Theta phase precession and the temporal representation of space : environmental and behavioural contributions to place cell firing in the rat
Place cells" fire when a rat enters a particular region of the environment - a rate code for space. As the rat traverses the "place field", the place cell fires earlier in each successive cycle of the theta EEG oscillation - a temporal code, known as "phase precession". Simultaneous recordings were made of place cell action potentials and the local EEG in rats shuttling between the ends of a stationary treadmill for food reward. Probe trials involved 1) changing the height of the walls bounding the ends of the treadmill, 2) compressing the treadmill by bringing the end walls together, and 3) changing the speed at which the treadmill moved. The effects of natural variability in running speed and firing rate were also investigated. Compressing the treadmill compressed place fields, with a proportional increase in phase precession slope. Phase precession was unaffected by other manipulations or natural variability in running speed and firing rate. The moving treadmill produced shifts in place field positions, suggesting that path integration based information plays a direct role in determining where place cells fire. O'Keefe and Recce (1993) proposed that the interference pattern of two oscillatory inputs produced phase precession. Alternatively it has been suggested (Harris et al., 2003; Buzsaki et al., 2002) that increased depolarisation as the rat approaches the centre of the field allows the cell to fire earlier in the theta cycle, where it would normally be inhibited. Here, the rate and temporal codes were shown to be dissociable, suggesting that a depolarisation model is insufficient to explain phase precession. Firing phase probably encodes information about position in space, with the rate of visual change in the environment determining the rate of one of the oscillators in a dual oscillator model. Firing rate is free to encode other variables, such as running speed.