ANÁLISIS ESTADÍSTICO

It has been demonstrated in the literature reviewed above that the evidence supporting a direct relationship between speech laterality and handedness is variable,

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mainly due to the differences in measurement and classification used in such research. However, as has been discussed, neurophysiological evidence demonstrates links in the neural processing components of praxis and speech, and neuro-computational models have suggested that the specialisation of left hemisphere regions for speech production is crucially mediated by the sequential processing of auditory stimuli and complex motor output. This is complemented by handedness and praxis research examining left hemispheric processing specificity for tasks which are complex and highly skilled, demonstrated through behavioural impairments in patients with left sided lesions. This combined experimental evidence is built upon theories that suggest an evolutionary trajectory for gesture as the basis of speech, which in itself is supported by research on sign language abilities in the deaf, and separately the motor and language milestones commonly interlinked in both typical and atypical development.

To that end it seems likely that tasks which tap into the sequential processing capacity of the left hemisphere may be useful in determining the extent to which speech and motor laterality overlap. Flowers and Hudson (2013) demonstrate the potential for skilled sequential motor tasks to predict the hemispheric lateralisation profile of

individuals. However, this evidence resulted from neurological populations (e.g. epilepsy patients) rather than neuro-typical participants, and so may be indicative of a difference in brain functioning due to atypical development in these patients rather than due to

similarities in the underlying neurological processing of the two functions. Therefore the first section of this thesis seeks to examine the relationship between speech lateralisation and hand skill in healthy adults, to determine whether sequencing based motor ability can predict direction of speech laterality. This study also enables the confirmation of fTCD as a reliable measure of laterality by testing the test-retest variability in speech laterality scores. The hypotheses in this study are that healthy participants will show a similar pattern of speech and motor laterality correlation to the Epilepsy patients tested

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previously, whereby those with smaller differences in performance between their hands will be more likely to show atypical speech profiles.

Secondly, the developmental profile of laterality has been looked at in detail for language development and handedness as separate functions, however, there have not been large scale investigations of the common developmental trajectory between these two functions (see section 2.2 and 2.3 above). It is still unclear whether theories suggesting motor action as the basis of speech production would mean that there are differences in the profile of hand skill and speech lateralisation as a function of age, or whether these two modalities develop independently, despite a possible common

underlying brain network. It is expected that language laterality would predominate to the left at the group level, but there is still conflicting data over the variability of LI scores in younger children and also whether handedness as assessed by sequencing based skilled manual tasks relates to profiles of speech lateralisation. This study hypothesises that younger children will show more variability in both their speech laterality and their motor skills than older children, representing an immature level of hemispheric specialisation for complex sequencing behaviours.

A novel question regarding the development of motor control and speech processes arises from previous research showing atypical speech lateralisation and handedness profiles in individuals with neurodevelopmental disorders. It has been shown that individuals with language and reading disorders such as Dyslexia (Illingworth and Bishop, 2009) and SLI (Whitehouse and Bishop, 2008) have atypical speech

representations, but it has not been investigated whether individuals with

neurodevelopmental disorders affecting motor coordination also have atypical cortical organisation for language. If it is accepted that speech production and fine motor coordination share common neural networks then it could be predicted that a disorder

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affecting one of these linked functions may also affect the neurological representations of the other function, even in the absence of behavioural deficits. What has not yet been explored is whether neurodevelopmental disorders affecting the motor system also impact upon the organisation of associated speech and language networks. This question will be addressed experimentally in chapter 6 of this thesis in a group of adults with DCD, where it is hypothesised that the DCD group will show more atypicality in their speech profiles than control participants.

Finally the literature above suggests that relatively little is known about how the various component processes and mechanisms inherent within the motor tasks deployed to measure lateralisation impact upon the profile of laterality observed. Many studies rely on preference based handedness measures and indirect measurements of speech

lateralisation (e.g. dichotic listening) to draw conclusions about the relative associations between motor and speech laterality. Chapters 7 and 8 will focus on revealing the

sequencing based mechanisms underlying motor skill and speech. Here it is hypothesised that tasks with high levels of sequential processing will show close associations with speech lateralisation indices, and will be more affected by overloading the lateralised system when requiring dual-focus on competing tasks.

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