Two classes of single-input X-cells in cat lateral geniculate nucleus. I. Receptive-field properties and classification of cells

Cells in the cat's dorsal lateral geniculate nucleus (LGN) were studied by presentation of visual stimuli and also by simultaneous recording of their ganglion cell inputs in the retina. This paper describes receptive-field properties and a new system of classification for LGN X-cells that appear to receive essentially only one excitatory retinal input. These X-cells were of two distinct classes. The visual responses of one class of cell (XS, single) replicated the basic form of the responses of a retinal X-cell. The other class of cell (XL, lagged) had responses with two remarkable features: their firing lagged 40-80 ms behind that of XS-cells or ganglion cells at response onset, and they fired anomalously at times when XS-cells or ganglion cells would not be firing. Thus, for a flashing spot, XL-cells were inhibited from firing after stimulus onset, during the time when XS-cells or retinal X-cells had an initial transient peak in firing; XL-cells generally had an anomalous peak in firing after stimulus offset, after XS-cells or retinal X-cells had stopped firing. For a moving bar, XS-cells or retinal X-cells responded primarily while the bar was in the receptive-field center, whereas most of a typical XL-cell's response occurred after the bar had left the receptive-field center. The latencies of various features in the visual responses were analyzed. For several visual response latencies, the distribution was clearly bimodal, thus objectively demonstrating the existence of two cell classes. Using only the latencies from spot and bar responses, over 90% of these single-input cells could be reliably identified as belonging to one of the two classes. The remaining cells (7 of 128) were intermediate between the two classes in some but not all respects; because they had some properties in common, these cells were kept in a separate group (XPL, partially lagged). The axons of both XS- and XL-cells could be antidromically activated from visual cortex. Cortical latencies were typically 0.7-2.0 ms for XS-cells but much longer, typically 2.4-5.0 ms, for XL-cells. It is possible that XL-cells have not previously been recognized as a separate class because cells with such long latencies have been recorded infrequently in the past. Responses to central flashing spots were more transient than those of retinal X-cells for most XS-cells and more sustained for most XL-cells.(ABSTRACT TRUNCATED AT 400 WORDS)