Some theories of memory propose that the hippocampus integrates the individual items and events of experience within a contextual or spatial framework. The hippocampus receives cortical input from two major pathways: the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC). During exploration in an open field, the firing fields of MEC grid cells form a periodically repeating, triangular array. In contrast, LEC neurons show little spatial selectivity, and it has been proposed that the LEC may provide non-spatial input to the hippocampus. Here, we recorded MEC and LEC neurons while rats explored an open field that contained discrete objects. LEC cells fired selectively at locations relative to the objects, whereas MEC cells were weakly influenced by the objects. These results provide the first direct demonstration of a double dissociation between LEC and MEC inputs to the hippocampus under conditions of exploration typically used to study hippocampal place cells.

The hippocampus receives its major cortical input from the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC). It is commonly believed that the MEC provides spatial input to the hippocampus, whereas the LEC provides non-spatial input. We review new data which suggest that this simple dichotomy between ‘where’ versus ‘what’ needs revision. We propose a refinement of this model, which is more complex than the simple spatial–non-spatial dichotomy. MEC is proposed to be involved in path integration computations based on a global frame of reference, primarily using internally generated, self-motion cues and external input about environmental boundaries and scenes; it provides the hippocampus with a coordinate system that underlies the spatial context of an experience. LEC is proposed to process information about individual items and locations based on a local frame of reference, primarily using external sensory input; it provides the hippocampus with information about the content of an experience.

Episodic memory, the conscious recollection of past events, is typically experienced from a first-person (egocentric) perspective. The hippocampus plays an essential role in episodic memory and spatial cognition. Although the allocentric nature of hippocampal spatial coding is well understood, little is known about whether the hippocampus receives egocentric information about external items. We recorded single units of rats from the lateral (LEC) and medial (MEC) entorhinal cortex, the two major inputs to the hippocampus. Many LEC neurons showed tuning for egocentric bearing of external items, whereas MEC cells tended to represent allocentric bearing. These results demonstrate a fundamental dissociation between the reference frames of LEC and MEC neural representations.

Summary Classical theories of associative memory model CA3 as a homogeneous attractor network because of its strong recurrent circuitry. However, anatomical gradients suggest a functional diversity along the CA3 transverse axis. We examined the neural population coherence along this axis, when the local and global spatial reference frames were put in conflict with each other. Proximal CA3 (near the dentate gyrus), where the recurrent collaterals are the weakest, showed degraded representations, similar to the pattern separation shown by the dentate gyrus. Distal CA3 (near CA2), where the recurrent collaterals are the strongest, maintained coherent representations in the conflict situation, resembling the classic attractor network system. CA2 also maintained coherent representations. This dissociation between proximal and distal CA3 provides strong evidence that the recurrent collateral system underlies the associative network functions of CA3, with a separate role of proximal CA3 in pattern separation.

The hippocampus is thought to represent nonspatial information in the context of spatial information. An animal can derive both spatial information as well as nonspatial information from the objects (landmarks) it encounters as it moves around in an environment. Here, we demonstrate correlates of both object-derived spatial as well as nonspatial information in the hippocampus of rats foraging in the presence of objects. We describe a new form of CA1 place cells, called landmark-vector cells, that encode spatial locations as a vector relationship to local landmarks. Such landmark vector relationships can be dynamically encoded. Of the 26 CA1 neurons that developed new fields in the course of a day’s recording sessions, in 8 cases the new fields were located at a similar distance and direction from a landmark as the initial field was located relative to a different landmark. We also demonstrate object-location memory in the hippocampus. When objects were removed from an environment or moved to new locations, a small number of neurons in CA1 and CA3 increased firing at the locations where the objects used to be. In some neurons, this increase occurred only in one location, indicating object +place conjunctive memory; in other neurons the increase in firing was seen at multiple locations where an object used to be. Taken together, these results demonstrate that the spatially restricted firing of hippocampal neurons encode multiple types of information regarding the relationship between an animal’s location and the location of objects in its environment.

