Contrasting opinions on the role of the right hemisphere in the recovery of language. A critical survey

Background: The role of the right hemisphere (RH) in the recovery of language is quite controversial. Aims: The aim of the present survey consisted in taking into account three main models advanced to explain the reconstitution of language systems: (1) the "perilesional hypothesis," which maintains that language recovery is mainly subsumed by left hemisphere (LH) tissue adjacent to the lesion; (2) the "right hemisphere hypothesis," which assumes that restitution of language entails an increased participation of the RH; and (3) the "disinhibition hypothesis," which maintains that recovery is facilitated by disruption of inhibitions exerted by RH regions over LH language areas. Methods & Procedures: The prognostic factors in poststroke aphasia are discussed first, focusing attention on factors that could subsume an increased participation of the RH to the recovery of language. Then results obtained with techniques of noninvasive brain stimulations of the RH are taken critically into account. Outcomes & Results: As for the prognostic factors, the following points are stressed: (1) the anatomical extension of the LH lesion plays an important role both on the degree and on the path of recovery; (2) the RH structures involved in the recovery of language are generally mirror structures of the damaged LH areas; and (3) the time elapsed since the onset of aphasia influences the contribution that contralateral and ipsilesional areas give to the recovery of language. As for results obtained with techniques of noninvasive brain stimulations of the RH, this point remains controversial, because most authors support the "disinhibition hypothesis," but theoretical and factual reasons suggest caution in the interpretation of these results. Conclusions: Since the meaning of the RH activations and of results obtained with techniques of noninvasive brain stimulations remain controversial, some tentative recommendations to clarify these issues are advanced.