Fig. 5.

Theta-related firing properties of pro-arousal slow-firing neurons in REM sleep. A–D: example of theta-related properties of 3 types of neurons recorded in the same REM episode. A, top: spectrogram of hippocampal LFP during the REM episode; bottom: smoothed instantaneous firing rates. B: phase preferences of the 3 individual neurons. The pro-arousal slow-firing neuron (proA) had weaker phase modulation. C: interspike interval (ISI; in s) distribution of the 3 neurons. D: autocorrelation during REM episode. The pro-arousal slow-firing neuron was not rhythmic-firing, whereas the other slow-firing neuron and the fast-firing neuron had characteristic rhythmic side peaks in their autocorrelation functions. Autocorrelation functions are normalized to their individual maximal values. E: population plots of phase-locking during REM sleep for the 3 groups of neurons. Units are sorted according to phase-locking strength (Rayleigh test critical value Z). Nonsignificant (P > 0.05) neurons are displayed as null (bottom). Pseudocolor indicates phase distribution (%). F: sorted phase-locking strength (Z values) for the 3 groups in E. Vertical line indicates significant phase-locking (to right of the line, P < 0.05). Among phase-locked neurons, Z values were generally small for the pro-arousal slow-firing neurons compared with the other 2 groups. G: ISI distribution for the 3 groups. Pro-arousal slow-firing neurons had a clear peak around 0.1–0.18 s. H: population plots of autocorrelation (pseudocolor, normalized to maximum of individual correlation). Units are sorted according to rhythmicity index. Many fast-firing neurons were rhythmic firing, unlike most pro-arousal slow-firing neurons.