Lateralization of processing spectrally-degraded music in the auditory cortex: An fNIRS study

Music processing is one of the most complex cognitive activities that human brain performs. The mechanism of music processing when musical sounds are perceived by listeners fitted with a cochlear implant (CI) is not well understood yet. The present study examined the effect of spectrally-degrading processing (via a noise-vocoding processing to simulate CI speech processing) on the hemispheric lateralization in music processing using functional near-infrared spectroscopy (fNIRS). The hemodynamic responses in both hemispheres caused by the perception of the original, 32-channel noise-vocoded and 16-channel noise-vocoded musical sounds were measured using fNIRS. The right-hemispheric lateralization in the original, 32-channel noise-vocoded and 16-channel noise-vocoded music processing was about 72%, 67%, 56% of all participants, respectively. The activation level of the auditory cortex caused by the perception of the original music was higher than that of the noise-vocoded music, and the activation level reduced when decreasing the number of channels in the noise-vocoder processing. The activation levels in the right auditory cortex in all conditions were higher than those in the left auditory cortex; however, the difference of the contrast values between the right and left hemispheres reduced when decreasing the number of channels in the noise-vocoder processing. Results in this work indicated that the spectrally-degrading processing in CI speech processing may diminish the dominant role of the right hemisphere in music processing.