The Neuropsychological and Neurophysiological Signatures of Age-Related Differences in Mind-Wandering

Abstract

Our inner mental life is subject to a constant, discursive, dynamically evolving stream of thoughts. Mind-wandering is the mind s default state, occupying up to half of all conscious cogitations, during which our thoughts momentarily stray from the here-and-now of perceptual experience to unrelated, self-generated mental content. Such attentional fluctuations may occur with and without deliberate intention. As a central facet of the human experience, mind-wandering has attracted incremental interdisciplinary research interest over the last decade or so. Despite the quotidian ubiquity of this mental phenomenon, mind-wandering research, particularly as it pertains to healthy ageing, remains exiguous. Although many cognitive abilities decline with advancing age, recent studies have demonstrated a consistent and perhaps, paradoxical finding of reduced mind-wandering propensity with age. Considering age-related cognitive decline in later life represents a leading cause of disease burden, loss of functional independence, and reduced quality of life, there is a research imperative to explore the impact of age on a broad spectrum of cognitive phenomena. Moreover, considering the possible adaptive and maladaptive corollaries of mind-wandering, the extent to which mind-wandering is disrupted, or influenced, by the natural ageing process is a timely issue. Therefore, the overarching purposes of the present work were two-fold: 1) To examine whether the natural ageing process influences mind-wandering frequency and phenomenology (in an age-comparative design); and 2) To investigate the shared and distinct neuropsychological and neurophysiological signatures of fluctuating attentional states as they unfold over time in younger and older adults. We employed a multi-faceted methodological approach involving healthy younger (18-35 years of age) and older (65-80 years of age) adult participants. In the first session, participants completed a battery of standardised cognitive and neuropsychological measures. In a second session, participants performed a non-demanding, computerised sustained attention task with built-in experience sampling probes and concurrent electroencephalography and pupillometry recording. Extending traditional research paradigms that utilised tasks with salient, sudden-onset, and predictably occurring targets, our task approach featured elements that circumvented exogenous attention capture and placed greater reliance on endogenous attentional control. The current task, as modified, was therefore well-suited for examining fluctuating attentional states. Moreover, to overcome methodological challenges for measuring mind-wandering owing to its ephemeral and covert phenomenology, we analysed convergent evidence from subjective, behavioural, electrophysiological, and pupillometric sources. Triangulation was employed to elucidate the neuropsychological and neurophysiological mechanisms underlying mind-wandering, with the capacity to differentiate younger and older adults. The high temporal resolution of these physiological recordings facilitated measurement of the discrete and dissociable neural signals that reflect the transitory shifts that occur between goal-directed thinking and mind-wandering as they unfold in real-time. Chapter 1 (Introduction) presents a comprehensive synthesis of the mind-wandering and ageing literature. It presents a critical review of the fundamental conceptual and methodological issues concerning the definition and measurement of mind-wandering and delineates the advantages and limitations of current methodological approaches. We discuss evolving theoretical frameworks and suggested phenomenological properties and neurocognitive underpinnings of age-related mind-wandering. We identify remaining gaps in the literature and propose the research objectives to be addressed in the current thesis. Chapter 2 (Empirical Paper 1) explored the frequency and phenomenology of mind-wandering as a function of age and examined the neuropsychological variables mediating age-related differences in unintentional and intentional mind-wandering. Our results replicated the finding of an age-related reduction in mind-wandering frequency and demonstrated that unintentional, but not intentional, mind-wandering was predicted by affective and motivational models. Despite evidence of declining executive resources with age, neither cognitive nor task demand variables further contributed to the relationship between age group and mind-wandering propensity. Further, an age-related behavioural difference in reaction time variability (RTV), a known index of oscillatory attention cycles, was observed and mediated the relationship between intentional mind-wandering and false alarms. Additionally, the large effect size for the age-related reduction in intentional mind-wandering suggests a particular tendency by younger adults to wilfully disengage from the task. Considering the trade-off in competing resources for mind-wandering and task performance, the relative group parity in performance suggests strategic differences in how younger and older adults approached the task. Together, this study showed that older adults tended to be more focused, less impeded by anxiety, and less mentally restless than younger adults. Notably, older adults appeared to mitigate the negative aspects of cognitive decline and potential performance decrements by increasing motivation and adopting a more efficient exploitative oscillation strategy to suspend the wandering mind when task focus was required. By contrast, younger adults utilised their greater resources to implement a more balanced oscillation strategy. They showed greater explorative tendencies indexed by more frequent mind-wandering (especially intentionally) and more variable performance. Intentional mind-wandering may therefore reflect an adaptive exploratory state that younger adults engage in more frequently without cost. This study showed that distinguishing between the presence, or absence, of intentionality has generated unique insights into age-related mind-wandering that can provide a basis for future research. Chapter 3 (Empirical Paper 2) explored the impact of ageing on the strategic trade-off between competing demands of task focus and mind-wandering, as expressed by the exploitation/exploration