The role of early left-brain injury in determining lateralization of cerebral speech functions

Abstract

Preparatory to craniotomy for the relief of medically refractory focal epilepsy, the lateralization of cerebral speech functions was determined by the Wada intracarotid Amytal test in 134 patients with clinical and radiologic evidence of an early left-hemisphere lesion. Their results were compared with those for 262 patients (140 right-handed, 122 left-handed), who were tested in a similar way. One-third of the patients with early lesions were still right-handed, and 81% of these right-handers were left-hemisphere dominant for speech. In the non-right-handers, speech was represented in the left cerebral hemisphere in nearly a third of the group, in the right hemisphere in half the group, and bilaterally in the remainder. Bilateral speech representation was demonstrated in 15% of the non-right-handers without early left-brain injury and in 19% of those with evidence of such early injury, whereas it was extremely rare in the right-handed groups. In addition, nearly half the patients with bilateral speech representation exhibited a complete or partial dissociation between errors of naming and errors in the repetition of verbal sequences after Amytal injection into left or right hemispheres. This points to the possibility of a functionally asymmetric participation of the two hemispheres in the language processes of some normal left-handers. The results of the Amytal speech tests in this series of patients point to locus of lesion as one of the critical determinants in the lateralization of cerebral speech processes after early left-brain injury. It is argued that in such cases the continuing dominance of the left hemisphere for speech in largely contingent upon the integrity of the frontal and parietal speech zones.