Neuromagnetic study on localization of speech sounds

Spatial processing of speech sounds by the human auditory cortex was studied measuring neuromagnetic responses utilizing magnetoencephalography (MEG). Realistic spatial sound environment was produced using modern stimulus generation methodology utilizing head-related transfer functions (HRTFs). In order to compare localization of speech sounds to that of nonspeech, the stimulus set involved three different stimulus types: 1) a semi-synthetic /a/-vowel, 2) a pseudo-vowel composed as a sum of sinusoids and 3) a wide band noise burst. Stimuli were filtered through HRTFs of eight horizontal equally spaced directions. The most prominent response, the cortically generated N1m, was investigated above the left and right hemisphere. We found, firstly, that cortical activity reflecting the processing of spatial sound stimuli was more pronounced in the right than in the left hemisphere. Secondly, we found that N1m amplitudes were largest for the /a/-vowel. However, behaviour of the N1m amplitude elicited by the pseudo-vowel was relatively similar to that of the /a/-vowel.