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This project has extended our knowledge on the age-related decrease in mind- wandering, age-related changes in thought content, the sensitivity of self-generated thoughts, and their malleability. Two studies specifically considered the impact of older adults’ distinct neurocognitive profile on self-generated thoughts for the first time. In fact, it can be concluded that age-related changes in self-generated thoughts are underlined by age-related changes in brain activity; specifically, within the DMN. Firstly, spectral power reflecting DMN activity and endogenous top-down processes (i.e. beta band), enabled the flexible adjustment of mental time travelling in young adults. However, the older adults’ beta band activity was suspected to underlie the absence of such flexibility in this population. Secondly, lower connectivity between the left temporal lobe and prefrontal regions of the DMN fostered spontaneous off-task thoughts and creative thinking in young and older adults respectively. Together, these findings indicated how mind-wandering experiences are influenced by the neurocognitive profile of older adults. While further work is needed to fully understand the role of the DMN network in mind-wandering in ageing, these studies suggest that the problem lays in the generation of these experiences.

Beyond neuroimaging, this thesis described in detail age-related changes in thought content. The behavioural findings of this project replicate the age-related decrease in off-task thoughts and extended our knowledge to the heterogeneity of self- generated thoughts in ageing. When compared to young adults, older individuals reported similar rates of positively biased thoughts (i.e. future-oriented thought, self- reflection), a smaller rate of negatively oriented thoughts (i.e. past-oriented thoughts, rumination), and more visual thoughts, as well as task-related interferences. These differences are informative due to the influence of thought content upon behavioural outcomes. Past-oriented thoughts often increase negative outcomes (e.g. task errors, negative mood; Banks et al., 2016; Poerio et al., 2013), while future-oriented thoughts

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enable more positive outcomes (e.g. efficient planning of personal goals, alleviate negative mood, fosters positive mood Baird et al., 2011; Engert et al., 2014; Ruby, Smallwood, Sackur, et al., 2013). Therefore, the mind-wandering profile of older adults adds to previous research describing older adults as positive and mindful individuals (Carstensen et al., 1999; Grühn et al., 2010; Splevins et al., 2009). Behavioural consequences of these changes in thought content should be investigated to clarify our understanding of ageing. Altogether, these findings addressed an important gap in the literature by further characterising the cognitive profile of older adults as well as refining our theories of the wandering mind. Moreover, this converging methods approach demonstrated the utility in combining new advances in both the neurosciences and traditional behavioural techniques.

With regards to the general population, this project also extended our knowledge about the influences of intrinsic and extrinsic factors on thought content. Intrinsic factors comprise individual differences, such as pathologies, culture or working memory capacity to name a few. As an example, research has outlined how working memory capacity positively impacts the flexible adjustment of self-generated thoughts (Rummel & Boywitt, 2014). In a similar vein, extrinsic factors, such as task difficulty, duration or variability, also modify participants’ experiences. Thus, difficult tasks significantly decrease the proportion of mind-wandering experiences (McVay & Kane, 2009). A contribution from the present project was to extend our knowledge regarding both intrinsic and extrinsic factors, and their impact on thought content. The impact of task difficulty on frequency was replicated and extended to thought content with task difficulty increasing verbal thoughts. Verbal thoughts were predicted by an increase of gamma power illustrating the binding of information and higher order processing (Fries, 2009; Jensen et al., 2007). Binding enables the integration and in-depth processing of information coming from different modalities. Indeed, the functional significance regarding binding processes has been outlined with evidence of complex mental tasks

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increasing gamma power (Fitzgibbon, Pope, Mackenzie, Clark, & Willoughby, 2004). Thus, imaging data provided evidence that thoughts in the form of words merely reflected verbal rehearsal, a strategy particularly useful during a demanding task and an important element in working memory function. Regarding intrinsic factors, individual differences such as one’s cultural affiliation and language were previously found to have notable impacts on cognition. For instance, time is conceptualised differently in Mandarin and English populations (Boroditsky, 2001), and French citizens tend to be more expressive than British citizens (Fortier & Moulin, 2015). This thesis outlined cultural difference between French and British native speakers in terms of frequency and content of self-generated thoughts. Together, describing the influence of different factors has informed us of the variability and sensitivity of thought content as well as the functionality of certain self-generated thoughts.

Finally, this project comprised one of the first intervention study aiming to change thought content in a population of young and older adults. Although self- generated thoughts are highly sensitive to various factors, limited prior work has directly aimed to modify thought content through experimentation. The successful transformation of self-generated thoughts has set the scene for a better understanding of the regulation and generation processes of mind-wandering. For example, minimising participants’ personal concerns should, and has, reduced the occurrence of self-referential thoughts and informed the generation process of mind-wandering. On the other hand, improving purely attentional control skills should bring greater insight about the management of different self-generated thoughts. Last, but not least, the contribution of this study also includes the future design of interventions aimed at patients facing difficulties with mind-wandering experiences. Several clinical conditions, for instance depression and anxiety disorder, would greatly benefit from interventions targeting detrimental self-generated thoughts, while preserving beneficial ones. Particularly, meditation practice, which has been successfully used with depressive and

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anxious patients (Goyal et al., 2014; Hofmann, Sawyer, Witt, & Oh, 2010), was found to significantly reduce negatively oriented thoughts while protecting positive ones (Chapter 6). Future work should maximise the benefits of such interventions by studying the relationship between changes in thought content and behavioural outcomes.