Hippocampal neurons show a strong modulation by theta frequency oscillations. This modulation is thought to be important not only for temporal encoding and decoding of information in the hippocampal system, but also for temporal ordering of neuronal activities on timescales at which physiological mechanisms of synaptic plasticity operate. The medial entorhinal cortex (MEC), one of the two major cortical inputs to the hippocampus, is known to show theta modulation. Here, we show that the local field potentials (LFPs) in the other major cortical input to the hippocampus, the lateral entorhinal cortex (LEC), show weaker theta oscillations than those shown in the MEC. Neurons in LEC also show weaker theta modulation than that of neurons in MEC. These findings suggest that LEC inputs are integrated into hippocampal representations in a qualitatively different manner than the MEC inputs. Furthermore, MEC grid cells increase the scale of their periodic spatial firing patterns along the dorsoventral axis, corresponding to the increasing size of place fields along the septotemporal axis of the hippocampus. We show here a corresponding gradient in the tendency of MEC neural firing to skip alternate theta cycles. We propose a simple model based on interference of delta oscillations with theta oscillations to explain this behavior. I N T R O D U C T I O NThe theta rhythm (6 -10 Hz) is the most prominent oscillatory activity in the hippocampus of freely moving rats. Neurons in the hippocampus show entrainment to these oscillations, with the preferred phase of firing advancing as the animal moves through the place field of the neuron. This phase precession is thought to be a temporal code, which organizes the spikes of neurons with overlapping place fields in subtheta timescales (O'Keefe and Recce 1993;Skaggs et al. 1996). These timescales are within the operating time window of spike timing dependent plasticity (Bi and Poo 1998). Entrainment of spiking activity to the ongoing theta rhythm is a prerequisite for this phase precession to operate. Rate and temporal codes have been shown to be dissociable and hypothesized to independently code for different variables in the hippocampus (Huxter et al. 2003;O'Keefe and Burgess 2005). Inputs that show theta-modulated activity are likely to be involved in the generation of this temporal code, whereas inputs that are not theta-modulated might contribute to the rate code in the hippocampus.Medial entorhinal cortex (MEC) and lateral entorhinal cortex (LEC) are the two major cortical inputs to the hippocam- Local field potentials (LFPs) in MEC show theta oscillations under a variety of conditions (Alonso and Garcia-Austt 1987a;Mitchell and Ranck Jr 1980). In contrast, the prevalence of theta oscillations in LEC in behaving animals is largely unknown. With respect to theta modulation of unit spiking, rat MEC neurons are known to exhibit such theta modulation Hafting et al. 2008;Ranck Jr 1973;Stewart et al. 1992). Alonso and Garcia-Austt (1987b) and Frank et al. (2001) showed theta modulat...

The medial temporal lobe (MTL) is involved in mnemonic processing. The perirhinal cortex (PRC) plays a role in object recognition memory, while the hippocampus is required for certain forms of spatial memory and episodic memory. The lateral entorhinal cortex (LEC) receives direct projections from PRC and is one of the two major cortical inputs to the hippocampus. The transformations that occur between PRC and LEC neural representations are not well understood. Here, we show that PRC and LEC had similarly high proportions of neurons with object-related activity (PRC 52/94; LEC 72/153), as expected from their locations in the “what” pathway into the hippocampus. However, LEC unit activity showed more spatial stability than PRC unit activity. A minority of LEC neurons showed stable spatial firing fields away from objects; these firing fields strongly resembled hippocampal place fields. None of the PRC neurons showed this place-like firing. None of the PRC or LEC neurons demonstrated the high firing rates associated with interneurons in hippocampus or medial entorhinal cortex, further dissociating this information processing stream from the path-integration based, movement-related processing of the medial entorhinal cortex and hippocampus. These results provide evidence for nonspatial information processing in the PRC-LEC pathway, as well as showing a functional dissociation between PRC and LEC, with more purely nonspatial representations in PRC and combined spatial-nonspatial representations in LEC.

We performed simultaneous single-neuron recordings from the hippocampus and the olfactory bulb of anesthetized, freely breathing rats. Odor response properties of neurons in the olfactory bulb and hippocampus were characterized as firing rate changes or respiration-coupled changes. A panel of five odors was used. The rats had not been exposed to the odors on the panel before the experiment. The olfactory bulb and hippocampal neurons responded to repeated odor presentations in two ways: first, by changes in firing rate, and second, by respiratory tuning changes. Approximately 60% of bulbar neurons, 48% of hippocampal CA1 neurons, and 12% of hippocampal CA3 neurons showed statistically significant responses. None of the odor-responsive neurons in either the bulb or hippocampus responded to all of the odors on the panel. Repeated 10 sec odor stimuli presented at the intervals of 20, 30, 60, 110, and 160 sec were used to analyze the effect of the interval on odor response properties of the recorded neurons. Bulbar neurons were relatively nonselective for odor interval. Hippocampal neurons showed unexpected selectivity for the interval between repeated odor presentations. CA1 and CA3 neurons responded to only one to three of the intervals in the range. On the basis of these findings, we postulate that the hippocampus has the ability to keep track of the time elapsed between consecutive odor stimuli. This may act as a neuronal substrate for habituation and for complex tasks such as odor-guided navigation.