short-range spatially coherent activities that are not promptly available with scalp recordings. In the present study, we examined the presence of gamma patterns during polysomnographically defined sleep states using intracranial EEG recordings, collected in parallel with macroscopic scalp EEG, of 20 subjects who required a clinical invasive evaluation for the treatment of their epilepsy. This relatively large sample size and broad spatial sampling in this study (with a total of 740 investigated intracranial electrode sites) provided an opportunity to evaluate the intracranial distribution of gamma activities over the whole cortex. Furthermore, the simultaneous recording of slow waves by scalp EEG allowed us to analyze different phasic modulations of gamma activities by global slow components. Results Gamma oscillations during SWS A group of 20 epileptic subjects were included in this study. Intracranial LFPs were recorded from the surface of the cortex (subdurally) or from depth electrodes stereotactic implanted in deeper cortical structures. Details of electrode placement, location of interictal and ictal epileptic abnormalities are outlined in Table S1. All subjects had 2 selected overnight recordings. Episodes of gamma activity (30–120 Hz) were automatically identified across all stages of vigilance using previous methodology [15], and detected events were then visually confirmed in the raw traces (see Materials and Methods and Figure 1). In particular, for the majority of investigated intracranial contacts (79%, 227 of a total of 286; n = 8 subjects), the density of detected events (see below) was significantly higher during SWS than during wakefulness or REM (p,0.05, two-tailed permutation-test). A representative example of the gamma detection degree in relation to the sleep-wake cycle for a single subject is given in Figure 2A. As expected, the time- frequency representation of the power for a scalp electrode (Figure 2A middle) showed a clear increment of the power in the low frequency range (,0.3–2 Hz) as the subject entered in the deepest stages of sleep (S2–S4). This increased rate of slow waves was in turn accompanied by an increment in the density of gamma events, measured as the number of detected events per time interval (20 seconds), presented here for seven intracranial contacts (Figure 2A bottom). In average, the mean density of gamma events during NREM sleep periods was 1.1460.45 (mean 6 SD) events per 20-seconds interval (n = 8 subjects), reflecting a large variability of gamma occurrences on each slow wave activity cycle. In order to illustrate the relation between the gamma activity and the presence of slow waves, Figure 2B illustrates typical recorded patterns during sleep stage 4. A predominant slow activity can be seen in raw signals for both, surface (FP1) and depth electrodes (here in the left orbito-frontal gyrus). Note the reversed polarities between the scalp and the intracranial activities, confirming what has been previously shown in animal studies [3]. The average main peak in the power spectra was estimated here at 0.8560.08 Hz (on average over n = 25 contacts) and the wave shapes at the scalp level fulfilled standard electrographic criteria of normal slow oscillations [16]. Concerning the signals filtered in the gamma-range, near-periodic bursts of increased amplitude could be observed in all intracranial contacts (but not at level of the scalp) at the depth-negative phase (respectively surface-positive phase) of the slow oscillations in the raw signals. A spectral analysis of the gamma envelopes indicated a main frequency peak of these bursts at 0.8860.15 Hz (n = 25 contacts), confirming a relation- ship between the occurrences of gamma bursts and the slow oscillations during SWS. In either the raw signals (without any Human Gamma Oscillations during Slow Wave Sleep previous filtering or pre-processing) or those filtered between 30 and 120 Hz, fast sinusoidal waves appeared in all intracranial contacts as discrete events which were clearly distinguishable from background activity (Figure 2C, bottom). The wavelet transformed- energy scalogram, which represents the spectral energy with respect to time and frequency, precisely characterizes the frequency components of single detected gamma events. For an individual recording contact, successive cycles of the slow oscillation were constituted by a series of pure oscillatory bursts appearing in narrow frequency bands in the low (,30–50 Hz) gamma range, high (,60–120 Hz) gamma range or by broad- band events composed of a few mixed frequencies (Figure 2C). When broad-band events were observed, the different oscillatory components could be superimposed, slightly overlapping, or completely non-overlapping in time (Figure 2C middle). Note that gamma activity is plainly observed in the deepest intracranial contacts, suggesting that these activities cannot be explained by superficial scalp muscle artifacts, which are regardless very low during SWS. Even though the intracranial electrodes in any individual subject explored a limited part of the brain, the relatively large sample size (20 subjects; see also Figure 6, yellow spheres) allowed us to analyze the anatomical distribution of gamma events. We found that gamma episodes were seen bilaterally during SWS in all investigated regions of the cerebral cortex (20/20 subjects). From all contacts located in the temporal lobe (451/740), 56% presented a significant higher density of gamma events during SWS than during other stages (comparing density during SWS vs. density in other stages together, p,0.05, two-tailed permutation-test). For the frontal lobe (142/740) and occipital lobe (76/740), higher detections occurred in 42% and 71% of all analyzed contacts, respectively. We found no significant correlations between the locations of gamma oscillations and the spatial extension of the seizure onset zone or propagation regions of epileptic spikes (58% of contacts presenting significant higher density of gamma oscillations during SWS were detected outside of these regions), suggesting that epilepsy is unlikely to be the main source of the observed oscillations. Phasic modulation of gamma oscillations by the slow waves To examine the possible modulation of gamma activity by the slow waves, we analyzed the temporal relationship between the phases of the slow wave and the occurrence of gamma activity. Over the entire recording session, the automatic detection algorithms independently identified slow waves and gamma oscillations for all investigated intracranial contacts (see Materials and Methods). Only slow waves where gamma events were detected inside a window of 2 seconds around the central peak of each intracranial slow wave were selected for analysis (see Figure 1 and Figure 3). Furthermore, we analyzed only contacts having a sufficient number of detected slow waves (.25; 233 of 740 contacts from 19 of 20 subjects). Analysis of individual gamma activities revealed three broad patterns of gamma modulation by the slow oscillation: In the first pattern, gamma events preferen- tially appeared during the positive phase of the scalp slow wave (‘‘IN-phase’’ pattern). In the second, gamma events tend to appear during the negative phase of the scalp slow wave (‘‘ANTI-phase’’ pattern). In the third pattern, there was no specific phase preference of the incidence of gamma events to the slow wave, meaning that gamma activity could be equally present during the positive as well as during the negative phase. Examples of phasic gamma modulations associated with IN-phase and ANTI-phase patterns are shown in Figure 3A and 3B for intracranial contacts in PLoS ONE | www.plosone.org 2 April 2012 | Volume 7 | Issue 4 | e33477