Neuronal mechanisms of repetition priming in occipitotemporal cortex: spatiotemporal evidence from functional magnetic resonance imaging and electroencephalography.
Repeated stimulus presentation (priming) is generally associated with a reduction in neuronal firing, macroscopically mirrored by a decrease in oscillatory electrophysiological markers as well as reduced hemodynamic responses. However, these repetition effects seem to be dependent on stimulus familiarity. We investigate the spatiotemporal correlates of repetition priming in cortical word-recognition networks and their modulation by stimulus familiarity (i.e., words vs pseudowords). Event-related functional magnetic resonance imaging results show reduced activation for repeated words in occipitotemporal cortical regions. Electroencephalogram recordings reveal a significant reduction of induced gamma-band responses (GBRs) between 200 and 350 ms after stimulus onset, accompanied by a decrease in phase synchrony between electrode positions. Pseudoword repetition, in contrast, leads to an activation increase in the same areas, to increased GBRs, and to an increased phase coupling. This spatiotemporal repetition by stimulus type interaction suggests that qualitatively distinct mechanisms are recruited during repetition priming of familiar and unfamiliar stimuli. Repetition of familiar stimuli leads to a "sharpening" of extrastriate object representations, whereas the repetition of unfamiliar stimuli results in the "formation" of a novel cortical network by means of synchronized oscillatory activity. In addition to isolating these mechanisms, the present study provides the first evidence for a possible link between induced electrophysiological and hemodynamic measures of brain activity.