In particular, the existence of hippocampal “time cells” that encode moments in temporally extended memories, much as place cells encode locations in spatially extended environments, suggests that time, not place, is the fundamental dimension of hippocampal representation Fluorouracil manufacturer that is common to navigation and memory. Furthermore, recent evidence revealed temporal organization in hippocampal ensembles that exists prior to experiences, to which learning attaches specific memories (Dragoi and Tonegawa, 2011). This observation of “preplay,” which anticipates subsequent replay, suggests that temporal organization is primary and may provide the scaffolding onto which

spatial and nonspatial memories are hung. The convergence of literatures on retrieval-associated replay in spatial memory and temporal organization in a broad variety of situations offers considerable promise for a comprehensive understanding of the role of the hippocampus in memory. H. Eichenbaum

is supported by NIMH MH094263, MH095297, MH51570, MH52090, ONR N00014-10-1-0936. “
“Multi-item messages must often be transmitted between brain regions. For instance, short-term memory may represent the last several events in the recent past; similarly, the sequence of events that constitute an episodic memory may be recalled from long-term memory. Handling such multi-item messages requires a neural code that specifies not only how items are represented, but also how different items are kept separate (e.g., the Selleckchem Dabrafenib longer pauses that separate letters Resminostat in the Morse code). Here, we evaluate the hypothesis that the neural code for multi-item messages is organized by brain oscillations. These oscillations can be observed in field potentials, a method of extracellular recording that provides a measure of average neural activity in a brain region (Buzsáki et al., 2012). Such recordings in rodents (Figure 1A) have

shown that gamma frequency (∼40 Hz) oscillations are nested within slow theta frequency (∼7 Hz) oscillations (Belluscio et al., 2012; Bragin et al., 1995; Colgin et al., 2009; Soltesz and Deschênes, 1993). A large number of experiments have investigated the role of theta/gamma oscillations, largely using physiological methods in rodents. More recently, the study of these oscillations in humans has become a focus of cognitive neuroscience (Axmacher et al., 2010; Canolty et al., 2006; Demiralp et al., 2007; Llinás and Ribary, 1993; Maris et al., 2011; Mormann et al., 2005; Sauseng et al., 2009; Voytek et al., 2010). The specific hypothesis that we will evaluate here is shown in Figure 1B (Lisman and Buzsáki, 2008; Lisman and Idiart, 1995). According to this coding scheme, the subset of cells that fire during a given gamma cycle (sometimes referred to as a cell assembly or an ensemble) form a spatial pattern that represents a given item.