Browsing by Author "Mononen, Riikka"

Memory is based on the brain’s ability to save information from the surroundings of the animal. Memory has three main functions: the encoding and consolidation of new memory traces and the retrieval. Oscillations are cyclic variations occurring in time. Neural oscillations are brain’s electric activity that can be viewed in cellular level or in the level of brain areas. The aim of this thesis is to review recent models about the role of neural oscillations in the encoding and consolidation of long term memories. Memory traces form when the connections of the neurons active in the encoding are strenghtened. New memory traces are stabilized to the long term memory in the consolidation processes. Consolidation contains both the strenghtening of local synaptic connections (synaptic consolidation) and the system consolidation, which solidifies the memory traces to the already existing information networks. According to the standard model of system consolidation new memory traces are saved simultaneously to the short term memory storage in the hippocampus and the long term storage in the cortex. The consolidation of the memory traces occurs when the memory traces in hippocampus are repeatedly reactivated which strengthens the analogous memory traces in the cortex. Neural oscillations are part of the memory processes both in awake state and sleep. In the awake state the main process is the encoding of new memories which for declarative memories is thought to occur mainly in hippocampus, mediated by for example gamma and theta oscillations. During the sleep the processes of the system consolidation are presumably in the main role, mediated by neural oscillations characteristic for different sleep stages (for example sharp-wave ripples and sleep spindles in slow wave sleep). Slow wave sleep is thought to be specifically important for the consolidation occurring in the sleep. On the other hand the role of lighter sleep stages may be greater than it has earlier been assumed. There is no complete understanding of the roles of different sleep stages and sleep time neural oscillations, and different partly contradictory, partly complementary models have been proposed.