Left-hemisphere control of oral and brachial movements and their relation to communication

Abstract The left cerebral hemisphere of the brain in man is known to be involved in both vocal and manual communication, as evidenced by speech and manual sign language disorders after left-hemisphere pathology. The left hemisphere also has important motor control functions in certain kinds of non-communicative oral and manual movements. Several tasks requiring oral and manual-branchial movements were presented for reproduction to patients with unilateral restricted lesions of the left or right hemisphere. Patients with left-hemisphere lesions were impaired relative to patients with equivalent right-hemisphere damage, thus confirming and extending earlier reports on the effects of more widespread left-hemisphere damage. Within the left hemisphere, frontal and parietal lesions had the most severe effect, with lesions in the central, occipital or temporal areas producing no or minimal impairment. The left frontal area was paramount in the control of oral movements, and the parietal lobe in control of hand movements, but both regions were required for the more demanding oral and manual tasks. Oral and manual control systems appeared more separable in the frontal than in the parietal region, suggesting that the latter may play a general programming role that is then enacted through the left frontal region. Speech-disordered (aphasic) patients were inferior to non-aphasic patients on oral and manual tasks. There was a close association between aphasia and the presence of oral or manual defects in left frontal and parietal lobe lesions, but not in temporal-lobe lesions. In addition, a speech task requiring reproduction of a single syllable closely paralleled the reproduction of a non-verbal oral movement, in that both depended critically on the left anterior region; aphasic patients with parietal or temporal lesions were relatively unaffected on either task. However, on reproduction of multisyllabic familiar phrases, there was no association with oral motor deficits. Aphasic patients with anterior and parietal lesions could reproduce these phrases relatively well, but patients with temporal-lobe lesions (and lesser impairment on motor tasks), had great difficulty. It is suggested that oral and manual apraxia, as well as aphasia, may be a manifestation of a basic motor selection problem in lesions of frontal and parietal lobes, but that the temporal region has some important acoustic-motor function in speech. When this temporal system is intact, it can, at least with overlearned material, bypass defective frontal or parietal oral motor systems.