The spatio-temporal receptive fields (RFs) of cells in the macaque monkey lateral geniculate nucleus (LGN) and striate cortex (V1) have been examined and two distinct sub-populations of non-directional V1 cells have been found: those with a slow largely monophasic temporal RF, and those with a fast very biphasic temporal response. These two sub-populations are in temporal quadrature, the fast biphasic cells crossing over from one response phase to the reverse just as the slow monophasic cells reach their peak response. The two sub-populations also differ in the spatial phases of their RFs. A principal components analysis of the spatio-temporal RFs of directional V1 cells shows that their RFs could be constructed by a linear combination of two components, one of which has the temporal and spatial characteristics of a fast biphasic cell, and the other the temporal and spatial characteristics of a slow monophasic cell. Magnocellular LGN cells are fast and biphasic and lead the fast-biphasic V1 subpopulation by 7 ms; parvocellular LGN cells are slow and largely monophasic and lead the slow monophasic V1 sub-population by 12 ms. We suggest that directional V1 cells get inputs in the approximate temporal and spatial quadrature required for motion detection by combining signals from the two non-directional cortical sub-populations which have been identified, and that these sub-populations have their origins in magno and parvo LGN cells, respectively.

W e (1-4) and many others (e.g., 5-8) have studied the chromatic response characteristics of cells in the lateral geniculate nucleus (LGN) and at the first processing stages within the striate cortex of the macaque monkey. We are revisiting the problem to test predictions from our recent color model (9) and from our psychophysical studies (10, 11) that certain transformations of color information should occur between the LGN and the cortex. Most previous studies of LGN and cortical cells, including our own earlier ones, used different stimuli in examining cells at the two levels, thus making direct comparisons between LGN and cortex difficult to quantify.To examine this issue more precisely, we have recorded from a considerable sample of cells in the LGN and striate cortex of macaque monkeys, by using identical stimuli in examining cells at these two successive processing levels. The stimuli also were identical in chromaticity to those used in our psychophysical studies (10, 11). These measurements should allow us to determine any changes in the chromatic information from the LGN to the cortex. Furthermore, they should also facilitate comparisons to psychophysical measurements of color appearance. MethodsMacaque monkeys (Macaca mulatta and M. fascicularis) were initially tranquilized with ketamine HCl (10-15 mg͞kg, i.m.). Anesthesia was maintained with a continuous i.v. infusion of sufentanil citrate (during surgery, 8-12 g͞kg͞h; during recording 5-8 g͞kg͞h). After surgery, paralysis was induced and maintained with pancuronium bromide (0.1-0.15 mg͞kg͞h, i.v.). Electrocardiogram, electroencephalogram, body temperature, and expired CO 2 were continuously monitored. All of the procedures were approved by the local Animal Care and Use Committee and were in accord with National Institutes of Health guidelines. Extracellular recordings were made from single neurons in the LGN and the striate cortex (V1). All recordings were from units whose receptive fields were within the central visual field. Action potentials (spikes) were recorded with a resolution of 1 msec. Visual stimuli were generated and controlled by a Sun͞TAAC (Sun Microsystems, Mountain View, CA) image processor, with on-line data analysis being performed by the Sun. The stimuli were presented on an NEC monitor (Nippon Electric, Tokyo) with a spatial resolution of 1,000 ϫ 900 pixels, a 66-Hz refresh rate, and a mean luminance of 70 cd͞m 2 .The chromatic response properties of each cell were characterized with drifting gratings that varied sinusoidally in chromaticity. These were presented for 2 sec each, in random order within a testing series. Typically, we first determined the spatial frequency tuning of a neuron and, in the case of cortical cells, the orientation tuning. The chromatic testing then proceeded with grating patches of the optimal spatial frequency and orientation. The grating patch was centered on the receptive field (RF) of the cell and was slightly larger than the classic RF. Many cortical cells and almost all LGN cells showed low-pass sp...

We considered the problem of determining how the retinal network may interact with electrical epiretinal stimulation in shaping the spike trains of ON and OFF ganglion cells, and thus the synaptic input to first-stage cortical neurons. To do so, we developed a biophysical model of the retinal network with nine stacked neuronal mosaics. Here, we describe the model's behavior under (i) electrical stimulation of a retina with complete cone photoreceptor loss, but an otherwise intact circuitry and (ii) electrical stimulation of a fully-functional retina. Our results show that electrical stimulation alone results in indiscriminate excitation of ON and OFF ganglion cells and a patchy input to the cortex with islands of excitation among regions of no net excitation. Activation of the retinal network biases the excitation of ON relative to OFF ganglion cells, and in addition, gradually interpolates and focuses the initial, patchy synaptic input to the cortex. As stimulation level increases, the cortical input spreads beyond the area occupied by the electrode contact. Further, at very strong stimulation levels, ganglion cell responses begin to saturate, resulting in a significant distortion in the spatial profile of the cortical input. These findings occur in both the normal and the degenerated retina simulations, but the normal retina exhibits a tighter spatiotemporal response. The complex spatiotemporal dynamics of the prosthetic input to the cortex that are revealed by our model should be addressed by prosthetic image encoders and by studies that simulate prosthetic vision.

We measured the spatial-frequency tuning of cells at regular intervals along tangential probes through the monkey striate cortex and correlated the recording sites with the cortical cytochrome oxidase (CytOx) patterns to address three questions with regard to the cortical spatial-frequency organization. (') Is there a periodic anatomical arrangement of cells tuned to different spatial-frequency ranges? We found there is, because the spatial-frequency tuning of cells along tangential probes changed systematically, varying from a low frequency to a middle range to high frequencies and back again repeatedly over distances of about 0.6-0.7 mm. (it) Are there just two populations ofcells, low-frequency and high-frequency units, at a given eccentricity (perhaps corresponding to the magno-and parvocellular geniculate pathways) or is there a continuum of spatial-frequency peaks? We found a continuum of peak tuning. Most cells are tuned to intermediate spatial frequencies and form a unimodal rather than a bimodal distribution of cell peaks. Furthermore, the cells with different peak frequencies were found to be continuously and smoothly distributed across a module. (iii) What is the relation between the physiological spatial-frequency organization and the regions of high CytOx concentration ("blobs")? We found a systematic correlation between the topographical variation in spatial-frequency tuning and the modular CytOx pattern, which also varied continuously in density. Low-frequency cells are at the center of the blobs, and cells tuned to increasingly higher spatial frequencies are at increasing radial distances.The primary transformation of visual information at the striate cortex level in cat and monkey appears to be the sharpening of the orientation and spatial-frequency tuning of cells (1-5). Retinal ganglion and lateral geniculate nucleus cells respond to a wide range of orientations and spatial frequencies, whereas striate cortex cells are usually much more narrowly tuned. In the striate cortex, most cells have both spatial frequency and orientation tuning and thus respond to only a limited two-dimensional spatial-frequency range, each acting in effect as a band-pass two-dimensional filter of patterns within a localized region of the visual field. Different cells within a cortical region respond to different two-dimensional frequency ranges, with the ensemble ofcells presumably covering the whole three-to five-octave range of spatial frequencies visible at that eccentricity (5). This physiological evidence is in good agreement with considerable psychophysical evidence for the presence of multiple two-dimensional spatial-frequency channels underlying human spatial vision (6-9).Early studies of the anatomical arrangement of cells in macaque striate cortex found evidence for a modular pattern to the cortical organization (10, 11). The cells within a slab of cortex -1-1.5 mm on a side all process the visual input from one small retinal region. Half of the cells within such a module receive their primary input ...