In biology, increased theta power seems to be coupled to the proc

In biology, increased theta power seems to be coupled to the processes of encoding (Klimesch, 1999, Sederberg et al., 2003 and Kendrick et al., 2011) and maintenance (Lee et al., 2005, Siegel et al., 2009 and Fuentemilla et al., 2010) of cortical memories. The view that theta oscillations during memory tasks

are related to assembly reactivations is supported by the observations that coding neurons are phase locked to theta during delay periods of working memory tasks with a preferred firing phase LGK974 carrying maximal information about the stimulus (Lee et al., 2005). In our network the preferred firing phases occurred when a specific assembly or subpopulation was maximally activated and the other ones maximally suppressed as a result of local feedback inhibition in the network. The model also shed light on the phenomenon of theta phase reset by a stimulus and recall (Gevins, 1997, Rizzuto et al., 2006 and Ito et al., 2012). For instance, consistently with our effect of theta wave generation driven by attractor memory activation, Rizzuto et al. (2006) observed in a working memory task stimulus-induced

reset of theta phase in many cortical regions. The contribution of theta reset phenomenon to establishing global synchrony that could hypothetically facilitate memory processes was emphasized. In addition, it was recently found that phase of delta/theta waves is locked to the onset of fixations in visual cortex (Ito et al., 2012) as observed in our cued pattern completion paradigm. The delta/theta rhythm in our network reflects the activation of a previously wired neuronal assembly accompanied by increase in firing rates due to the recurrent connectivity within this assembly. In this light, theta oscillations are driven by cell assemblies rather than the opposite. Still however, the slow frequency could also, in other circumstances, reflect general excitability of the network 3-mercaptopyruvate sulfurtransferase (Lakatos et al., 2005 and Neymotin et al., 2011) governed by intrinsic connectivity and cell properties (White et al., 2000). We hypothesize that this is

the case during learning. In this scenario, the gamma oscillation dynamics would underlie the selection of a local winning subpopulation based on response properties and the external input to that particular site. The intrinsic slow rhythm coherent over distance, on the other hand, would facilitate the Hebbian process of forming spatially distributed assemblies − attractors similar to the ones stored in the proposed network that could be used in several memory paradigms. In other words, theta oscillations would provide a window for bursting and wiring within a cortical area, and the neural mechanisms underlying gamma activity would mediate control of burst rates and selection of local winners within an area of around 0.5 mm. Multi-neuron spatiotemporal firing patterns, called precise firing sequences (Abeles and Gerstein, 1988, Abeles et al.

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