Concordant and discordant coding of spatial location in populations of hippocampal CA1 pyramidal cells

Pyramidal cells in the rat hippocampus commonly show place-related activity, but it has been difficult to understand the factors that govern them. A particularly important question is whether individual cells have identifiable correlates that can be manipulated independently of the correlates of other cells. Recently Tanila et al. examined the activity of small ensembles of hippocampal cells in rats running on a plus-maze with distinct intra- and extramaze cues. When the two sets of cues were rotated 90 degrees in opposite directions, some cells followed the intramaze cues, others followed the extramaze cues, and others "remapped" unpredictably; moreover, all possible combinations were seen within simultaneously recorded ensembles. In the current study, CA1 pyramidal cell population activity was recorded from four rats in a similar paradigm, using a recording system that permitted the analysis of ensembles of 4-70 simultaneously recorded units. The results were consistent with the data from the earlier study in showing an increase in remapping over time and in showing some place fields following one of the defined sets of cues while others remapped. When the possibility of random remapping was controlled for, however, the analysis did not show significant numbers of place fields following both sets of cues simultaneously. Furthermore, all rats initially showed fully concordant responses with all place fields following the local cues. For two rats, this pattern continued until a new configuration was introduced at which time all fields switched to follow the distal cues. Taken together, the results are difficult to reconcile with the hypothesis that individual hippocampal cells encode information about different subsets of cues in the environment.