Investigating the contribution of the right hemisphere to language processing in the damaged and healthy brain

Acquired language disorders after stroke are strongly associated with left hemisphere damage. When language difficulties are observed after right hemisphere damage, patients are commonly considered to have atypical functional anatomy (i.e. crossed aphasia). On the other hand, fMRI studies have reported right hemisphere activation when neurologically-normal participants perform language tasks, and have shown that the right hemisphere contributes to recovery of language function after left hemisphere damage. In this thesis I investigated (i) the degree to which language difficulties after right hemisphere stroke can reflect disruption to typical functional anatomy and (ii) how the damaged areas contribute to normal language processing. In Study 1 (Chapter 3), I investigated a group of patients with unilateral strokes that damaged either the right or the left hemisphere. The most frequently impaired language task was auditory sentence-to-picture matching after right hemisphere strokes, and spoken picture description after left hemisphere strokes. In 9 right hemisphere stroke patients, performance on the auditory sentence-to-picture matching task was selectively impaired and could not be explained by poor perceptual (visual or auditory) or linguistic processing (semantic, phonological or syntactic). I therefore hypothesised that the behavioural difficulties experienced by those patients arose as a consequence of impaired non-linguistic executive functions that are needed to support language processes. In Study 2 (Chapter 4), I investigated the lesions of the 9 patients with selective deficits in the auditory sentence-to-picture matching task, and found that they had significantly more damage to subcortical regions and parts of the superior longitudinal fasciculus impinging on the right inferior frontal sulcus compared to other right hemisphere stroke patients who were not impaired on the sentence comprehension task. Having identified these regions, their function (e.g. linguistic or executive) can be investigated using functional neuroimaging in neurologically-normal participants. In Study 3 (Chapter 5), I used fMRI to investigate whether any parts of the right hemisphere regions associated with impaired sentence comprehension, in Study 2, were activated when neurologically-normal participants performed similar language tasks to those administered to right hemisphere stroke patients in Study 1 (including the auditory sentence-to-picture matching task). I found that, within the brain areas derived from Study 2, the right inferior frontal sulcus and right mediodorsal thalamus were normally activated by auditory sentence-to-picture matching but there was no evidence that these regions were exclusively performing linguistic functions. In Study 4 (Chapter 6), I investigated the contribution of the identified regions further by using a new fMRI study of one-back matching tasks that varied demands on semantic and non-semantic working memory. By systematically integrating neuropsychological, lesion and fMRI data, I conclude that the right inferior frontal cortex and right mediodorsal thalamus contribute to non-semantic working memory capacity that is needed to accurately perform a range of language functions. This account helps to explain why auditory sentence-to-picture matching impairments occur after right hemisphere damage.