Neural markers of attentional parameters in the ageing brain

The studies presented in the current PhD thesis aimed at investigating the neural correlates of distinct aspects of visual attention, i.e., spatial bias, efficiency of top-down control and visual processing speed and how they change with healthy ageing. The presented studies are theoretically grounded on Bundesen's Theory of Visual Attention, which provides a mathematical framework enabling quantifying various visual attention functions as the independent parameters of the model. A more recent neural interpretation of these attentional processes – NTVA – served as a basis for investigating attentional processing in combination with neurophysiological techniques. In the presented thesis, we combined TVA-based partial- and whole-report tasks with briefly presented letter arrays that enable mathematically independent estimations of the attentional parameters with simultaneous EEG recording. Extracting event-related lateralizations (ERL) from the signal provided fine-graded neurophysiological signatures of the attentional processes involved in the tasks. In the first empirical study, we combined a partial report task with ERP measurement and aimed at investigating how attentional spatial bias and top-down control efficiency are reflected in the electrophysiological brain activity of healthy participants of between 20 and 35 years of age. We divided participants into groups with more and less efficient top-down control and left- and rightward spatial bias, respectively. First, an ERL component – the PCN wave – reflecting interindividual differences in top-down control efficiency was identified by comparing participants with more relative to less efficient top-down control. Participants with more, compared to less efficient top-down control showed higher PCN amplitudes. Second, we identified asymmetries in earlier visual ERLs reflecting the direction of the spatial bias by comparing participants with a biastowards left or towards right hemifield. Our results show that these two distinct functions of attention are exclusively reflected by the amplitudes of the distinct ERLs. In the second study, we aimed at investigating whether top-down control efficiency is affected by healthy ageing and whether it is similarly reflected in older adults by the amplitude of the lateralized components PCN and Ppc, as reported for younger adults. To pursue this goal we employed an identical methodology as in the first study to additionally test older adults ranging between 33 and 77 years of age. To control for a potential influence of (the amount of) cognitive reserve, we included crystallized IQ as a common proxy for cognitive reserve in the analyses. We divided participants into individuals with more relative to less efficient top-down control and lower versus higher level of cognitive reserve. We found comparable top-down control efficiency values across age groups and, furthermore, that across all age groups, participants with more efficient top-down control showed higher amplitudes of both the Ppc and the PCN waves. We further found that the amount of cognitive reserve modulated the relation between age, top-down control efficiency, and ERLs amplitudes. More precisely, only in participants with higher crystallized IQ Ppc amplitude was higher in participants with more efficient compared to less efficient top-down control. In contrast, in participants with lower crystallized IQ the PCN amplitude decreased significantly with age. In the third study, we combined a whole report task with ERP assessment. We aimed at investigating whether and how visual processing speed is reflected in the neural activity of the brain as indexed by ERLs, and whether this potential relation is affected by healthy ageing. We assessed visual processing speed in a sample of participants ranging from 20 to 78 years of age. Participants were divided into groups with relatively low versus high visual processing speed. Data revealed that visual processing speed declines with age. Further, we identified two lateralized components indexing individual differences in visual processing speed: N1pc and PCN. Both components had increased amplitudes in participants with higher processing speed. Furthermore, amplitudes of the N1pc was increased and latency of the PCN was delayed in older, compared to younger, participants. These age-related changes in the components reflecting visual processing speed may suggest that younger versus older participants employ different mechanisms while performing the whole-report task. More precisely, younger participants may encode the entire display at the early stage of processing and then select relevant information, whereas older ones rather preselect relevant information during an initial phase. The evidence collected across the three ERP studies provides valuable insights into the relation between efficiency of attentional functions, brain activity, and healthy ageing. We found that trajectories of age-related changes of distinct attentional functions are substantially different. Age also does not affect all electrophysiological correlates of functions of attention equally. Finally, our studies show that the relation between efficiency of attentional functions and their neurophysiological correlates is influenced by the amount cognitive reserve.