Contributions of self-motion information and theta phase precession to the spatial metric of the dorsal and middle hippocampus

Abstract

This dissertation explores the relationships between self-motion, place field size, and theta phase precession with a primary focus on providing insight into the interactions between theta phase precession and place field size. The present data indicate that place field size increases along the dorsal to ventral axis of the hippocampus. Pyramidal neurons of the middle hippocampus, with larger place fields, exhibit a lower intrinsic burst frequency compared to dorsal place cells. Moreover, the firing rate of neurons in the middle hippocampus show a weaker relationship with running velocity compared with cells in the dorsal CA1 region suggesting that there is a decrease in the self-motion input to this region. By defining place fields as phase shifts up to, but not exceeding 360 degrees, the rate of phase precession is found to significantly correlate with place field size. Moreover, this definition revealed that approximately 10% of the pyramidal neurons will have place fields that overlap in space. Applying this critereon to interneurons reveals that a subset shows a similar spatial metric to those of pyramidal cells, inheriting the activity profiles and spike-phase relationships of the pyramidal cells that they are putatively monosynaptically coupled to. Finally, a reliable reconstruction of the look-ahead phenomenon provides preliminary evidence that suggests an increase in place field size as velocity increases.The results are presented to imply that the influence of the self-motion signals is graded along the dorsal-ventral axis of the hippocampus. These self-motion signals are capable of influencing the neuronal spike times of both pyramidal cells and interneurons on short-time scales of a theta cycle or less. Despite these short-time scale spike timing control mechanisms, preliminary data is presented that the influence of self-motion information with velocity is not enough to maintain a fixed place field size.