Theta and gamma coherence: Measuring changes in global and local synchrony within the hippocampal formation

The hippocampus is important for creating memories. Complex cognitive processes such as memory require the coordinated firing of large ensembles of neurons on millisecond timescales. Theta (4-12 Hz) and gamma (40-100 Hz) oscillations are rhythmic extracellular voltage fluctuations reflecting synchronized cellular and network activity. Theta and gamma act in concert to synchronize networks of neurons at timescales relevant for synaptic plasticity. These oscillations are strongly tied to the behavior of the animal and are most prominent during awake, attentive behavior, and during REM sleep. Synchronized theta and gamma oscillations across brain structures are related to cognitive performance. Here we use an animal model (rat) to investigate changes in hippocampal synchrony. Animal models provide a means of exploring the degree of synchrony within and across brain structures, and allow for an assessment of changes in synchrony induced via pharmacological manipulations and sensory and motor events. ^ The overarching goal of this thesis is to use theta and gamma coherence at distinct sites within the hippocampus as a metric of temporal synchronization of spatially segregated neuronal populations. The major factors determining hippocampal synchrony are: (1) the topography of the anatomical projections from the EC, (2) the extrinsic and intrinsic glutamatergic projections of the hippocampus, and (3) the neuromodulatory influences promoted via the medial septum. We investigate the contribution these factors to the local and global synchrony of the HF in the rat by: (1) examining the variability of theta and gamma coherence as a function of anatomical connectivity, (2) modulating the activity of the NMDA glutamate receptor, and (3) modulating the activity of medial septal inputs. The findings reported suggest theta and gamma are altered independently following pharmacological manipulations that lead to memory impairments, and suggest the potential to dissect the relationship between EEG changes and cognitive phenomenon.