Inter-regional synchronization in the hippocampal system by oscillations: unit and LFP studies

This dissertation investigated how several different types of oscillation organize neural activity in the hippocampus and the entorhinal cortex. I used recordings performed by myself and others to explore the nature of the ripple, sharp wave, fast gamma, theta, and spindle oscillations, the manner in which these oscillations propagate through the hippocampal network, and how behavior affects the properties of these oscillations. The first study concerned two types of high frequency oscillation in CA1: fast gamma and ripples, their relationship to sharp waves, the nature of their emergence in CA3 and propagation to CA1, and their interaction with slow oscillations during sleep. I found that the ripple oscillation emerges de novo in CA1 rather than being transmitted wave-by-wave from CA3 to CA1. I also found that there is an inverted-U shaped relationship between the peak frequency of ripple oscillations and the amplitude of sharp waves. The second study extended the observations of the first study by taking the behavioral state of the animal into account. I found that the ripples that occur intermidst active behavior oscillate at a higher frequency than those that occur during sleep and quiescence, and that the relationship between sharp wave amplitude and ripple frequency differs depending on behavioral state. I also found that ripples that are detected during running are due to recording artifacts, whereas ripples occasionally occur during REM sleep. The third study examined the nature of sleep spindles in the hippocampus, and is an exploration (and ultimately, a rejection) of the hypothesis that the mechanisms of sleep spindles are the same. I found that many neurons prefer different oscillation phases of theta and spindles, suggesting that different sets of inputs drive neurons. I also found that principal cells are not rhythmically entrained by spindle oscillations; this and the lack of phase precession during spindles led to the conclusion that spindles should not be considered as a special case of theta.Ph. D.Includes bibliographical referencesIncludes vitaby David W. Sulliva