Lateralization of dichotic speech stimuli is based on specific auditory pathway interactions: neuromagnetic evidence

The first 2 authors contributed equally to the present study Dichotic listening (DL) is a neuropsychological technique for the study of functional laterality. Based on behavioral patient studies, the ‘‘structural theory’ ’ states that lateralization of the auditory input during DL is allowed by an inhibition of the ipsilateral pathways. We aimed here at extending this theory to provide a neurophysiological basis of verbal DL. We investigated the magnetic responses of the primary auditory cortices elicited by dichotic consonant--vowel syllables. Dichotic stimuli consisted of 2 syllables pairs, a ‘‘compet-ing’ ’ one composed by syllables with high spectral overlap (/da / and /ba/) and a ‘‘noncompeting’ ’ pair (/da / and /ka/). One of the syllables in each pair was delivered at 2 intensities, whereas the other did not change. A reduced increase of source intensity in response to dichotic pairs at the 2 levels was assumed to indicate pathway inhibition effects. We obtained that the left ipsilateral pathwa