Evidence of sharp frequency tuning in the human auditory cortex.

The frequency tuning of the human auditory cortex was studied by masking 100-ms tones of 1 and 2 kHz by continuous white-noise maskers with frequency notches around the tone frequencies. The subjects ignored the stimuli and concentrated on a reading task. The neuronal activity elicited by the test tones in the auditory cortex was measured with a 24-channel neuromagnetometer. The masker affected the amplitude and latency of the neuromagnetic N100m response, peaking about at 100 ms after stimulus onset, in a systematic way: the wider the notch, the shorter was the latency and the larger the amplitude. The source location of N100m in the auditory cortex did not depend on the notch width. Auditory filters at 1 and 2 kHz were modelled by a single-parameter rounded-exponential [Roex(p)] filter, based on the amplitude changes of N100m. The filters revealed sharp tuning of the auditory cortex, resembling that obtained in psychoacoustical masking studies. The results demonstrate that frequency tuning of the neurons or neuron ensembles in the human auditory cortex can be studied completely noninvasively. Moreover, since the stimuli were ignored by the subjects, the filter shape is not affected by the criterion adopted by the subject in the discrimination task.