The classification of repeating stimuli as either old or new is a general mechanism of everyday perception. However, the cortical mechanisms underlying this process are not fully understood. In general, mnemonic processes are thought to rely on changes in oscillatory brain activity across several frequencies as well as their interaction. Lower frequencies, mainly theta-band (3-7 Hz) and alpha-band (8-14 Hz) activity, are attributed to executive control and resource management, respectively; whereas recent studies revealed higher frequencies, e.g. gamma-band (> 25 Hz) activity, to reflect the activation of cortical object representations. Furthermore, low-frequency phase to high-frequency amplitude coupling (PAC) was recently found to coordinate the involved mnemonic networks. To further unravel the processes behind memorization of repeatedly presented stimuli, we applied a continuous item recognition task with up to five presentations per item (mean time between repetitions ~ 10 s) while recording high-density EEG. We examined spectral amplitude modulations as well as PAC. We observed theta amplitudes reaching a peak at second presentation, a reduction of alpha suppression after second presentation, decreased response time, as well as reduced theta-gamma PAC (3 to 7 to - 30 to 45 Hz) at frontal sites after third presentation. We conclude a shift from an explicit- to an implicit-like mnemonic processing, occurring around third presentation, with theta power to signify encoding of repetition-based episodic information and PAC as a neural correlate of the coordination of local neural networks.
Keywords: Cross-frequency coupling; EEG; Oscillations; Working memory.