Gamma band responses to target and non-target auditory stimuli in humans.

We studied the EEG oscillatory changes in the gamma band during auditory oddball paradigms in two different conditions (counting targets and reading). A time-frequency analysis was performed for standard and target stimuli. The study revealed an early (26-59 ms) phase-locked oscillation. Around 200 ms, a non-phase locked response was found for standard and target stimuli in temporal posterior electrodes. At about 360 ms, a phase-locked oscillation was observed only after target stimuli in the "counting targets" condition. During the "reading" task this late activity was not found, and energy increases were lower than during "counting" task. The early oscillation may be related to the sensory processing of the stimuli. The response around 200 ms may be involved in auditory mismatch and/or memory retrieval, and late activity is probably a P300-related response. Attention enhances all these activities.