The ability to recognize a previously experienced stimulus is supported by two processes: recollection of the stimulus in the context of other information associated with the experience, and a sense of familiarity with the features of the stimulus. Although familiarity and recollection are functionally distinct, there is considerable debate about how these kinds of memory are supported by regions in the medial temporal lobes (MTL). Here, we review evidence for the distinction between recollection and familiarity and then consider the evidence regarding the neural mechanisms of these processes. Evidence from neuropsychological, neuroimaging, and neurophysiological studies of humans, monkeys, and rats indicates that different subregions of the MTL make distinct contributions to recollection and familiarity. The data suggest that the hippocampus is critical for recollection but not familiarity. The parahippocampal cortex also contributes to recollection, possibly via the representation and retrieval of contextual (especially spatial) information, whereas perirhinal cortex contributes to and is necessary for familiarity-based recognition. The findings are consistent with an anatomically guided hypothesis about the functional organization of the MTL and suggest mechanisms by which the anatomical components of the MTL interact to support the phenomenology of recollection and familiarity.

Neuronal oscillations of different frequencies can interact in several ways. There has been particular interest in the modulation of the amplitude of high-frequency oscillations by the phase of low-frequency oscillations, since recent evidence suggests a functional role for this type of cross-frequency coupling (CFC). Phase-amplitude coupling has been reported in continuous electrophysiological signals obtained from the brain at both local and macroscopic levels. In the present work, we present a new measure for assessing phase-amplitude CFC. This measure is defined as an adaptation of the Kullback-Leibler distance-a function that is used to infer the distance between two distributions-and calculates how much an empirical amplitude distribution-like function over phase bins deviates from the uniform distribution. We show that a CFC measure defined this way is well suited for assessing the intensity of phase-amplitude coupling. We also review seven other CFC measures; we show that, by some performance benchmarks, our measure is especially attractive for this task. We also discuss some technical aspects related to the measure, such as the length of the epochs used for these analyses and the utility of surrogate control analyses. Finally, we apply the measure and a related CFC tool to actual hippocampal recordings obtained from freely moving rats and show, for the first time, that the CA3 and CA1 regions present different CFC characteristics.

Summary The hippocampus is critical to remembering the flow of events in distinct experiences and, in doing so, bridges temporal gaps between discontiguous events. Here we report a robust hippocampal representation of sequence memories, highlighted by “time cells” that encode successive moments during an empty temporal gap between the key events, while at the same times encoding location and ongoing behavior. Furthermore, just as most place cells “remap” when a salient spatial cue is altered, most time cells form qualitatively different representations (“re-time”) when the main temporal parameter is altered. Hippocampal neurons also differentially encode the key events and disambiguate different event sequences to compose unique, temporally organized representations of specific experiences. These findings suggest that hippocampal neural ensembles segment temporally organized memories much the same as they represent locations of important events in spatially defined environments.

Recent neurobiological studies have begun to reveal the cognitive and neural coding mechanisms that underlie declarative memory--our ability to recollect everyday events and factual knowledge. These studies indicate that the critical circuitry involves bidirectional connections between the neocortex, the parahippocampal region and the hippocampus. Each of these areas makes a unique contribution to memory processing. Widespread high-order neocortical areas provide dedicated processors for perceptual, motor or cognitive information that is influenced by other components of the system. The parahippocampal region mediates convergence of this information and extends the persistence of neocortical memory representations. The hippocampus encodes the sequences of places and events that compose episodic memories, and links them together through their common elements. Here I describe how these mechanisms work together to create and re-create fully networked representations of previous experiences and knowledge about the world.

Recent studies on the hippocampus and the prefrontal cortex have considerably advanced our understanding of the distinct roles of these brain areas in the encoding and retrieval of memories, and of how they interact in the prolonged process by which new memories are consolidated into our permanent storehouse of knowledge. These studies have led to a new model of how the hippocampus forms and replays memories and how the prefrontal cortex engages representations of the meaningful contexts in which related memories occur, as well as how these areas interact during memory retrieval. Furthermore, they have provided new insights into how interactions between the hippocampus and prefrontal cortex support the assimilation of new memories into pre-existing networks of knowledge, called schemas, and how schemas are modified in this process as the foundation of memory consolidation.

Firing patterns of hippocampal complex-spike neurons were examined for the capacity to encode information important to the memory demands of a task even when the overt behavior and location of the animal are held constant. Neuronal activity was recorded as rats continuously alternated left and right turns from the central stem of a modified T maze. Two-thirds of the cells fired differentially as the rat traversed the common stem on left-turn and right-turn trials, even when potentially confounding variations in running speed, heading, and position on the stem were taken into account. Other cells fired differentially on the two trial types in combination with behavioral and spatial factors or appeared to fire similarly on both trial types. This pattern of results suggests that hippocampal representations encode some of the information necessary for representing specific memory episodes.

Phase-amplitude cross-frequency coupling (CFC) between theta (4 -12 Hz) and gamma (30 -100 Hz) oscillations occurs frequently in the hippocampus. However, it still remains unclear whether thetagamma coupling has any functional significance. To address this issue, we studied CFC in local field potential oscillations recorded from the CA3 region of the dorsal hippocampus of rats as they learned to associate items with their spatial context. During the course of learning, the amplitude of the low gamma subband (30 -60 Hz) became more strongly modulated by theta phase in CA3, and higher levels of theta-gamma modulation were maintained throughout overtraining sessions. Furthermore, the strength of theta-gamma coupling was directly correlated with the increase in performance accuracy during learning sessions. These findings suggest a role for hippocampal theta-gamma coupling in memory recall.associative memory ͉ brain rhythms ͉ local field potential B rain oscillations have been classically divided into specific frequency ranges associated with multiple cognitive processes (1, 2). Oscillations in these frequency bands may occur simultaneously and can interact with each other (3, 4), suggesting that this coupling may reflect a higher-order representation (5, 6). In one type of interaction, the phase of low-frequency rhythms modulates the amplitude of higher-frequency oscillations (3). This type of cross-frequency coupling (CFC) is called phase-amplitude modulation, and its best known example occurs in the rodent hippocampus between the theta (4-12 Hz) phase and the amplitude of gamma (30-100 Hz) oscillations (6-9). Based on this finding, theoretical work has suggested that gamma and theta oscillations coordinate in support of a neural code (10-15). According to this view, events are represented by distinct neuronal ensembles, each contained within a distinct gamma cycle, and entire episodes are encoded by a succession of event-specific gamma cycles embedded into each theta cycle (12-15). These theories are elegant and appealing, but there is a paucity of evidence linking the existence of theta-gamma coupling to behavior (but see ref. 16). Therefore, it remains unclear whether the hippocampal theta-gamma coupling possesses any functional role (13).Here, we investigated coupling between theta and gamma rhythms as rats learned which of two stimuli was rewarded depending on the environmental context in which the stimuli were presented. Learning in this type of conditional discrimination task depends on hippocampal function (17, 18). On each trial, a rat initially explored one of two environmental contexts, then the two stimuli were placed into different corners of the environment and the rat was required to choose the correct stimulus for that context to receive a reward (Fig. 1A). In our preliminary studies, we found that the initial context exploration period is essential for learning the context-dependent choice. Therefore, we focused on the prominent theta and gamma activity that is prevalent during this period. We fou...

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