Phase organization of network computations

Abstract

Coupled oscillations are hypothesized to organize the processing of information across distributed brain circuits. This idea is supported by recent evidence, and newly developed techniques promise to put such theoretical framework to mechanistic testing. We review evidence suggesting that individual oscillatory cycles constitute a functional unit that organizes activity in neural networks, and that oscillatory phase (defined as the fraction of the wave cycle that has elapsed relative to the start of the cycle) is a key oscillatory parameter to implement the functions of oscillations in limbic networks. We highlight neural manipulation techniques that currently allow for causal testing of these hypotheses.

Figure 1. Oscillatory cycles and phase organize place cell activity in hippocampus

The overlapping place fields of five principal cells are represented schematically and color-coded. The corresponding vertical bars represent the spikes from each place cell as the animal traverses the receptive fields from left to right. Below is the raw local field potential (LFP), overlaid with the same signal filtered in the theta band (4–12Hz, black), to illustrate the theta rhythm that dominates during locomotion and exploration. As the animal moves across each place field, the spikes of that cell occur at progressively earlier phases of the hippocampal theta rhythm (phase precession; e.g., green spikes). As a consequence, for any given theta cycle, the spikes of place fields extending behind (‘A’) and ahead (‘C’) of the animal occur at different phases, effectively generating a compressed representation of a spatial trajectory (theta sequence, spikes in ‘ABC’). Note that, without the phase information, the position within a receptive field could not be estimated from the spikes of that cell alone; also, without theta sequences, the representation of the same spatial trajectory would require computation over several cycles.