Langauge laterality in impaired and typical language functioning

Cerebral lateralisation for language refers to the well-established finding of a bias in language processing towards the left hemisphere. A longstanding question in the field concerns whether such lateralisation is beneficial for language functioning, and in particular whether its disruption may be an aetiological risk factor for disorders of speech and language. The current thesis aimed to take a new approach to this question by considering the importance of within individual variability in lateralisation. The first part of this thesis considers optimal practice for measurement of lateralisation for language using functional magnetic resonance imaging (fMRI). Chapters two and three present systematic reviews of this literature which discuss the challenges in calculating a laterality index (LI), and in selecting a language activation paradigm respectively. Recommendations are made for optimal laterality measurement with a view to promoting standardisation in protocols across the field. The second part of this thesis investigates differences in lateralisation and interhemispheric interactions in a group of adults with developmental disorders affecting language. My first experimental study (Chapter four) established that this group demonstrated significantly impaired language skills compared to a group of typically developing control adults. The second experiment of my thesis (Chapter five) went on to investigate the relationship between language skills and patterns of lateralisation within this same sample. Three language tasks were used with functional transcranial Doppler ultrasound (fTCD) to measure consistency in lateralisation across different language functions. Contrary to predictions, inconsistency in lateralisation was not associated with poorer language skills across the whole sample. However, the developmental disorder group demonstrated a higher incidence of inconsistent lateralisation, weaker correlations between task LIs, and increased variability in lateralised responses over trials within a phonological task. These findings suggest that inconsistent lateralisation does not directly cause impaired language skills, but nevertheless is more likely to occur in individuals with developmental disorders associated with poorer language functioning. In a further experiment (Chapter six), interhemispheric transfer during word recognition was investigated in adults with dyslexia and typically reading adults using a visual half-field (VHF) paradigm. Mixed evidence for a deficit in interhemispheric transfer in dyslexia was found. Both dyslexic and control participants were able to benefit from simultaneous presentation of identical word stimuli to both visual fields, an effect which is attributed to interhemispheric cooperation. However, dyslexic participants demonstrated poorer performance for left visual field words, suggesting an impairment in the transfer of visual word form information from the right to the (dominant) left hemisphere. Finally, agreement between fTCD and VHF measures of laterality was investigated by administering both methods within the same participants (Chapter seven). Although moderate levels of concordance were found between dominance classifications obtained with these methods, correlations between their LIs were low and non-significant. This may be attributable to the VHF method’s sensitivity to efficiency of interhemispheric transfer in addition to asymmetry of word processing. Overall, the findings of this thesis demonstrate that a multidimensional view of lateralisation that considers the importance of interactions between the hemispheres is informative for understanding how processing of language may be altered in developmental disorders.