Background acoustic noise and the hemispheric lateralization of speech processing in the human brain: magnetic mismatch negativity study.

The present study explored effects of background noise on the cerebral functional asymmetry of speech perception. The magnetic equivalent (MMNm) of mismatch negativity (MMN) elicited by consonant-vowel syllable change presented in silence and during background white noise was measured with a whole-head magnetometer. It was found that in silence MMNm to speech stimuli, registered from the auditory cortex, was stronger in the left than in the right hemisphere. However, when speech signals were presented in white noise background, MMNm in the left hemisphere diminished while that in the right hemisphere increased in amplitude and dipole moment. These results confirm that in silence, speech signals are mainly discriminated in the left hemisphere's auditory cortex. However, in noisy conditions the involvement of the left hemisphere's auditory cortex in speech discrimination is considerably decreased, while that of the right hemisphere increases.