Hemispheric balance in processing attended and non-attended vowels and complex tones

We compared cortical processing of attended and non-attended vowels and complex tones, using a whole-head neuromagnetometer, to test for possible hemispheric differences. Stimuli included vowels [a] and [i], spoken by two female Finnish speakers, and two complex tones, each with two pure tone components corresponding to the first and second formant frequencies (F1-F2) of the vowels spoken by speaker 1. Sequences including both vowels and complex tones were presented to eight Finnish males during passive and active (phoneme/speaker/complex tone identification) listening. Sequences including only vowels were presented to five of the subjects during passive listening and during a phoneme identification task. The vowel [i] spoken by speaker 1 and the corresponding complex tone were frequent, non-target stimuli. Responses evoked by these frequent stimuli were analyzed. Cortical activation at approximately 100 ms was stronger for the complex tone than the vowel in the right hemisphere (RH). Responses were similar during active and passive listening. Hemispheric balance remained the same when the vowel was presented in sequences including only vowels. The reduction of RH activation for vowels as compared with complex tones indicates a relative increase of left hemisphere involvement, possibly reflecting a shift towards more language-specific processing.