The Role of the Right Hemisphere in Speech Production

Introduction

For decades, researchers have attempted to answer whether or not language "shifts" to the right hemisphere following focal damage to the left hemisphere (Moore & Papanicolaou, 1992; Ogden, 1989). A related question is, how much could the right hemisphere contribute to the recovery from language deficits following stroke (i.e., aphasia) and which areas would be responsible for the increase in functional communication and language capabilities for comprehension and production (Thomas, Altenmuller, Marckmann, Kahrs, & Dichgans, 1997). Research addressing these questions has been inconclusive and controversial.

Many researchers have suggested that following damage to the left hemisphere language shifts to the homologous areas of the right hemisphere (Gold & Kertesz, 2000; Heiss et al., 1999). Evidence of this compensatory strategy has been well documented using various methodologies with regard to language comprehension (for an in-depth review, see Ogburn Yeager & Rubin, 2005).

Moore and Weidner (1975) found a significant left ear advantage (LEA), which indicates right hemisphere dominance, in patients with aphasia. However, the LEA was not apparent until six months post insult, which suggests recovery of function is associated with a shift to the right hemisphere for language processing.

Pettit and Noll (1979) documented a group of aphasic listeners with LEAs, whereas a non-brain damaged control group demonstrated typical right ear advantage (REA). The control group REA was thought to reflect left hemisphere language dominance. However, for the aphasic group, a transfer of hemispheric specialization for language due to the LEA seemed apparent. Moore and Papanicolaou (1988) found significant REAs for a group of non-brain damaged listeners, listeners who had temporary dysarthria due to transient ischemic attack, and listeners who had experienced a right hemisphere stroke. However, they reported a significant LEA for the majority of their aphasic participants (8 out of 11), which they assumed was a result of the right hemisphere's increased contribution.

In a single subject design using PET technology, Gold and Kertesz (2000) found that the right hemisphere along with other areas in the left inferior temporal lobe had come to subserve lexico-semantic processes in an individual with large scale damage to the left hemisphere and subsequent global aphasia. In a semantic-orthographic comparison task, their non-aphasic control subject demonstrated left perisylvian and extrasylvian activation, whereas the patient with global aphasia demonstrated activation in the homologous areas of the right hemisphere. In the individual with aphasia, specific areas activated were the right middle frontal gyrus, superior temporal gyrus, angular gyrus, supramarginal gyrus and the precuneus. Semantic decisions revealed activation of the right inferior and middle temporal gyri, whereas in the control patient, the left superior temporal gyrus was activated. Gold and Kertesz concluded that the improvement in the processing of lexical-semantic information was due to recruitment of right hemispheric regions.