Abstract
Mnemonic operations are vital in our everyday life and comprise a vast array of sub-processes that need to be co-ordinated constantly to ensure smooth functioning. The goal of this work was to investigate the superimposed control component responsible for the co-ordination of those individual sub-processes. Furthermore, by using neuroscientific methods like electroencephalography (EEG) and transcranial magnetic stimulation (TMS), the underlying oscillatory signatures were explored and their behavioural relevance tested. It will be shown that increase in theta frequency (4-8 Hz) over medial frontal cortical areas is a local indicator of increasing task demand and task complexity. Moreover, findings will be presented that indicate that the phase of this local oscillation (frontal-midline theta, FMtheta) enables task-relevant processes in posterior cortical areas (represented by increased activity in the gamma frequency range, 30-80 Hz) to flexibly access prefrontal cognitive resources. This mechanism is implemented in a way that bursts of posterior gamma frequency are locked to specific phases of a FMtheta cycle. The higher the task complexity, i.e. the more prefrontal control processes are needed for its execution, the closer to the excitatory phase of FMtheta (trough) the posterior gamma bursts are nested. In contrast, when only very little cognitive control needs to be employed, the posterior fast frequency bursts are locked near the inhibitory FMtheta phase (peak) rather. By using repetitiveTMS it was furthermore possible to show the behavioural relevance of this mechanism and how the disruption thereof has a highly selective and immediate deteriorating effect on working memory performance. Moreover, it will be demonstrated that this mechanism does not only depend on task difficulty but can be influenced and controlled voluntarily by information prioritisation. Finally, based on these findings a model will be introduced that could potentially explain the mechanism of such flexible allocation of and access to prefrontal control processes.