A clear difference in spectral power was revealed (p < 1 × 10−41, paired t test), pointing to significant differences in underlying neuronal activity, and indicating that nonconcordant events are see more indeed regional spindles. Furthermore, the analysis of those cases where target channels did not exhibit any increase in spindle spectral power above the noise level (Experimental

Procedures) revealed that 32% of all nonconcordant events were local in the strongest sense—that is, a full-fledged spindle occurred in the seed channel while spectral power in the target channel was not different from chance. Importantly, the occurrence of local spindles was independent of local slow waves, since spindles occurring in isolation (i.e., not associated with a slow wave within ± 1.5 s) constituted 53.7% ± 3.1% of all events and 79.8% ± 0.8% of

such “isolated” spindles were detected in less than 50% of brain regions. In addition, comparing homotopic regions revealed that 40.4% ± 1.7% of DAPT spindles were observed only in one hemisphere (mean ± SEM across nine pairs), indicating that differences between anterior and posterior regions could not account for spindle locality. Next, we quantified the involvement in spindle events by computing the number of brain structures in which each spindle was observed. The distribution of involvement in sleep spindles was skewed toward fewer regions (Figure 5C), indicating that spindles were typically spatially restricted. Mean involvement for sleep spindles was 45.5% ± 0.3% of brain regions (n = 50 depth electrodes). Moreover, 75.8% ± 0.9% of spindles were many detected in less than 50% of regions, indicating that most spindles were local given the definition above. Finally, as was the case for slow waves, the spatial extent of spindles was significantly correlated with spindle amplitude

(Figure 5D; r = 0.62; p < 0.0001; n = 177). Increasing evidence suggests that early and late NREM sleep differ substantially in underlying cortical activity (Vyazovskiy et al., 2009b). Hence, it was of interest to determine whether the spatial extent of slow waves and spindles changes between early and late NREM sleep. To this end, we focused on episodes of early and late NREM sleep in five individuals exhibiting a clear homeostatic decline of SWA during sleep (Figure S1). We identified separately slow waves, spindles, and K-complexes, which are isolated high-amplitude slow waves that are triggered by external or internal stimuli on a background of lighter sleep (Colrain, 2005). We examined for each type of sleep event separately how its spatial extent varied between early and late sleep (Figure 6). Slow waves became significantly more local in late sleep as compared to early sleep (Figure 6A, involvement of 30.4% ± 0.57% in early sleep versus 25.0% ± 0.62% in late sleep; p < 2.