Neural oscillatory synchronization has been proposed as a dynamic link between the different brain areas engaged into the same perceptual or cognitive process. This hypothesis has mainly been supported by studies in the visual modality, either at the unit level in animals or at a macroscopic level from human scalp EEG/MEG. In this latter case, induced gamma oscillations have been proved to be strongly associated with perception and rehearsal of coherent objects and to be modulated by attentional top-down processes. Similar oscillations have been observed in other sensory modalities but have been much less studied.
We studied the temporo-spatial characteristics and the attentional modulation of induced beta/gamma oscillations in the auditory system. For that purpose, we recorded intracranial EEG in the temporal cortex of epileptic patients in several situations, including passive listening and active discrimination tasks. Focal sources of gamma oscillations have been found in the 20-90 Hz range, between 150 and 350 ms after stimulus onset: one in the primary auditory cortex and another one in higher level areas (superior temporal gyrus, most lateral part of planum temporale). These gamma sources showed differential modulations by attention. With similar timing, stimulus induced decrease of on-going beta activity (15-20 Hz) was also observed at different focal location of the supra-temporal cortex. The respective time-courses of these beta/gamma oscillatory activities and their modulation by attention are described and discussed.