Speech perception and auditory P300 potentials after section of the posterior half of the truncus of the corpus callosum.

A 47 year old woman who underwent surgical section of the posterior half of the corpus callosum because of meningioma in the right lateral ventricle was studied from the aspects of speech perception and auditory evoked potentials. The pure tone audiometry showed normal hearing levels for both ears. The monosyllable perception test disclosed that the left ear had lower correct percent than the right ear. The dichotic listening test demonstrated remarkable extinction in response to verbal stimuli in the left ear. Auditory brainstem, middle latency and slow vertex responses were normal for both ears, without right and left difference. P300 event related potentials were recorded by three different pairs of stimuli: pure tone (1 KHz vs. 2 KHz), words (Aka vs. Kuro), and monosyllable (PA vs. BA). Both P300 for pure tone and word stimuli to each ear showed large positive potentials within 200-500 msec but P300 for monosyllable stimuli to each ear did not show obvious potentials within 200-500 msec. These results suggest that the posterior half of the truncus of the corpus callosum can be related to transfer of speech information between right and left hemispheres.