In young adults, spatial attention typically manifests in a processing advantage for the left side of space (“pseudoneglect”)(Bowers & Heilman, 1980), whereas older adults tend to display no strongly lateralised bias, or a preference towards the right side of space (Benwell, Thut, Grant, & Harvey, 2014; Schmitz & Peigneux, 2011). However in addition, in recent studies, pseudoneglect has also been found to be maintained into old age (Brooks, Darling, Malvaso, & Della Sala, 2016; Brooks, Sala, & Darling, 2014; Friedrich, Hunter, & Elias, 2018). This suggests that the traditional view of an attenuated spatial asymmetry bias with increasing age may be too simplistic and that the spatial biases observed could be sensitive to a range of influences besides age. In addition, the traditionally observed shifts in spatial asymmetry in older adults have been attributed to neuroanatomical changes in the right hemisphere (with age), however as yet, there is limited evidence linking neurophysiological results to such behavioural shifts.To this end, for older adults, spatial attention research lacks systematic investigation of intra- and inter-task consistency. In the first of the four experiments of this thesis, I built on an earlier study which investigated young adults (Learmonth, Gallagher, Gibson, Thut, & Harvey, 2015, see 2018), and addressed this issue by investigating the magnitude and direction of spatial asymmetry in older adults aged between 60 to 86 years in five commonly used spatial tasks (line bisection, landmark, grey and grating scales and lateralised visual detection). I also compared the obtained spatial biases to a driving task. Results confirmed a stable retest reliability of all spatial tasks across two testing days in older adults. The line bisection and greyscales tasks elicited significant left spatial biases, in accordance with pseudoneglect, while the other tasks showed no significant biases to either side of space. Interestingly, in the driving task a right bias emerged for older adult and was stable across testing sessions. Yet, it failed to correlate with the other spatial measures. In comparison to the young adults’ sample from Learmonth et al. (2015, 2018), only the landmark task was age sensitive. However, none of the task showed significant inter task correlations. This replicates the findings of Learmonth et al. (2015, 2018) for an older age group. So in view of my findings of no significant inter-task correlations, as well as the inconsistent directions of the observed spatial biases for the older adults, I present supporting evidence that pseudoneglect is a multi-component phenomenon and highly task sensitive. Each task may reflect a distinct neural mechanism, likely to be impacted differently by age or other non- spatial modulators. In fact, the influence of other non -spatial modulators on spatial attention was the central topic of the other three experiments presented in Chapters 3 and 4. I employed a dual task paradigm (Chapters 3 and 4) and electroencephalography (EEG) (Chapter 4 only) to investigate behaviourally as well as neurophysiologically if an increase in attentional load has a reducing effect on spatial asymmetry and whether this would be more pronounced with old age. Interestingly, for the last experiment including EEG (Chapter 4) in particular, results showed that although older adults perform similarly well to young adults on a behavioural level, changes are visible on a neuronal level. Specifically, I found that older adults showed an age related reduction in the right hemisphere, for right lateralized targets at the early stages of stimuli processing, indexed by the N1 component, which was absent in young adults. Moreover, the results suggest that older adults used additional neuronal recruitment in the later stages of stimuli processing (P3), to compensate for increased task difficulty and increased resource allocation, likely improving the behavioural results of the older adults so that they were similar to young adults. The work presented in this thesis thus suggest that ageing per se does not result in an attenuated spatial asymmetry (that would be seen as equal to a decline in spatial attention ability). Instead I would argue that, independently of age, a set of underlying non spatial factors (such as load for example) influence the magnitude and direction of spatial asymmetry. In addition, additional neuronal recruitment and intrinsic mechanisms are used in older adults to compensate for possible deficits and this results in maintained performance in this age group.
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