Examining targeted brain stimulation to improve vigilant attention in right-hemispheric stroke

Abstract

Neglect is a disabling neuropsychological syndrome frequently observed following right-hemispheric stroke. Affected individuals present with attentional deficits, ranging from a difficulty in orienting towards the contralesional space to a generalised difficulty with maintaining attention over time. Neglect may be persistent - particularly its non-lateralised component. This thesis focused on investigating the efficacy of a potential treatment involving non-invasive targeted brain stimulation to improve vigilant attention. In a randomised double-blind sham-controlled crossover study, healthy individuals across the lifespan and stroke patients with attentional impairments received real and sham transcranial direct current stimulation (tDCS) whilst performing a vigilance task. A high-definition montage was used to constrain current delivery over the right dorsolateral prefrontal cortex, a key region of the vigilance network. Results show that, at the group level, targeted tDCS improved target detection across all groups. By examining performance across temporal epochs, it was noted that tDCS did not impede worsening of performance with increasing time-on-task. The superiority of tDCS was however found throughout the task, outlasting stimulation delivery. A lesion anatomy study indicated that task performance was related to lesion location rather than volume. In addition, variability in patients' response to treatment was observed and linked to lesion profile, revealing that damage to specific brain regions caused lack of tDCS response. Finally, a concurrent tDCS-fMRI study was conducted to examine brain network response to tDCS. Brain stimulation did not affect local connectivity, but rather influenced functional connectivity within large-scale networks in the contralateral hemisphere. This finding emerged across groups using different analysis approaches, confirming its robustness. This systematic behavioural and imaging investigation supports a role of tDCS to improve non-lateralised deficits of neglect, which could be harnessed in future clinical trials. Furthermore, it sheds light on network response to precise cortical targeting, revealing its widespread effect.