Memory formation by neuronal synchronization.

Cognitive functions not only depend on the localization of neural activity, but also on the precise temporal pattern of activity in neural assemblies. Synchronization of action potential discharges provides a link between large-scale EEG recordings and cellular plasticity mechanisms. Here, we focus on the role of neuronal synchronization in different frequency domains for the subsequent stages of memory formation. Recent EEG studies suggest that synchronized neural activity in the gamma frequency range (around 30-100 Hz) plays a functional role for the formation of declarative long-term memories in humans. On the cellular level, gamma synchronization between hippocampal and parahippocampal regions may induce LTP in the CA3 region of the hippocampus. In order to encode spatial locations or sequences of multiple items and to guarantee a defined temporal order of memory processing, synchronization in the gamma frequency range has to be accompanied by a stimulus-locked phase reset of ongoing theta oscillations. Simultaneous gamma- and theta-dependent plasticity leads to complex learning rules required for realistic declarative memory formation. Subsequently, consolidation of declarative memories may occur via replay of newly acquired patterns in so-called sharp wave-ripple complexes, predominantly during slow-wave sleep. These irregular bursts induce longer lasting forms of synaptic plasticity in output regions of the hippocampus and in the neocortex. In summary, synchronization of neural assemblies in different frequency ranges induces specific forms of cellular plasticity during subsequent stages of memory formation.