Independent rate and temporal coding in hippocampal pyramidal cells

摘要

Hippocampal pyramidal cells use temporal as well as rate coding to signal spatial aspects of the animal’s environment or behaviour. The temporal code takes the form of a phase relationship to the concurrent cycle of the hippocampal EEG theta rhythm (​; ). These two codes could each represent a different variable ^,. However, this requires that rate and phase can vary independently, in contrast to recent suggestions ^, that they are tightly coupled: both reflecting the amplitude of the cell’s input. Here we show that the time of firing and firing rate are dissociable and can represent two independent variables, viz, the animal’s location within the place field and its speed of movement through the field, respectively. Independent encoding of location together with actions and stimuli occurring there may help to explain the dual roles of the hippocampus in spatial and episodic memory or a more general role in relational/declarative memory^,.(a) Behavioural task: rat shuttles back and forth along linear track between food rewards contained in cups attached to moveable walls. (b) False-colour firing field of a place cell created from multiple runs in the eastward direction. (c) EEG theta rhythm and place cell firing (in red) for the same cell on a single eastward run. Ticks above the spikes indicate + to − zero crossings (0°/360° phase) for each theta wave, lines through theta waves indicate 270°. Bursts of spikes occur at higher than theta frequency causing each successive burst to move to an earlier phase of the theta cycle, despite initially rising, then falling firing rate. Theta cycle phase of spikes from multiple runs is plotted against position (d), time (e) and instantaneous firing rate (f) in the place field. g) Phase (adjusted for circularity, see Methods) is better correlated with location than with time or firing rate across the population of cells. Here and in subsequent figures, vertical bars represent +/− s.e.m.