Binaural interaction in the human auditory cortex revealed by neuromagnetic frequency tagging: no effect of stimulus intensity.

Frequency tagging of magnetoencephalographic signals was recently introduced as a new tool to study binaural interaction in the human auditory cortex [Fujiki et al., J. Neurosci. 22 (2002) RC205]. As the method has potential value for assessing brain plasticity in patients with unilateral hearing deficits, we studied binaural interaction in 10 healthy adults at different intensity levels. Cortical steady-state fields were measured with a 306-channel whole-scalp neuromagnetometer to amplitude-modulated sounds (carrier frequency 1 kHz), presented monaurally or binaurally at 45, 60 and 75 dB SL. The modulation frequencies were 39.1 Hz for the right ear and 41.1 Hz for the left. During binaural stimulation, the ipsilateral responses were suppressed more than the contralateral ones in both hemispheres, and the hemispheric balance shifted towards the contralateral hemisphere for inputs from both ears. The patterns of binaural interaction were similar at all three stimulus intensities. These data could be useful in examining patients who suffer from auditory disorders as well as in revealing basic mechanism of human auditory processing.