framework. Neurophysiological measures of endogenous attention revealed age-related reductions in pre-target alpha variability coupled with reduced pre-target mean pupil diameter (PD) and higher post-target PD amplitudes, suggesting steadier attentional engagement with age. Signal analysis in the pre-probe interval provided support for perceptual decoupling, the process whereby attention is disengaged from sensory input and redirected inward toward self-generated mental content. Specifically, older adults exhibited greater sensory evidence representation (higher mean amplitude for the steady-state visually evoked potential, SSVEP) during focused compared to mind-wandering states. Younger adults displayed greater variability in the SSVEP sensory and alpha attentional signals, as well as higher mean PD amplitudes preceding mind-wandering relative to focused states. As such, younger adults pursued more intermittent sensory encoding and fluctuating attention during mind-wandering. An age-related reduction in alpha variability prior to mind-wandering further supported a less pronounced transition from exploitative to exploratory states by older adults, even when mind-wandering was reported. Neural indices of perceptual decision formation (centro-parietal positivity (CPP)), sensory evidence encoding (SSVEP), and motor preparation (mu/beta) showed that younger and older participants similarly tracked the exogenously driven feature changes of target evolution over time. Older adults, however, more faithfully tracked the downward trajectory of the visual stimulus (indexed by reduced mean pre-target SSVEP amplitude) and further, demonstrated earlier initiation of sensory evidence accumulation (earlier onset CPP). Against the backdrop of reduced executive resources with age, these findings suggest that older adults employ a more exploitative strategy, attending more consistently to the task. Conversely, younger adults transitioned in and out of an exploratory state more frequently, as corroborated by their increased mind-wandering frequency, and greater variability in evidence encoding and attention. Given that they did not incur relative performance costs, younger adults may have more resources to oscillate between focused and mind-wandering states more optimally. Chapter 4 (Empirical Paper 3) investigated time-on-task changes in momentary attentional fluctuations and deteriorations by examining the temporal evolution of mind-wandering, behavioural performance, and pupil dynamics serially over time for younger and older adults. The task s simplified perceptual requirements meant that performance was relatively non-demanding over shorter timescales but became increasingly challenging over a prolonged duration. Indeed, our task was sensitive to time-on-task performance decrements and showed increased unintentional and intentional mind-wandering frequencies over shorter (within-block) and longer (across-blocks) timescales. Older adults exhibited a linear decrease in self-reported focus across the 8 task blocks, indicating a slow decline in their exploit mode over time. In contrast, younger adults demonstrated a sudden drop after block one, but an absence of subsequent change, signaling that younger adults regulated their exploit/explore ratio more efficiently according to task demands. Further, both groups received a boost in performance and better focus after brief between-block breaks. Following breaks, younger adults exhibited a lower propensity to intentionally mind-wander suggesting that they deliberately explored the mind-wandering space when their task motivation waned by the end of each block. By contrast, older adults remained as focused before and after the break. Endogenous baseline PD was analysed as a proxy psychophysiological measure of locus coeruleus noradrenaline (LC-NA) neuromodulatory activity, representing one potential mechanism through which brain states may flexibly shift between different serial exploit/explore modes. Pre-target PD was analysed according to the different inter-trial-intervals (ITI), namely 3-, 5-, and 7-seconds. Younger adults gradually reduced their PD as time unfolded before the target (especially at 5 and 7 seconds), dropping out of a relatively exploratory state and returning to an exploitative state just in time. Conversely, older adults demonstrated steadier PD before targets, consistent with their more exploitative approach. Together these behavioural and pupillary findings propound a more explorative oscillation strategy in younger adults, adaptively shifting back-and-forth from the task to competing thoughts more frequently than older adults over distinct timescales. By contrast, older adults marshalled their more limited cognitive resources more predominately toward the task, fixed in an exploit mode and prioritising task-relevant information, to mitigate performance costs. Chapter 5 (General Discussion) concludes this thesis with a critical discussion of the principal findings and contributions, and the theoretical and practical implications, of the present work. We outline outstanding challenges in the field and identify future research directions. Our study is the first, to our knowledge, to concurrently contrast competing theories of age-related mind-wandering to directly compare the relative contributions of dominant models in the field. The research provides new insight into the influence of the natural ageing process on mind-wandering, highlighting the adaptive strategies and positive qualities adopted by older adults leading to a beneficial reduction in mind-wandering and equivalent performance with younger adults, despite evidence of age-related cognitive decline. We suggest that this represents an adaptive quality of successful ageing; namely, older adults suspend the wandering mind to allay potential costs when the context demands it. Younger adults, on the other hand, explore the mind-wandering space and adaptively oscillate between competing strategies. Together, our research highlights the nature, neuropsychological and neurophysiological correlates, and subcortical contributions to fluctuating and deteriorating attentional states. Our findings provide new insight into how unintentional and intentional mind-wandering processes change with age and over time. Dissecting the mechanisms underlying different attentional processes may provide important indications of successful ageing that inform future interventions.