Cerebral asymmetries : handedness and the right hemisphere

There are well-known, but poorly understood, links between left cerebral language dominance and hand preference. Approximately 95% of right-handers and 70% of non-right-handers have language lateralised to the left hemisphere. In contrast, virtually nothing is known about handedness and cerebral dominance for a number of different specialisations linked with the right cerebral hemisphere. This thesis examines several of these asymmetries, including face, emotional, attentional, and body processing, inright-handed and non-right-handed groups using both behavioural and neuroimaging techniques. The main aims of these investigations were to quantify the frequencies of these biases, and to examine possible links between each of these asymmetries and speech/language, to see if they 'anti-localise' in the two hemispheres in a complementary fashion. To do this, a large pool of language 'atypicals' (individuals with right hemisphere dominance for language) were identified for inclusion in the neuroimaging experiments. An important foundation of this work advocates the use of proportional and individual level analyses, rather than the usualexclusive reliance on typicalinferential statistics that focus on measures of central tendency. First, a large-scale battery of perceptual tests, which included measures of language, emotional, attentional, and face-related asymmetry, was administered to a large sample of right-handers and non-right-handers (Chapter 2). These efforts were coupledwith a large-scale functional neuroimaging series, quantifying cerebral asymmetries for emotional prosody, emotional vocalisations, bodies, neutral and emotional faces,as well as for language (Chapter 3). The final empirical chapter attempts to predict the neuroimaging asymmetry groups from behavioural measures of asymmetry(Chapter 4). The results from this thesis confirms the links of the 'target specialisations'to the right hemisphere for the majority of individuals. Intriguingly, it also suggests that there are moderating effects of handedness, with non-right-handers having a more varied laterality profile whilst right-handed participants were largely complementary for all functions measured. The atypically lateralised individuals had the most varied asymmetry profiles, in spite of remarkably similar asymmetry for language with the right-handed and non-right-handed language typical groups. These results are discussed in terms of models of hemispheric specialisation, the use of perceptual tests to aid in the identification of individuals with rare laterality patterns, and future studies important for a full appreciation of cerebral dominance and human handedness.