Stochastic properties of spontaneous unit discharges in somatosensory cortex and mesencephalic reticular formation during sleep-waking states.

1. We compared renewal and Markovian characteristics of neuronal discharge sequences in deeper layers (V and VI) of the somatosensory area I (SI) and the mesencephalic reticular formation (MRF) of unanesthetized cats in the following five behavioral states: active wakefulness with movements (AW), motionless quiet waking (QW), slow-wave sleep (SWS), and paradoxical sleep with and without rapid eye movements (REM-PS and non-REM-PS). 2. The profile of the pooled renewal properties for the shorter interspike-interval component predicted by the exponential tail of interval distribution during a sleep-wakefulness cycle was quite different from that for the total intervals including the longer ones; e.g., the mean rate of the SI neuronal group for the shorter interval component during SWS was as high as that during REM-PS in spite of the lowest rate for the total intervals. 3. In the SI and MRF neurons, the group mean of the degree of Markovian characteristics of the shorter intervals was much larger during AW, SWS, and REM-PS then QW and non-REM-PS, indicating that successive interspike intervals of spontaneous activity are time dependent in the AW, SWS, and REM-PS states, while they tend to be independent during QW and non-REM-PS. This tendency was more conspicuous for SI than MRF. On the other hand, the longer intervals in the five states did not contribute to the revealment of the Markov property. 4. The time-independent activities during QW and non-REM-PS were markedly similar especially for SI, suggesting that these activities may be regarded as autochthonous ones inherent to the neuron and its surrounding network under study. 5. The degree of Markovian properties was much higher in the MRF neurons than in the SI neurons during each of the states of vigilance except for AW, suggesting that the MRF neuronal group has a tonically modulated unit activity even in the absence of external stimuli.