ABSTRACT
Based on findings from animal research, the hypothesis is examined that the encoding and retrieval of new information is reflected by a task related increase (synchronization) in theta power. The findings from several studies support this hypothesis, and show that only those items were correctly remembered that exhibited a significant increase in theta power during the encoding process. During the actual retrieval processes, a significant increase in band power was found only for correctly remembered words. In contrast to the theta band, remembered and not-remembered items revealed a different and more complex pattern of results in the lower and upper alpha band. These and similar findings support the view that synchronous neural activity within the theta frequency range reflects the encoding and retrieval of new (episodic) information, and is induced in the cortex via hippocampo-cortical feedback loops. KEYWORDS: Theta; Brain oscillations; Pace maker; Encoding; LTP; Episodic memory
1. INTRODUCTION
In contrast to the alpha rhythm, most of what is known about the theta rhythm stems from animal research. Therefore, the most important properties of the hippocampal EEG will be reviewed first. Most interestingly, when studying results of experiments linking animal behaviour with the hippocampal EEG, it becomes evident that hippocampal theta power increases with increasing task demands just as does theta band power in the human scalp EEG. With respect to the functional meaning of the theta rhythm, evidence from divergent fields such as electrophysiology, anatomy, and neuropsychology point towards a specific relationship between hippocampal theta activity and the encoding of new information. Animal research shows that theta, as the dominant rhythm in the hippocampal formation, is closely related to the strength of long-term potentiation (LTP), which is generally considered a synaptic memory mechanism for the encoding of new
information. Scalp recordings from human subjects show that the encoding of new information, and the retrieval from short-term memory (STM), is accompanied by a significant increase in theta power. The reviewed evidence suggests that theta oscillations may provide the basis for the encoding of new information. It may even be suggested that-in a functional sense-memory traces are actually stored in the form of nested oscillations within the theta frequency range (Lisman and Idiart, 1995; Klimesch, 1996).
