Induced gamma activity during various memory tasks in the human EEG

Gruber, T.
Department of Experimental Psychology, Leipzig, Germany

It has been proposed that learning is accomplished by the formation of cell assemblies and synchronous activity among the neurons of such a network. In a series of studies we have tested the hypothesis that oscillatory activity in the Gamma Band is a signature of different memory processes. In particular, the focus was directed on rapid perceptual learning, recognition memory, and repetition priming. Using multi-channel EEG the following results were found: (1) Retrieval processes in a perceptual learning paradigm led to an increase of Gamma power and phase synchrony. (2) Gamma Band activity is crucial not only during retrieval but also for the establishment of a valid memory trace. (3) Repetition priming of familiar objects resulted in repetition suppression in the Gamma Band, possibly related to a 'sharpening' in cortical representations. Repetition of unfamiliar objects led to repetition enhancement, which suggests that repetition effects depend on the presence or absence of pre-existing object representations. From this series of studies, it was concluded that memory retrieval/formation is indeed based on networks, 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 EEG.