Dichotic listening in split brain subjects: Ipsilateral suppression reconsidered

Abstract

One current view of the mechanism underlying the ear advantage in dichotic listening has two components. The first is the relative superiority of contraolateral over ipsilateral auditory pathways, the second, the supression of ipsilateral information. While the notion of superiority simply implies an advantage for contralateral information, supression suggests that the information is unavailable. Three commissurotomy patients were given dichotic tests using consonant—vowel syllables and digits, as well as a binaural fusion test, in order to evaluate their ability to use information presented to the ear ipsilateral to the language hemisphere. Consonant—vowel syllables produced complete or near complete extinction of left ear information, while the digits test revealed a substantial increase in the use of left ear information. The fusion task produced performance greater than what would be expected by probability summation of the performance of each ear alone. The data suggest that in the absence of the corpus callosum, information from contralateral and ipsilateral ears can be integrated. Furthermore, the inability of the subjects to use information presented to the ipsilateral ear appears to be a function of the degree to which the competing items are acoustically similar, rather than a general suptession mechanism.