Oscillatory Brain Activity as Underlying Neural Mechanism of Human Memory

Abstract

In 1949, the famous Psychologist Donald Hebb proposed that learning is accomplished by dynamical binding of cell assemblies. Neuronal synchronization in the gamma band range is discussed as a plausible mechanism to integrate the activity within and between the elements of such a network. The main goal of the studies described in this thesis, was to examine if oscillatory activity in the human electroencephalogram is a signature of different memory processes. In particular the focus was directed on the formation of a cell assembly due to operant learning, implicit recall of a cell assembly due to perceptual learning or perceptual priming, repetition suppression (repetition priming), explicit recall due to short-term memory, and associative learning. Using multi-channel electroencephalography the following results were found: Firstly, the formation of a cell assembly led to an integration of activity within and between task specific modalities. Secondly, implicit recall processes led to an activation of a cell assembly, which stores the features of a percept close to the cortical areas that mediate the perception of those features. Finally, in contrast to implicit memory, explicit forms of memory recall activated a cell assembly, which included the storage sites of a learned percept and related higher order monitoring areas.From this series of studies, it was concluded that memory recall and formation is indeed based on cell assemblies, which are established by neuronal synchrony amongst their elements. Induced gamma band responses and phase synchrony are a signature of activity within such a cell assembly in the human electroencephalogram.