Oscillatory brain activity in the human EEG during indirect and direct memory tasks.

Up to today, the neuronal bases of activating and establishing a memory trace are not well understood. Several findings point towards the idea that the activation of an engram is mirrored in induced oscillatory bursts in the gamma frequency range (> 20 Hz; induced gamma band responses; iGBRs). In the present study, we further investigated this hypothesis. Volunteers performed two tasks on repeated pictures of familiar and unfamiliar objects. They either made a familiarity decision with repetition being task irrelevant (indirect memory task), or a recognition judgment with repetition being task relevant (direct memory task). Furthermore, we complemented iGBR analyses by investigating other brain responses known to be modulated by mnemonic manipulations, namely, evoked gamma oscillations, lower frequency oscillations, and event-related potentials (ERPs). The results obtained for the indirect task replicated previous findings of repetition suppression of iGBRs for repeated familiar stimuli and an increase of iGBRs for repeated unfamiliar objects. These effects might be linked to the 'sharpening' of a cell assembly representing a familiar object and to the formation of a new object representation for unfamiliar stimuli, respectively. In contrast, the direct task revealed no repetition-related modulations of iGBRs. Thus, modulations of iGBRs are not a mere automatic consequence of repeated stimulus processing but might rather mirror changes within cortical object representation according to use-dependent plasticity principles. Furthermore, evoked gamma responses, lower frequency bands, and late components of the ERPs correlated with more specific subprocesses during mnemonic functioning.