Contributions of the left and right hemisphere in language: investigating the effects of unilateral brain damage (stroke) on metaphor processing

It is widely accepted that the left hemisphere of the brain is specialised and dominant for language comprehension and production and that those with left hemisphere damage often display profound language disruption (Geschwind, 1965). The importance of the left hemisphere is shown by communication problems or extreme difficulty in producing speech following damage to this brain region. In contrast, following right hemisphere damage, disruption to language is less perceptible to the casual observer. The evidence base currently available acknowledges a critical role for the right hemisphere in processing inferred or implied information by maintaining relevant facts and/or suppressing irrelevant ones but the exact role of the right hemisphere and its coordination with the left is open for debate (Johns, Tooley and Traxler, 2008). Two theories have been proposed to explain communication/language difficulties in individuals with right hemisphere damage: (i) the “coarse semantic coding” hypothesis and (ii) the “suppression deficit” hypothesis. The “coarse semantic coding” hypothesis proposes that damage to the right hemisphere causes an over reliance on fine coding assumed to be undertaken by the left hemisphere in the comprehension of language, implying the recall of most literal interpretations. The “suppression deficit” hypothesis proposes that damage in the right hemisphere means multiple activations of meanings of words are not attenuated leading to ineffective suppression of inappropriate interpretations. This project investigated competing evidence for each of these hypotheses by studying the processing abilities of individuals with depressed unilateral brain function caused by stroke or innovatively produced by transcranial DC stimulation (tDCS), on semantic judgement tasks using metaphorical language. The results demonstrated the strongest of evidence for the coarse semantic coding hypothesis when the data from participants with damage to the right hemisphere, both caused by stroke and simulated by tDCS was considered. Overall, the study has furthered the understanding of the role of the right hemisphere in language comprehension and demonstrated the contribution of the tDCS methodology in the field.