Left ear performance in dichotic listening following commissurotomy.

Contribution of the left ear stimulus to dichotic listening performance following commissurotomy was studied in five patients. In two tasks, subjects were asked to identify in writing both members of a pair of competing stimuli, either digits or CV syllables. A third task required subjects to integrate high and low frequency components of a single word presented dichotically. Left ear performance was at chance level for CV syllables but exceeded 80% for four out of five patients on the digit stimuli. All patients showed evidence of being able to utilize left ear information in the dichotic fusion task. Results indicate that apparent supression of left ear material in the dichotic task is a function of spectral-temporal overlap between competing stimuli. KIMURA’S [I, 21 neurological model of the mechanisms underlying ear asymmetries observed during dichotic listening has subsequently generated a vast quantity of research involving dichotic listening as a tool for investigating functional brain asymmetries (for reviews, see BERLIN and MCNEIL [3] and HAGGARD [4]). Much of the utility of the dichotic paradigm in this context is predicated upon the assumption that it provides a means of lateralizing an auditory input to one hemisphere, in spite of the fact that the auditory pathways project bilaterally from the level of the superior olivary nucleus upward [5]. KIMURA [2] proposed that ipsilateral ear-cortex projections are partially occluded during dichotic presentation so that the most functional routes are from each ear to the contralateral hemisphere. She attributed the right ear advantage observed among neurologically intact right handed subjects to the superior connections from the right ear to the hemisphere specialized for speech and language. In contrast, the left ear input would be projected to the left hemisphere along the occluded ipsilateral pathways, placing it at a disadvantage relative to the right ear. While KrhluRA [2] did not consider the role of the commissural pathways in mediating identification of the left ear inputs, SPARKS and GESCHWIND [6] subsequently suggested that the most likely route for the left ear items was first to the right hemisphere followed by callosal transfer to the left hemisphere speech centers. They cited two pieces of evidence in support of their argument. First, temporal lobe lesions in the nondominant hemisphere had been shown to produce deficits in the report of material presented to the contralateral ear 171. If left ear projections to the left hemisphere were primarily ipsilateral, they noted, left ear performance would not be expected to decrease as a consequence of right hemisphere damage. Second, a patient tested by SPARKS and GESCHWIND [6] who had undergone sectioning of the corpus callosum for relief of intractable epilepsy showed a right ear advantage considerably greater than that reported for neurologically intact subjects. Sparks and Geschwind used the term “extinction” to refer to the very low level of performance for left ear inputs in this patient. Again, disruption of left ear performance would not be expected if the