Cerebral lateralisation of speech production and motor skill

The association between praxis and language is longstanding in neuropsychology, with evidence revealing that left hemisphere lesions often lead to combined impairments in motor control and speech (Rasmussen and Milner, 1975; Goldenberg, 2013). Strong left hemisphere asymmetry for language is a robust finding at the population level (e.g. Knecht et al 2000a) and similarly the cortical activation patterns of manual praxis for skilled tasks also reveal a left hemisphere bias (Buxbaum et al, 2005; Haaland et al, 2004). As such, common neural mechanisms are thought to underlie both speech and motor skill, especially actions involving fine motor control of the hands. However, evidence for a clear causal relationship between handedness and speech laterality has proven somewhat weak and inconsistent, due to the wide variation in measurement and classification approaches used (Groen, et al, 2013). A suggestion by Flowers and Hudson (2013) is that motor and speech laterality are related where they involve a common feature of motor output, namely the co-ordination of sequences of movements or utterances to execute a plan or intention so as to achieve a goal; either limb movement or expression of an idea (e.g. Grimme, et al, 2011). The research conducted here investigates speech and motor lateralisation from the hypothesis that sequencing based tasks will be best able to elicit the predicted left hemisphere activation patterns. Five empirical chapters are presented detailing a number of studies involving healthy adults, typically developing children and adults with Developmental Coordination Disorder. The research uses an emerging technique in cognitive neuropsychology; functional Transcranial Doppler (fTCD) sonography, to explore hemispheric laterality of speech and motor skill. Measurements of the degree of activation in each of the hemispheres during language tasks, and the use of a skill-based motor task to determine handedness, are the primary indicators of lateralisation used throughout this thesis. Results from the first 3 chapters 4 reveal that 1) atypical patterns of speech laterality are linked to greater performance differences on motor skill tasks; 2) that whilst hand preference is established early on in childhood the relative performance ability between the non-preferred and preferred hands develops linearly with age; 3) adults with developmental coordination disorder display atypical patterns of laterality of speech networks. The final 2 empirical chapters employ novel neuroimaging paradigms to investigate the mechanisms underlying the links between speech and motor sequencing. Results show that the pegboard task elicits left hemisphere dominant activation regardless of the hand used, unlike other motor tasks with similar properties. Finally a dual task paradigm demonstrates that speech production suffers greater impairments than motor skill when performed simultaneously, providing support for theories proposing a gestural origin to speech. The data are discussed in terms of the specialisation of the left hemisphere for higher order sequential processing, in the context of a lateralised speech-praxis centre model.