Verbal and Non-Verbal Cerebral Processing in Man for Audition

Abstract

Hemispheric asymmetries were investigated with various auditory techniques in several groups of subjects. The first study was a dichotic listening experiment in which two separate musical chords were presented simultaneously one to each ear of right-handed males. The subjects were required to listen to the chord stimuli and then recognize them from a multiple choice of four chords heard immediately following the dichotic presentation. More chords were recognized from the left ear than from the right implying right cerebral dominance for this task. In a similar test, dichotic presentation of melodies showed no difference between the ears. It was hypothesized that the subjects in this case were identifying the tune segments on the basis of rhythmic rather than pitch cues. It was suggested that the right hemisphere is superior to the left in processing stimuli that are non-temporal. Musical expression was investigated in patients who had transiently lost the function of one hemisphere following intracarotid amytal injection. It was observed that after right hemisphere depression, singing was devoid of pitch at a time when speech was only minimally disturbed. Conversely, singing was much less affected than speech after left hemisphere depression. This differential effect of amytal depression is supportive of the idea that the right hemisphere is used for pitch control in singing whereas the left hemisphere is used expressly for speech. Singing was also studied in two young patients with surgical hemispherectomies for non-infantile causes. One patient who had a right hemisphere removal with no evidence of aphasia, sang most songs poorly. He also failed pitch discrimination tests wherein he could not distinguish two tones that were separated by an interval of less than one musical step. Another patient with a left hemispherectomy produced the opposite results. She had great difficulties in expressive speech yet could sing with excellent pitch control and intonation. These cases support the previous conclusion that the right hemisphere is necessary for correct pitch production in singing. Dichotic listening studies on patients with complete surgical division of the corpus callosum indicated that the right hemisphere also had some capacity to understand and manually express verbs and verbal commands. This was evidenced in instances where only the command presented to the left ear was manually performed at a time when another command presented simultaneously to the right ear was the only one that was verbally reported. The indication is that the right hemisphere understood and performed the required action when the left hemisphere was apparently unaware. However, it was also shown that for most dichotic verbal tests the left hemisphere still has dominant control over the right. Dichotic listening studies also indicated that the left hemisphere could separately monitor stimuli in the ipsilateral along with stimuli in the contralateral pathway. This was contradictory to previous conclusions that the contralateral pathway suppresses the ipsilateral in dichotic competition. Response time studies carried out in these callosum-sectioned patients investigated organization of the two cortical systems that separately analyzed stimuli from the two ascending paths. It was found that response times for repeating words to the right ear were faster than for words in the left ear. Control tests showed the cause of this difference was not in delay of transmission in ascending routes, nor in differences of perception in the two systems. It was deduced .that the cause was an asymmetrical process of memory retrieval for translation into motor impulses to the speech apparatus.