The representation of space in the rat hippocampus as revealed using new computer-based methods

While there is agreement that the hippocampus plays an important role in brain function, the details are hotly debated. Extracellular recordings from freely moving rats have provided significant but not conclusive evidence that the rodent hippocampus is specific to map based spatial navigation. The spatial hypothesis is supported by a well developed theory, and can be readily tested. These experiments require methods for accurately and simultaneously measuring the activity patterns of multiple single hippocampal neurons with high resolution in both space (location of the animal) and time of spike firing. Computer based methods are described which improve the accuracy of chronic hippocampal recording. Experiments using these methods reveal differences, from previous studies, in the characteristics of hippocampal single neurons and the relationship between extracellular electrical activity and the animal's spatial location. In particular, the receptive field of each putative hippocampal pyramidal cell (place cell) is smaller and more localised. Smaller firing regions improve the performance of an associative memory for places, perhaps located in the hippocampus (region CA3). During displacement behaviours in the rat, hippocampal EEG has a striking sinusoidal activity pattern, the theta rhythm. In contrast to published data the frequency of the theta rhythm is shown to be correlated with the speed of locomotion of the rat. In addition the frequency of the theta rhythm is shown to predict the speed of the animal's movement. The correlation between speed and EEG frequency may explain how place cell firing maintains spatial specificity with changes in movement parameters. Experiments were conducted to study the detailed firing relationship between the theta rhythm and the activity of single hippocampal neurons in the freely moving rat. The preferential firing phase of single place cells was found to shift systematically as the animal ran through the place field. This firing phase is shown to be spatially coded to the location in the place field. These results change the way in which place cell firing is interpreted