Year: 2013
Type: dissertation
Abstract: This thesis investigated the relationship between semantically-driven behaviour and executive control.Patients with multimodal semantic deficits (referred to here as semantic aphasia or SA) can access detailed semantic information in tasks that provide strong external constraints on processing, suggesting intact semantic representations, but they have poor performance on tasks that require controlled semantic retrieval or inhibition of irrelevant semantic relationships.Additionally, these patients show deficits on non-semantic assessments of executive functioning which correlate with their performance on semantic tasks.In the current study, we explored the hypothesis that semantic control is underpinned by domain-general cognitive control mechanisms.Group of patients with a primary impairment of executive control (referred to here as dysexecutive syndrome or DYS) were compared with SA cases on a range of semantic tasks that differently manipulate semantic control and on non-semantic executive tasks.The results showed that DYS cases exhibit multimodal semantic impairments that are qualitatively similar to the pattern in SA patients (and highly contrasting with the pattern seen in semantic dementia).(1) Both groups were consistent on an item-by-item basis across different modalities (i.e., judgements to the same concepts presented as words and pictures) but inconsistent between different types of semantic tasks, even when these probed the same concepts.(2) There were minimal effects of familiarity and frequency on comprehension across different range of tasks.(3) Performance on semantic tasks was strongly affected by manipulating control demands -DYS and SA cases showed comparable effects of semantic distance between the probe and target, the strength of distracters and semantic ambiguity.(4) Both groups showed 'refractory' effects in comprehension, when the same set of semantically related items was presented repeatedly at a fast rate.The DYS group were more influenced than SA cases by speed of presentation, and this factor interacted with the semantic relatedness of the items in the set.Finally, we evaluate the effects of transcranial direct current stimulation (tDCS) on synonym judgement tasks that vary in semantic control requirement in healthy participants, with the ultimate aim of evaluating whether this technique could be used to improve comprehension, providing a potential rehabilitation tool for patients with semantic aphasia.
Author: Azizah Almaghyuli
Language: English
Subfield: Cognitive Neuroscience
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W1568648462