Human Gamma Oscillations during Slow Wave Sleep Figure 5. Histograms presenting the distribution of the preferred phases for all contacts associated with IN-phase (122 from 15 subjects, top histogram) and ANTI-phase (21 from 7 subjects, bottom histogram) patterns, for the first detection time (left) and the maximal amplitude time (right). Phases of the corresponding SW are given for reference (blue lines on top). doi:10.1371/journal.pone.0033477.g005 event to be detected at the same time in two contacts would be 25% lower if the separation between them were increased by 10 mm. The cortical distribution of channels presenting a co- detection probability over 50% revealed that the frontal lobe had a greater tendency to express gamma co-activations, followed by the temporal lobe and the cingulate cortex (Figure 7C, blue bars). In contrast, the parietal, occipital and insular cortices only displayed a very low tendency of gamma co-activations. We next examined whether gamma activities that co- occurred in space at two different places were also oscillating in phase (i.e. were phase-locked). For this, we selected all channels with a co-detection probability over 50% (2121 pairs of contacts from 14 subjects and from a total of 28417 pairs over all analyzed frequency bands in the gamma range) and we calculated the windowed phase-locking value (PLV) and the cross-correlation coefficient (CC), both quantities giving values varying from 0 to 1, where 1 means perfect synchrony (see Materials and Methods). In a large number of cases, significant synchronization could be identified during gamma co-activa- tions by both measurements (p,+0.001, two-tailed permutation- test; for PLV and CC, in 83.5% and 86% of the cases respectively). Similarly to gamma co-activations, we found that the number of synchronous pairs fell off quickly with distance (Figure 7D and Figure S2A). Nevertheless, in contrast to gamma co-activations, the strength of the synchronization did not decrease with the distance, suggesting that strong synchrony could happen, with a low probability, between two widely separated contacts (Figure 7E and Figure S2B). The cortical distribution of channels presenting significant synchronous activities (for both PLV and CC) revealed that the frontal lobe had a greater tendency to express more gamma synchronization followed by the cingulate and the temporal ones (Figure 7C, yellow and red bars). We finally examined the relationship between the phases of the slow wave and the co-activations of gamma oscillations. In particular, from all channels presenting significant synchrony, we evaluated whether co-activations tended to systematically appear at a particular phase of the slow wave (see Materials and Methods). From all analyzed co-activations, we found that only 2.8% presented a systematic temporal relationship with the slow wave (p,0.01, Rayleigh test for circular uniformity). Examples of contact pairs presenting this infrequent phenomenon are shown in Figure 7F. For these pairs of contacts, we estimated the phase- locking of the gamma oscillations calculated across all corre- sponding slow waves. As illustrated in these examples and observed most of the time (65.4%, representing pairs of contacts from 8 subjects), the phasic modulation of gamma synchroniza- tion corresponded to IN-phase and ANTI-phase patterns (Figure 7G). The preferred phases together with the preferred times were estimated respectively to 0.74 radians (,20.4475 sec) PLoS ONE | www.plosone.org 7 April 2012 | Volume 7 | Issue 4 | e33477