DICHOTIC-LISTENING IN PATIENTS WITH PARTIAL SECTION OF THE CORPUS-CALLOSUM

Patients with a complete section of the corpus callosum have been obse rved to exhibit strong left-ear suppression when different speech stim uli are presented to both ears simultaneously (so-called dichotic list ening). Data concerning the locus of corpus callosum damage that cause s strong left-ear suppression remains scanty. In the present investiga tion, a consonant-vowel syllable dichotic listening test was given to Jive right-handed patients with partial sections of the corpus callosu m, which were located using MRI and accurately defined measurement pro cedures. The following two measurement methods were used: (i) the genu -splenium (G-S) method in which a lesion was localized in the anteropo sterior dimension relative to the total length of the corpus callosum, defined as the distance between the most anterior point of the genu t o the most posterior point of the splenium; and (ii) the rostrum-splen ium (R-S) method, which takes into account the curvature of the corpus callosum, and in which a lesion was localized relative to the total l ength of the corpus callosum, defined as the length of the curved line from the tip of the rostrum to the end of the splenium. Results were compared with scores from 50 normal control subjects. Strong left-ear suppression was observed in two patients, who had surgical sections of the posterior 15.5-18.5% of the corpus callosum as measured with the G-S method, or the posterior 20-24% of the corpus callosum as measured with the R-S method. The suppression phenomemon persisted for more th an 10 years post-surgery. On the other hand, the remaining three patie nts, who had lesions anterior to the posterior 17-28% of the corpus ca llosum as measured with the G-S method or 20-33% as measured with the R-S method exhibited no left-ear extinction. Despite the common assump tion that damage to the posterior part of the trunk of the corpus call osum causes strong left-ear suppression, the results from the G-S meth od indicated that damage to the splenium defined as the posterior one- fifth of the segment between the anterior-most and posterior-most poin ts of the corpus callosum, cause strong left-ear suppression. By the R -S method, results showed that damage to the splenium (the posterior o ne-fifth of the curvature of the corpus callosum) and possibly the par t extending to the most posterior part of the trunk (the posterior one -quarter of the curvature) causes strong left-ear suppression.