Lateralisation of language functions in the brain: Effects of handedness

O'Regan, Louise (2020) Lateralisation of language functions in the brain: Effects of handedness. PhD thesis, University of Nottingham.

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Abstract

Evidence suggests that an individuals’ handedness is intrinsically linked to their brain asymmetry for speech and language functions. The best estimates suggest that the left hemisphere is dominant for language in 95% of right-handers and 70-85% of left-handers. Atypical or reversed language laterality patterns have been observed in 4% of strong right-handers and up to 27% of strong left-handers, which points to increased variability due to non-right-handedness and confirms that handedness is a useful tool to study individual differences in brain organisation.

Many studies investigating the relationship between handedness and language dominance rely heavily on speech production tasks. However, speech production represents just one aspect of language, which is a highly complex and multifaceted cognitive function. Therefore, the relationship between handedness and hemispheric asymmetries for less commonly-studied functions such as word comprehension, lexical decision, semantic processing and word matching is poorly understood. This is possibly due to the fact that these functions are not as strongly lateralised as speech production.

The studies presented in the current thesis compared left- and right-handers on a variety of language tasks. Use was made of divided visual field (DVF) methodology, in order to examine both hemispheric differences and interhemispheric communication during task performance. The DVF paradigm is a technique that reliably measures hemispheric asymmetries in healthy humans. Hemispheric asymmetries were investigated using three behavioural metrics: accuracy, reaction time and laterality indices.



The findings presented in this thesis show that right-handers tend to benefit more from left-sided lateralisation for language comprehension, particularly of abstract as compared to concrete information, whereas the left-handers demonstrate a less focal profile, characterised by increased input from the right hemisphere during language-related tasks. We also found that left-handers rely on interhemispheric interactions during word processing to a greater degree than right-handers.

The findings of this thesis offer insights into the relationship between handedness and the lateralisation of a variety of language functions in the brain. It is argued that the study of handedness is important as handedness represents a distinct form of individual difference that permits us to explore differences in cognitive performance and enables us to further our understanding of the neural mechanisms that underpin cognitive processing.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Serrien, Deborah
Keywords: handedness, hemispheric asymmetry, cerebral lateralisation, language, divided visual field
Subjects: B Philosophy. Psychology. Religion > BF Psychology
Faculties/Schools: UK Campuses > Faculty of Science > School of Psychology
Item ID: 60533
Depositing User: O'Regan, Louise
Date Deposited: 27 Aug 2020 09:48
Last Modified: 27 Aug 2020 10:00
URI: http://eprints.nottingham.ac.uk/id/eprint/60533

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