Movement-Related Changes in Hippocampal and Cortical Local Field Potentials

Abstract

Large amplitude-low frequency oscillations, such as the 4-12Hz theta rhythm prominent in the hippocampus, have been consistently shown to entrain faster, lower amplitude oscillations such as gamma. As local field potentials are primarily shaped by synaptic transmembrane potentials, high-amplitude, low-frequency interactions are thought to serve as a mechanism by which the brain facilitates coordination across regions. As theta has been reported in various cortical and subcortical brain regions, it has been suggested to support a global read-out of synaptic coordination. However, the origin of cortical theta remains a subject of debate. Decades of neurophysiology research have linked the hippocampal theta rhythm to voluntary movement. A consistent observation has been a robust correlation between the amplitude and frequency of hippocampal theta and running speed. Recently, however, it has been suggested that acceleration- and not running speed- is the dominating influence on theta frequency. As acceleration is the rate of change in velocity, a challenge is to decouple the interdependence of these variables. The goal of this dissertation is two-fold: 1) Determine the relative contributions of velocity and acceleration to hippocampal theta, and 2) Determine whether theta found in the cortex is the result of volume conduction from the hippocampus or locally generated. Study 1 investigated theta frequency and amplitude of the local-field potential recorded from the stratum pyramidale, stratum radiatum, and stratum lacunosum moleculare of the CA1 subregion, considering both speed and acceleration in tandem. In unconfined animals, it was found that running speed carries nearly all the variance in theta frequency and power, with a minute contribution from acceleration. Study 2 focused on the visual cortex, and the origins of theta rhythms present there. While visual cortical theta was tightly coupled to hippocampal theta and showed similar modulation by running velocity, conclusive evidence for local generation of theta was not found. Instead, the evidence suggests that cortical theta is comprised, at least in part, of theta volume conducted from the hippocampus. These results lay the foundation for future studies seeking to elucidate further the mechanisms underlying the generation of cortical theta and its role in inter-region communication.