in their properties suggests that they have different functions in vision. The Y cells in the magnocellular layers form a third functional group with spatial properties distinctly different from the X cells.

We used a variety of statistical measures to identify the point process that describes the maintained discharge of retinal ganglion cells (RGC's) and neurons in the lateral geniculate nucleus (LGN) of the cat. These measures are based on both interevent intervals and event counts and include the interevent-interval histogram, rescaled range analysis, the event-number histogram, the Fano factor, Allan factor, and the periodogram. In addition, we applied these measures to surrogate versions of the data, generated by random shuffling of the order of interevent intervals. The continuing statistics reveal 1/f-type fluctuations in the data (long-duration power-law correlation), which are not present in the shuffled data. Estimates of the fractal exponents measured for RGC- and their target LGN-spike trains are similar in value, indicating that the fractal behavior either is transmitted form one cell to the other or has a common origin. The gamma-r renewal process model, often used in the analysis of visual-neuron interevent intervals, describes certain short-term features of the RGC and LGN data reasonably well but fails to account for the long-duration correlation. We present a new model for visual-system nerve-spike firings: a gamma-r renewal process whose mean is modulated by fractal binomial noise. This fractal, doubly stochastic point process characterizes the statistical behavior of both RGC and LGN data sets remarkably well.