The septal and temporal poles of the hippocampus differ markedly in their anatomical and neurochemical organization. Although it is well established that the internal representation of space is a fundamental function of hippocampal neurons, most of what is known about spatial coding in the hippocampus of freely moving animals has come from recordings from the dorsal one-third (largely for technical convenience). The present study therefore compared the spatial selectivity of CA1 neurons in the dorsal and ventral hippocampi of rats during performance of a food reinforced, random search task in a square chamber containing simple visual landmarks. Neural activity was recorded in the dorsal and ventral hippocampi of opposite hemispheres in the same rats, in many cases simultaneously. As in dorsal hippocampus, ventral CA1 units could be classified as "complex spike" (pyramidal) cells or "theta" interneurons. Both dorsal and ventral theta cells fired at relatively high rates and with low spatial selectivity in the apparatus. Of the population of complex spike cells in the ventral hippocampus, a significantly smaller number had "place fields" than in the dorsal hippocampus, and the average spatial selectivity was of significantly lower resolution than that found among dorsal hippocampal complex spike cells. Thus, a septotemporal difference of spatial selectivity was found in the CA1 field of the rat hippocampus, complementing many other anatomical and neuropharmacological studies. A number of possible functional interpretations can be suggested from these results, including a computational advantage of representing space at different scales or a preeminence of essentially nonspatial information processing in the ventral hippocampus.

Hippocampal 'place' cells and the head-direction cells of the dorsal presubiculum and related neocortical and thalamic areas appear to be part of a preconfigured network that generates an abstract internal representation of two-dimensional space whose metric is self-motion. It appears that viewpoint-specific visual information (e.g. landmarks) becomes secondarily bound to this structure by associative learning. These associations between landmarks and the preconfigured path integrator serve to set the origin for path integration and to correct for cumulative error. In the absence of familiar landmarks, or in darkness without a prior spatial reference, the system appears to adopt an initial reference for path integration independently of external cues. A hypothesis of how the path integration system may operate at the neuronal level is proposed.

Brain temperature changes accompany exploratory behavior and profoundly affect field potential amplitudes recorded in hippocampus. The waveform alterations in fascia dentata include a reduction in population spike area, which might be explained by fewer granule cells firing in response to a given stimulus or by an alteration in the size or shape of the individual action potentials. This study was designed to assess these alternate possibilities. In experiment 1, changes in the shape and firing rates of single cells recorded in the fascia dentata of awake rats were compared with changes in the population spike before and after a bout of activity. Single-unit amplitudes were significantly reduced following exploration, and there was a small (< 3%) change in unit spike-width. These changes, however, were insufficient to account, in a linear fashion, for the entire decline in the population spike. In experiment 2, radiant heat was used to manipulate brain temperature in anesthetized rats. As in the first experiment, the magnitude of change in the extracellular units was much smaller than the change in population spike amplitude. The spontaneous firing rates of the cells were also modified by brain temperature changes. In experiment 3, the polysynaptic, contralateral commissural response (which covaries with changes in the ipsilateral population spike at a fixed temperature) was measured as a function of either exploratory behavior or radiant heat. The relationship between the ipsilateral population spike and corresponding polysynaptic commissural response was altered following exploration and passive warming in a manner consistent with a reduction in net granule cell output, reduced transmission efficacy through the polysynaptic circuit, or a combination of these. Taken together these data suggest that at least two factors contribute to temperature-dependent changes in the perforant path-evoked population spikes recorded in the fascia dentata: changes in the size of individual action potentials and alterations in discharge of action potentials in response to a given stimulus.