Modulation of oscillatory brain activity and evoked potentials in a repetition priming task in the human EEG

Cortical object representations seem to require the formation of neural cell assemblies. The physiological correlate of cell assembly activity may be seen in synchronized neural activity in the gamma band range. The improvement in perceiving and identifying an object by experience is commonly referred to as repetition priming. One possible neural mechanism for repetition priming is 'repetition suppression' within a cell assembly coding the stimulus. The present electroencephalogram study was designed to investigate oscillatory brain activity when line drawings of concrete objects were repeated either immediately after a first presentation or after intervening a number of different stimuli. Results showed a broad posterior distribution of induced gamma band responses (GBRs) after the initial picture presentation. Repeated presentations of the same picture led to a significant decrease of induced gamma power. Furthermore, repeated presentations of the same object resulted in a decrease in phase synchrony between distant electrode sites. No significant repetition effects were found in the alpha or beta frequency range. The event-related potential (ERP), which was also modulated by priming, showed a different scalp distribution compared with induced GBRs. In addition, ERP repetition effects decayed at larger intervals between initial and repeated presentations, whereas induced GBRs were not modulated as a function of stimulus lag. We concluded that the decrease in amplitude of induced GBRs and the reduction of gamma phase synchrony between pairs of electrodes after repeated picture presentations might be linked to a 'sharpening' mechanism within a cell assembly representing an object.