We explored neuronal activity in the -range (> 40 Hz) in the primary and caudomedial auditory cortex of anesthetized macaque monkeys. Stimuli were tone bursts of 100-500 ms duration, which were presented at sound pressure levels of 40-60 dB and were varied over a wide range of frequencies. These stimuli induced -oscillations, not phase-locked to the onset of stimulation, in 465 of 616 multiunit clusters and at 321 of 422 sites at which field potentials were recorded. Occurrence of -activity was stimulus-dependent. It was mostly seen when the stimulus was at the units' preferred frequency. -activity at different sites of the primary and the caudomedial auditory field could be synchronized at near-zero phase. Synchrony depended on the spatial distance and on the receptive field similarity of pairs of units. The results indicate that stimulus-induced -oscillations originate from sources in the auditory cortex. They further suggest that -oscillations may provide a mechanism utilized in many parts of the sensory cortex, including the auditory cortex, to integrate neurons according to the similarity of their receptive fields.

Fig.: Stimulus induced -oscillations in the auditory cortex of the macaque monkey. (A) Field potentials in the caudomedial field recorded during and after 10 presentations of a 760-Hz tone burst (presence indicated by gray shading). Note the oscillatory patterns commencing about 100 ms after tone onset. The lowest trace gives the average of the 10 traces shown above. (B) Average amplitude spectra of the field potentials shown in (A) observed in different time windows. The time windows were 0-300 ms before (black line) and 100 - 400 ms after tone onset (blue line). Note the stimulus-induced enhancement of field potential frequencies between 40.7 and 71.2 Hz in the latter time window. (C) Auto correlogram of the discharges shown in (B), calculated from the time window 100 - 400 ms after tone onset. Note the oscillatory structure of the auto correlogram indicating periodic neuronal firing. The decrease of correlation with increasing delay is due to the rectangular analysis window.