Year: 2018
Type: dissertation
Study design: Observational Study
Abstract: The corpus callosum is the largest fiber bundle in the human brain and mediates communication between cerebral hemispheres through a combination of excitatory and inhibition interactions. The balance between excitation and inhibition has been clearly defined for certain perceptuomotor processes, but not with language. This dissertation outlined the interhemispheric interactions involved in sentence comprehension in aphasia patients (studies 1 and 2) and word recognition in healthy adults (studies 3 and 4). Aphasia is commonly associated with right hemisphere activation, which has been attributed to both inhibitory and excitatory mechanisms. Study 1 demonstrated a positive relationship between left hemisphere dysfunction and right hemisphere language activation, which could have resulted from either disinhibition or excess recruitment of the right hemisphere. Study 2 examined these two hypotheses using Dynamic Causal Modelling of fMRI. Healthy controls showed no evidence for interhemispheric homotopic inhibition, but instead demonstrated excitatory right-to-left connectivity between primary auditory cortices. Critically, auditory connectivity was associated with language recovery in patients; the worst performing patients lacked typical right-to-left excitation, and showed atypical left-to-right excitation. Studies 1 and 2 provide two very interesting results: 1) healthy language processing requires asymmetrical right-to-left perceptual transfer and 2) right hemisphere activation in aphasia results from atypical excitatory recruitment. Interhemispheric interactions between perceptual regions was further explored in study 3 using a lateralized lexical decision task with MEG. In line with results from study 2 and the broader word recognition literature, the results showed an asymmetrical transfer of visual information from right to left hemisphere. Importantly, the results add to the current literature by demonstrating transfer between primary visual cortices, at one of the earliest processing stages of word recognition. Finally, study 4 showed that simultaneous bilateral word recognition was facilitated when word pairs were semantically related. These findings again suggest that the interhemispheric interactions in word recognition are primarily excitatory. The four studies in this dissertation found consistent evidence for interhemispheric excitation in language processing. While there is clear evidence for inhibition in some language processes, the current studies suggest that both sentence comprehension and visual word recognition are primarily associated with excitatory interhemispheric interactions.
Source: TSpace (University of Toronto)
Author: Ronald Chu
Language: English
Subfield: Cognitive Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2897101237