First, changes “across sleep” were defined as differences between the first and the last non-REM episodes in a sleep session. Second, changes in “within non-REM” episodes refer to differences between the first and the last thirds of each non-REM. Third, changes in “within REM” episodes refer to differences between the first and the last thirds of each REM. Finally, we examined the relationship between these categories. Since non-REM sleep is characterized by alternating periods
of population activity and inactivity in both the neocortex (Steriade et al., 1993) and hippocampus (Ji and Wilson, 2007; Isomura et al., 2006), we defined active periods as those in which smoothed gamma and epsilon band (30–300 Hz) LFP activity was at
least 0.5 SDs above the mean for at least 50 ms. Conversely, inactive periods learn more were detected as those in which gamma and epsilon band activity was 0.5 SDs below the mean for at least 50 ms (see Supplemental Experimental Procedures, see also Figure S2 for an analogous spike-based analysis). The incidence of active periods decreased, whereas the incidence of inactive periods increased significantly from the first to the last non-REM episodes of each session (i.e., across sleep; Figure 1B; Table S1). The firing rates of both pyramidal cells and interneurons decreased significantly across sleep (Figure 1B). These findings are in Crizotinib mw accord with the two-process model of sleep and indicate similarities between sleep-related activity of neurons between the neocortex and hippocampus (Borbély, 1982; Tononi and Cirelli, 2006; Vyazovskiy et al., 2009). During sleep, the Non-receptor tyrosine kinase hippocampal neural population fires synchronously during sharp-wave ripple events and relatively asynchronously between ripples (Buzsáki et al., 1992). The discharge rate of pyramidal neurons between ripples decreased significantly across sleep (Figure 1B), similar to the decrease in global firing rate. Conversely, the mean firing rate
of pyramidal cells within the short-lived ripple events increased during the course of sleep (Figure 1B). This increase in ripple-related activity across sleep was the result of an increase in the percentage of ripples within which pyramidal cells participated (i.e., fired at least one spike) rather than an increase of the within-ripple firing rates of individual neurons in individual ripples (Figure 1B; Figure S3). Concurrent with the increase of within-ripple participation, the coefficient of variation of within-ripple firing rate across cells decreased (Figure 1B; Figure S3), suggesting that the within-ripple participation was more evenly distributed across the population of pyramidal cells at the end compared to the beginning of sleep. Synchrony, as measured by the correlation strength of pyramidal cell pairs in nonoverlapping 100 ms bins (Wilson and McNaughton, 1994), also increased across sleep (Figure 1B), probably due to the more consistent participation of pyramidal cells in ripples.
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