Changes in gamma-band power and phase synchronization with the difficulty of a visual oddball task.

We examined local and global differences in neuronal association patterns due to task difficulty. Induced and evoked gamma-band activity (GBA) was investigated using electroencephalograms recorded during visual oddball tasks of two difficulty levels. Significant phase synchronization (PS) in the gamma-band was determined by a double-threshold strategy, devised to detect prominent increases with respect to prestimulus baseline and surrogate data. A considerable increase in the early evoked GBA was observed for both difficulties, and this tended to be delayed for the hard task. Induced GBA was increased at the P300 latency period, but no difficulty-related change was observed. Intense PS among anterior and posterior regions along the midline was observed in a 30-38 Hz band, which was enhanced in the easy task throughout task execution, although the overall pattern of connectivity was similar for both difficulties. This may be the underlying basis of the faster and more accurate response to the easy task. The results showed that the most apparent change in the gamma-band associated with difficulty was observed in the PS pattern, not in the spectral power itself. This may imply that the difficulty-related change in cortical information processing was implemented as a change in the spatiotemporal pattern of interregional connectivity.