O'Keefe and Recce [ I 9931 Hippocampus 3:317-330 described an interaction between the hippocampal theta rhythm and the spatial firing of pyramidal cells in the CAI region of the rat hippocampus: they found that a cell's spike activity advances to earlier phases of the theta cycle as the rat passes through the cell's place field. The present study makes use of large-scale parallel recordings to clarify and extend this finding in several ways: 1) Most CA1 pyramidal cells show maximal activity at the same phase of the theta cycle. Although individual units exhibit deeper modulation, the depth of modulation of CAI population activity i s about 50%. The peak firing of inhibitory interneurons in CAI occurs about 60" in advance of the peak firing of pyramidal cells, but different interneurons vary widely in their peak phases. 2) The first spikes, as the rat enters a pyramidal cell's place field, come 90"-120" after the phase of maximal pyramidal cell population activity, near the phase where inhibition is least. 3) The phase advance is typically an accelerating, rather than linear, function of position within the place field. 4) These phenomena occur both on linear tracks and in two-dimensional environments where locomotion is not constrained to specific paths. 5) In two-dimensional environments, place-related firing is more spatially specific during the early part of the theta cycle than during the late part. This is also true, to a lesser extent, on a linear track. Thus, spatial selectivity waxes and wanes over the theta cycle. 6 ) Granule cells of the fascia dentata are also modulated by theta. The depth of modulation for the granule cell population approaches 1 OO%, and the peak activity of the granule cell population comes about 90" earlier in the theta cycle than the peak firing of CAI pyramidal cells. 7) Granule cells, like pyramidal cells, show robust phase precession. 8) Cross-correlation analysis shows that portions of the temporal sequence of CA1 pyramidal cell place fields are replicated repeatedly within individual theta cycles, in highly compressed form. The compression ratio can be as much as 1O:l.These findings indicate that phase precession is a very robust effect, distributed across the entire hippocampal population, and that it is likely to be inherited from the fascia dentata or an earlier stage in the hippocampal circuit, rather than generated intrinsically within CA1. It i s hypothesized that the compression of temporal sequences of place fields within individual theta cycles permits the use of long-term potentiation for learning of sequential structure, thereby giving a temporal dimension to hippocampal memory traces.

The correlated activity of rat hippocampal pyramidal cells during sleep reflects the activity of those cells during earlier spatial exploration. Now the patterns of activity during sleep have also been found to reflect the order in which the cells fired during spatial exploration. This relation was reliably stronger for sleep after the behavioral session than before it; thus, the activity during sleep reflects changes produced by experience. This memory for temporal order of neuronal firing could be produced by an interaction between the temporal integration properties of long-term potentiation and the phase shifting of spike activity with respect to the hippocampal theta rhythm.

When rats forage for randomly dispersed food in a high walled cylinder the firing of their hippocampal "place" cells exhibits little dependence on the direction faced by the rat. On radial arm mazes and similar tasks, place cells are strongly directionally selective within their fields. These tasks differ in several respects, including the visual environment, configuration of the traversable space, motor behavior (e.g., linear and angular velocities), and behavioral context (e.g., presence of specific, consistent goal locations within the environment). The contributions of these factors to spatial and directional tuning of hippocampal neurons was systematically examined in rats performing several tasks in either an enriched or a sparse visual environment, and on different apparati. Place fields were more spatially and directionally selective on a radial maze than on an open, circular platform, regardless of the visual environment. On the platform, fields were more directional when the rat searched for food at fixed locations, in a stereotypic and directed manner, than when the food was scattered randomly. Thus, it seems that place fields are more directional when the animal is planning or following a route between points of special significance. This might be related to the spatial focus of the rat's attention (e.g., a particular reference point). Changing the behavioral task was also accompanied by a change in firing location in about one-third of the cells. Thus, hippocampal neuronal activity appears to encode a complex interaction between locations, their significance and the behaviors the rat is called upon to execute.

The neocortex, hippocampus, and several other brain regions contain populations of excitatory principal cells with recurrent connections and strong interactions with local inhibitory interneurons. To improve our understanding of the interactions among these cell types, we modeled the dynamic behavior of this type of network, including external inputs. A surprising finding was that increasing the direct external inhibitory input to the inhibitory interneurons, without directly affecting any other part of the network, can, in some circumstances, cause the interneurons to increase their firing rates. The main prerequisite for this paradoxical response to external input is that the recurrent connections among the excitatory cells are strong enough to make the excitatory network unstable when feedback inhibition is removed. Because this requirement is met in the neocortex and several regions of the hippocampus, these observations have important implications for understanding the responses of interneurons to a variety of pharmacological and electrical manipulations. The analysis can be extended to a scenario with periodically varying external input, where it predicts a systematic relationship between the phase shift and depth of modulation for each interneuron. This prediction was tested by recording from interneurons in the CA1 region of the rat hippocampus in vivo, and the results broadly confirmed the model. These findings have further implications for the function of inhibitory and neuromodulatory circuits, which can be tested experimentally.

Populations of hippocampal neurons were recorded simultaneously in rats shuttling on a track between a fixed reward site at one end and a movable reward site, mounted in a sliding box, at the opposite end. While the rat ran toward the fixed site, the box was moved. The rat returned to the box in its new position. On the initial part of all journeys, cells fired at fixed distances from the origin, whereas on the final part, cells fired at fixed distances from the destination. Thus, on outward journeys from the box, with the box behind the rat, the position representation must have been updated by path integration. Farther along the journey, the place field map became aligned on the basis of external stimuli. The spatial representation was quantified in terms of population vectors. During shortened journeys, the vector shifted from an alignment with the origin to an alignment with the destination. The dynamics depended on the degree of mismatch with respect to the full-length journey. For small mismatches, the vector moved smoothly through intervening coordinates until the mismatch was corrected. For large mismatches, it jumped abruptly to the new coordinate. Thus, when mismatches occur, path integration and external cues interact competitively to control place-cell firing.When the same box was used in a different environment, it controlled the alignment of a different set of place cells. These data suggest that although map alignment can be controlled by landmarks, hippocampal neurons do not explicitly represent objects or events.