Learned Hippocampal Sequential Coding through Phase Precession

The hippocampal formation is necessary for spatial learning and memory in rats. One aspect of this cognitive function, the ability to remember path sequences, is thought to be achieved through phase precession. The two components of phase precession are a strong theta rhythm (8-12 Hz) in the local field potential and place-selective neurons, which fire selectively while the rat is in a particular area or “place field” of an enclosed arena. As a rat enters and leaves each place field, the respective neuron will fire most strongly in the center and less strongly in the periphery, but in addition to changing firing rate the cell will also fire at progressively earlier phases of the theta rhythm. This ultimately allows for the recent path of the rat to be represented by neuronal firing within one theta cycle, possibly reducing the time between place cell firing to allow for effective spike timing dependent plasticity. We test whether phase precession is required for sequence learning in a small recurrent network of Izekevich neurons with place fields either around the circumference of a closed circular track or forming a grid across a closed square arena with synaptic weights updated according to a spike timing dependent plasticity rule. In either environment, we find that phase precession of place cell responses, as opposed to purely spatially tuned responses, is required for heteroassociative recall and spatial sequence learning.