Auditory cortex and anterior cingulate cortex sources of the early evoked gamma-band response: relationship to task difficulty and mental effort.

High frequency oscillations in the 40 Hz (gamma-band)-range are involved in the synchronization of brain regions, e.g., in cognitive functions. It has been suggested that the auditory evoked gamma-band response (GBR) is affected by attention and apart from auditory cortex activity a frontal or anterior cingulate cortex (ACC) generator could be involved. It was the aim of the present study to address three questions: (1) is there a neural generator of the early evoked GBR in the dorsal (d)ACC? (2) Are there different activation patterns in the dACC and the auditory cortex areas in response to task difficulty? (3) Is it possible to detect an influence of early ACC-gamma-band activity (GBR timeframe) to later auditory information processing (N1 timeframe)? In the present EEG/ERP-study we have investigated 30 healthy subjects using six auditory reaction tasks with increasing difficulty and mental effort demands. In the MANOVA analysis we found a significant main effect of task difficulty on both the GBR amplitude (F=7.75; p<0.001) and the auditory evoked N1 potential (F=7.00; p<0.001) with higher amplitudes in the more difficult tasks. In the LORETA region of interest (ROI) analysis, this effect was only due to increased dACC-activity during the GBR-timeframe. For the ROI analysis during the N1 timeframe, in addition to a strong effect of task difficulty in the dACC a similar main effect was found in the auditory association area 22. These findings are in line with a top-down influence of dACC-activity to the auditory association area 22 during the early evoked GBR.