Assessing language lateralisation using functional transcranial Doppler sonography

This thesis uses functional transcranial Doppler sonography (fTCD) to investigate hemispheric asymmetries in brain activity during language processing. FTCD is a simple method that provides a measure of relative lateralisation. Given its portability and tolerance for movement, it allows physiological activity and behaviour to be measured simultaneously in understudied paediatric populations. The fi rst half of the thesis describes three methodologically motivated studies with adults. The results indicated that the strength of lateralisation is affected by experimental manipulations of task and stimuli. A particularly influential factor was the intensity of phonological lexical search required. There was also an effect of stimulus pace, suggesting that difficulty or effort may also play a role in driving the strength of lateralisation. The second half of the thesis provides the main theoretical contributions to the literature in three developmental studies. The fi rst of these found no evidence of increases in the strength of lateralisation between the ages of three-and-a-half years and four-and-a-half years. The second study found typical left-lateralisation during language production in a heterogeneous group of children born deaf. This study provides preliminary evidence that auditory input is not a contributory factor to the development of language lateralisation. The final study used a paced picture naming task with children. Concordance was measured between fTCD during this novel task and an established narrative task. The data also suggested that LIs measured by fTCD are most likely to relate to offine measures when the tasks share cognitive or linguistic demands. In summary, this thesis contributes to a growing body of research demonstrating that fTCD is a useful tool to investigate hemispheric lateralisation. It is of particular use with those populations for whom other neuroimaging modalities are not suitable. It is often these groups of participants who can offer unique insights into language processing and the underlying neural systems