Top-down attentional processing enhances auditory evoked gamma band activity.

In contrast to animal studies, relatively little is known about the functional significance of the early evoked gamma band activity in humans. We investigated whether evoked and induced 40 Hz activity differentiate automatic, bottom-up aspects of attention from voluntary, top-down related attentional demands. An auditory novelty-oddball task was applied to 14 healthy subjects. As predicted, more evoked gamma was found for the target condition than in the two task-irrelevant conditions. Since gamma band activity was not enhanced for novel stimuli, the evoked gamma response cannot be explained with a simple concept of stimulus arousal. Neither induced gamma nor the degree of 40 Hz phase-locking were different between the experimental conditions. Taken together, our data emphasize the role of evoked gamma band activity for top-down attentional processing.