In the lateral geniculate body (LGB) of urethane-anesthetized and Flaxedil-immobilized rats, unit responses of principal cells (P-cells) were recorded by using flash stimulation of the eyes. By measuring response latencies to electrical stimulation of the optic tract, all P-cells were classified into the fast or slow group according to the criterion established by Fukuda. Flash-evoked unit responses consisted of the two kinds of discharges, i. e. the early discharge (ED) with latencies less than 100 msec and the late discharges of spike burst (LDs) often recurring twice or more (LD
1, LD
2 and so on). Cells' firings were suppressed during the interval between ED and LD, and those between successive LDs. The ED- and LD
1-latencies, measured in dark with strong flashes, were shorter in the fast cells than in the slow ones. The interval between ED and LD
1 was shorter in the fast cells than in the slow ones. It was common to the both types of P-cells that as flash intensity was reduced, the ED-latency increased and the LD
1-latency decreased. However, the ED-LD
1 interval remained shorter in the fast cells than in the slow ones. Upon shifting from dark to light, the LD
1-latency markedly increased or decreased in the slow cells, whereas such was not seen in the fast cells. These findings seem to provide a further support for the classification of P-cells into the fast and slow types.
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