Previous theoretical and empirical studies have indeed shown that

functional interactions between brain regions are particularly crucial for cognitive processes and can occur in the absence of changes in local activity parameters, such as discharge phosphatase inhibitor library rate and oscillation amplitude (Hipp et al., 2011; Lima et al., 2011). Recent advances in EEG and MEG approaches have now allowed the noninvasive mapping of changes in the large-scale networks during perceptual and higher cognitive processes (Figure 2). Support for the distinction between local oscillatory versus long-range synchronization processes comes from studies that have examined the frequencies at which neuronal ensembles oscillate. Local processes tend to be associated with increased oscillations at gamma-band frequencies (25–200 Hz) while long-range interactions tend to involve a larger spectrum of frequency bands comprising theta (4–7 Hz), alpha (8–12 Hz), and beta (13–25 Hz) frequencies (von Stein and Sarnthein, 2000). One reason could be that larger networks cannot support selleck products synchronization with very high temporal precision as a result of long conduction times. This is because lower frequencies put fewer constraints on the precision of timing since the phases of increased and reduced excitability are longer (Kopell et al., 2000). Recent theoretical (Vicente et al., 2008) and empirical work (Buschman and Miller, 2007), however,

indicates that long-range synchronization can also occur at substantially higher frequencies (>30 Hz) and that even zero phase-lag synchronization is compatible with conduction Tolmetin delays. It is therefore conceivable that the nesting of local high-frequency oscillations in more global, lower-frequency oscillations serves

the binding of local processes into more integrated global assemblies. This possibility is supported by the growing evidence on the existence of cross-frequency coupling, the amplitude, frequency or phase of high-frequency oscillations being modulated by slower oscillatory processes (Canolty et al., 2006; Canolty and Knight, 2010; Jensen and Colgin, 2007; Palva et al., 2005). Neuron clusters can participate in several networks oscillating at different frequencies by engaging in partial coherence with both of them. This concatenation of rhythms has been observed in the hippocampus for gamma- and theta-band oscillations (Wang and Buzsáki, 1996), between different cortical laminae (Roopun et al., 2008) and for both low- and high-frequency activity (Canolty et al., 2006; Jensen and Colgin, 2007; Palva et al., 2005). Much work has been devoted to the analysis of synaptic mechanisms and circuits that support the generation of oscillatory activity and its synchronization over short and long distances, respectively, which makes it possible to relate abnormalities of these dynamic phenomena to specific neuronal processes (Sohal et al., 2009; Traub et al., 2004; Vicente et al., 2008; Wang and Buzsáki, 1996).