Prefrontal Contributions to Attention in Ageing and Stroke: Implications for Cognitive Reserve
Citation:BROSNAN, MÉADHBH BRÍD, Prefrontal Contributions to Attention in Ageing and Stroke: Implications for Cognitive Reserve, Trinity College Dublin.School of Psychology.PSYCHOLOGY, 2017
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Abstract M?adhbh B. Brosnan Prefrontal contributions to attention in ageing and stroke: implications for cognitive reserve. Global life expectancy is increasing dramatically with a corresponding rise in the prevalence of pathological ageing conditions such as dementia and stroke. Maintaining high levels of cognitive function in the face of dementia-related neuropathology or following acquired brain damage is of pressing societal importance. The ability to maintain endogenous levels of attentional engagement is a domain-general process underlying many higher-order cognitive capabilities. Deficits in this capacity are evident across a range of neurodegenerative diseases, interfere with functional recovery following right hemisphere stroke, and are associated with an increased likelihood of falling in older adults. Cognitive reserve refers to the phenomenon where older adults with more cognitively stimulating environments, achieved through education, complex professions, or social engagements, present with less clinical symptomatology in a range of conditions, such as dementia, given the degree of disease-related neuropathology. In healthy ageing, superior levels of attentional engagement have been proposed to promote plasticity processes contributing to cognitive reserve. Sustained attention is postulated to contribute to cognitive reserve such that high levels of attention, necessitated by cognitively demanding environments, lead to greater environmental enrichment. This promotes plasticity processes which in turn contribute to a neuroprotective effect against disease. The neural underpinnings of this effect have recently been proposed to result from the continuous engagement of the right lateralised fronto-parietal networks (FPN) and the repeated activation of the neurotransmitter noradrenaline (norepinephrine), which plays a pivotal role in arousal and plasticity. Throughout a lifetime, the repeated activation of this system strengthens the right-lateralised network creating a neuro-cognitive buffer against disease. Although the right-lateralised fronto-parietal networks play a well-established role in aspects of attention, it remains to be seen whether plasticity of the underlying networks can be harnessed directly in older adults to improve this cognitive function. The first aim of this thesis was therefore to assess whether activity in the right prefrontal cortex (PFC), a flexible dynamic cortical region, could be upregulated in older adults to modulate facets of attentional function that are crucial contributors to cognitive reserve. Following damage to the right hemisphere, the capacity for sustained attention is reduced, and this interferes with functional recovery. Our understanding of these deficits and how best to ameliorate them is coarse. The second aim of the thesis therefore, was to explore how damage to the right hemisphere effects the temporal dynamics of sustained attention, and whether plasticity of the right PFC could be targeted in chronic stroke to ameliorate deficits in engagement. The first empirical chapter was designed to examine how behavioural and electrophysiological markers of sustained attention would change in older adults when activity in the right prefrontal cortex was directly upregulated using transcranial Direct Current Stimulation (tDCS), a non-invasive brain stimulation technique. Two separate experiments are presented where tDCS was employed to increase neuronal excitability of the right PFC while sustained attention was monitored. In Experiment 1 tDCS was administered during simultaneous electroencephalography (EEG) recordings in group of cognitively healthy older adults with suboptimal sustained attention performance. During tDCS, fewer lapses in attention occurred and electrophysiological markers of frontal engagement and early visual attention were enhanced, suggesting that attentional improvements were achieved through modulating processes involved in top-down endogenous control and early visual attention. In Experiment two, the extent to which the observed improvements in accuracy generalized to a very different sustained attention paradigm was examined. Consistent with Experiment 1, fewer attentional lapses occurred during tDCS. These findings suggest the right prefrontal cortex plays a critical role in supporting sustained attention in ageing, and that this region may be targeted in older adults to increase attentional engagement. The speed at which visual information can be processed is an aspect of visual attention that is increasingly considered a biomarker of cognitive decline. The second experimental chapter explored the relationship between visual attention, hemisphere asymmetries, and cognitive reserve, and assessed whether visual processing speed could be enhanced in older adults by increasing activity in the right prefrontal cortex. For this purpose, computational modeling of visual attention was combined with tDCS. Using a formal mathematical framework visual attention processes were quantified in older adults, unbiased by age-related motor slowing. During the performance of a lateralised whole-report task, older adults with higher levels of cognitive reserve showed a stronger processing speed asymmetry towards objects in the left hemifield. This is in contrast to younger adults who showed a rightward processing speed asymmetry, thus suggesting functional reorganization involving the right hemisphere in older adults with high levels of cognitive reserve. Correspondingly, when activity in the right PFC was increased in older adults using tDCS, processing speed improved. In older adults with lower levels of reserve, tDCS altered their processing speed asymmetry within the left and not right hemifield, thus mimicking that of their high reserve peers. This chapter suggests that tDCS is a promising target region to increase cognitive reserve in later years and provides further evidence in support of the first experimental chapter - that the DLPFC can be targeted in ageing to enhance top-down modulation over visual attention processes. In the third empirical chapter, using a behavioural paradigm that elicits steep decrements in sustained attention performance, the temporal dynamics of attentional engagement were explored in stroke patients with unilateral damage to the right hemisphere. Relative to neurologically healthy older adults, right hemisphere damage was associated with sharp drops in sustained attention over a short temporal window (less than 3 1⁄2 minutes). This vigilance decrement is within a shorter temporal window than previously noted in the literature and holds valuable practical information for informing neurorehabilitation protocols for RH stroke patients. In the final experimental chapter, the feasibility of upregulating activity in the right prefrontal cortex to improve sustained attention deficits in patients with right hemisphere damage was assessed. The same tDCS protocol (from Chapter 1, Experiment 2) that improved sustained attention performance in neurologically healthy older adults was applied to a group of stroke patients with unilateral right hemisphere damage. It was hypothesized that right prefrontal tDCS would increase excitability in the perilesional and residual areas of the damaged right hemisphere to improve sustained attention. However, preliminary data suggests that tDCS disrupted sustained attention. Further work will be necessary to determine whether prefrontal tDCS to the damage hemisphere is disrupting a natural compensatory processes of the contralesional network in chronic stroke patients with persisting sustained attention deficits. The work presented in this thesis constitutes important information regarding the role of the right hemisphere in facets of attention, that can be employed to inform neurorehabilitation programs to enhance attentional function in healthy ageing and following stroke. The implications of this work and future directions are outlined in the discussion.
This work was supported by the European Union FP7 Marie Curie Initial Training Network Individualised Diagnostics & Rehabilitation of Attention Disorders (grant number 606901).
Author: BROSNAN, MÉADHBH BRÍD
Publisher:Trinity College Dublin. School of Psychology. Discipline of Psychology
Type of material:Thesis
Availability:Full text available