Title

Novel Space and the Hippocampal Formation: Modulation of Theta and Gamma Oscillations

Date of Completion

January 2012

Keywords

Biology, Neuroscience|Psychology, Behavioral|Psychology, Physiological

Degree

Ph.D.

Abstract

The hippocampal formation [entorhinal cortex (EC), hippocampus (HPC)] is essential for memory formation. One important component in the establishment of a memory is the ability to distinguish between familiar and novel stimuli. A variety of evidence indicates that novel experience engages hippocampal physiology and promotes successful encoding. Hippocampal theta and gamma local field potentials reflect the dynamic synchronization of afferent inputs impinging upon hippocampal neurons. Importantly, oscillations within these networks allow for a temporal window in which neurons can fire, and in turn encode, store, and retrieve information. Historically the hippocampus has been viewed as a homogenous structure with any transverse subcomponent containing equivalent circuits. However, mounting behavioral, anatomical, and physiological data have put this view into question. Specifically, disparate behavioral deficits have been identified following lesions of the septal vs. temporal hippocampus. Further, neuroanatomical evidence seems to support a more modular view of the HPC. This is most evident within the circuits connecting the EC and the HPC. This dissertation looked to examine hippocampal physiology across anatomically distinct sub-regions of the HPC and EC. Specifically, these experiments tested the hypothesis that a novel spatial environment would increase theta power and coherence across the long axis of the hippocampus and within the EC. We compared theta and gamma local field potential signals (power, coherence) within the dentate (DG), CA1 and EC while rats navigated a runway in a familiar environment, on a modified path and in a novel space. Locomotion in novel space was related to increases in theta and gamma power at all CA1 and DG sites. The increase in theta and gamma power was concurrent with an increase in theta and gamma coherence across the long axis of CA1; however, there was a significant decrease in theta coherence across the long axis of the DG. Within the EC there was a novelty related increase in both theta and gamma coherence and gamma power, however theta power decreased. These findings suggest that during a novel experience the entire HPC network receives a more coherent input, which may contribute to the successful encoding of novel sensory stimuli. ^