Unit activities of the lateral geniculate body (LGB) were studied in free behaving cats during arousal, light sleep (sleep with high-voltage, slow EEG) and deep sleep (sleep with low-voltage, fast EEG). The LGB units were classified into two types according to their response patterns to stimulation of the optic chiasm and the visual cortex; by both types of stimulation the P units were fired singly at short latencies and the I units were fired repetitively at long latencies.
1. Spontaneous activity of the P units changed markedly upon alternation of the behavioral state. During arousal the P units showed well-spaced regular discharges. During light sleep the grouped discharges consisting of 2-5 spikes became manifest intermingled with sporadic discharges. In deep sleep spontaneous activity was accelarated with occasional bursts of spikes (deep sleep bursts) lasting about 0.5 sec.
2. The deep sleep waves of the LGB, which were mass activity of spiky form seen during deep sleep, were found to be correlated to the deep sleep burst of the P unit spontaneous activity in their negative phase. Suppression of the unit activity was seen during their positive phase.
3. The rate of spontaneous activity of the P units was lowest during light sleep and increased from arousal to deep sleep.
4. The firing probability of the P units to chiasmatic and visual cortical stimulation was lowest during light sleep and increased from arousal to deep sleep. This was true of the firing probability to the spontaneous synaptic bombardment.
5. The I units did not alter significantly the rate of spontaneous activity and the strength of responsiveness to visual cortical stimulation when sleep changed from one type to another. Responsiveness of the I units to chiasmatic stimulation was higher during deep sleep than during light sleep, suggesting that orthodromic activation of the I units is mediated via the P units.