Novel space and the hippocampal formation : modulaton of theta and gamma oscillations

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.