Neuronal correlates of repetition priming of frequently presented objects: insights from induced gamma band responses.

The activation of cortical object representations requires the integration of dispersed cortical areas, signified by induced oscillatory bursts of activity > 20 Hz (induced Gamma Band Responses; iGBRs) at approximately 300 ms after stimulus onset. A well established marker of the functional dynamics within such cell assemblies is the suppression of iGBR amplitudes after the repetition of familiar stimuli. This effect is commonly interpreted as a signature of 'sharpening' processes within conceptual networks, which are behaviourally mirrored in repetition priming effects. Furthermore, it was demonstrated that the repetition of unfamiliar stimuli leads to iGBR increases indicating the 'formation' of a new cell assembly. A limitation of previous experiments was that only small numbers of repetitions were used. Thus, in the present EEG study we presented familiar and unfamiliar stimuli 10 times. We were able to replicate sharpening effects within conceptual networks representing familiar stimuli. Furthermore, we observed a gradual increase of iGBRs elicited by repeated unfamiliar stimuli. Interestingly, this formation effect did not turn into a sharpening effect after many repetitions (i.e. after an unfamiliar stimulus became familiar). Thus, we conclude that sharpening and formation effects rely on qualitatively different networks representing familiar and unfamiliar material.