A methodological scoping review of the integration of fMRI to guide dMRI tractography. What has been done and what can be improved: A 20-year perspective.

Combining MRI modalities is a growing trend in neurosciences. It provides opportunities to investigate the brain architecture supporting cognitive functions. Integrating fMRI activation to guide dMRI tractography offers potential advantages over standard tractography methods. A quick glimpse of the literature on this topic reveals that this technique is challenging, and no consensus or "best practices" currently exist, at least not within a single document. We present the first attempt to systematically analyze and summarize the literature of 80 studies that integrated task-based fMRI results to guide tractography, over the last two decades. We report 19 findings that cover challenges related to sample size, microstructure modelling, seeding methods, multimodal space registration, false negatives/positives, specificity/validity, gray/white matter interface and more. These findings will help the scientific community (1) understand the strengths and limitations of the approaches, (2) design studies using this integrative framework, and (3) motivate researchers to fill the gaps identified. We provide references toward best practices, in order to improve the overall result's replicability, sensitivity, specificity, and validity.

Functional network and structural connections involved in picture naming.

We mapped the left hemisphere cortical regions and fiber bundles involved in picture naming in adults by integrating task-based fMRI with dMRI tractography. We showed that a ventral pathway that "maps image and sound to meaning" involves the middle occipital, inferior temporal, superior temporal, inferior frontal gyri, and the temporal pole where a signal exchange is made possible by the inferior fronto-occipital, inferior longitudinal, middle longitudinal, uncinate fasciculi, and the extreme capsule. A dorsal pathway that "maps sound to speech" implicates the inferior temporal, superior temporal, inferior frontal, precentral gyri, and the supplementary motor area where the arcuate fasciculus and the frontal aslant ensure intercommunication. This study provides a neurocognitive model of picture naming and supports the hypothesis that the ventral indirect route passes through the temporal pole. This further supports the idea that the inferior and superior temporal gyri may play pivotal roles within the dual-stream framework of language.

Functional Connectivity of Successful Picture-Naming: Age-Specific Organization and the Effect of Engaging in Stimulating Activities.

Aging is a lifelong process that starts at birth. Throughout the course of their life, individuals are exposed to various levels of stimulating activities. A higher level of engagement in such activities is suspected to protect against the normal course of cognitive aging or the cognitive manifestations of age-related brain diseases. However, the exact mechanism underlying such protective action remains unclear. The concept of the neurocognitive reserve was introduced to refer to the hypothesis that engagement in stimulating activities shapes brain structure and function, thus indirectly allowing for better preserved cognitive abilities. Although it is known that word production is among the best-preserved cognitive abilities in aging, the underlying neurofunctional mechanisms that allow this relative preservation are still unknown, and it is still unclear how engagement in stimulating activities affects these processes. The objective of this study is to describe the brain functional connectivity patterns associated with picture-naming abilities in younger and older adults with varying levels of engagement in stimulating activities, as a proxy for neurocognitive reserve. A mediation analysis was applied to determine whether the association between reserve proxies and naming accuracy is dependent on task FC. Results show that naming accuracy depends on the posterior cingulate cortex (PCC) functional decoupling in both younger and older adults but through different pathways. While high-performing older adults rely on the asynchronization of this area from motor speech regions' activity, the best-performing younger adults rely on the functional decoupling with language-related regions. Mediation analysis reveals that the PCC decoupling mediates the relationship between the level of engagement in stimulating activities and naming accuracy in younger adults, but not in older adults. These findings suggest that reserve-related mechanisms may be more critical for naming in early adult life, while older adults' neurofunctional organization may benefit more from a lifetime of acquired knowledge.

Differential language network functional connectivity alterations in Alzheimer's disease and the semantic variant of primary progressive aphasia.

Patients with Alzheimer's disease (AD) and semantic variant primary progressive aphasia (svPPA) can present with similar language impairments, mainly in naming. It has been hypothesized that these deficits are associated with different brain mechanisms in each disease, but no previous study has used a network approach to explore this hypothesis. The aim of this study was to compare resting-state functional magnetic resonance imaging (rs-fMRI) language network in AD, svPPA patients, and cognitively unimpaired elderly adults (CTRL). Therefore, 10 AD patients, 12 svPPA patients and 11 CTRL underwent rs-fMRI. Seed-based functional connectivity analyses were conducted using regions of interest in the left anterior temporal lobe (ATL), left posterior middle temporal gyrus (pMTG) and left inferior frontal gyrus (IFG), applying a voxelwise correction for gray matter volume. In AD patients, the left pMTG was the only key language region showing functional connectivity changes, mainly a reduced interhemispheric functional connectivity with its right-hemisphere counterpart, in comparison to CTRL. In svPPA patients, we observed a functional isolation of the left ATL, both decreases and increases in functional connectivity from the left pMTG and increased functional connectivity form the left IFG. Post-hoc analyses showed that naming impairments were overall associated with the functional disconnections observed across the language network. In conclusion, AD and svPPA patients present distinct language network functional connectivity profiles. In AD patients, functional connectivity changes were restricted to the left pMTG and were overall less severe in comparison to svPPA patients. Results in svPPA patients suggest decreased functional connectivity along the ventral language pathway and increased functional connectivity along the dorsal language pathway. Finally, the observed connectivity patterns are overall consistent with previously reported structural connectivity and language profiles in these patients.

Linking mind wandering tendency to risky driving in young male drivers.

Risky driving is a significant contributor to road traffic crashes, especially in young drivers. Transient mind wandering states, an internal form of distraction, are associated with faster driving, reduced headway distance, slower response times, reduced driver vigilance, and increased crash risk. It is unclear whether a trait tendency to mind wander predicts risky driving, however. Mind wandering is also associated with poor executive control, but whether this capacity moderates the putative link between mind wandering tendency and risky driving is uncertain. The present study tested whether mind wandering tendency predicts risky driving behaviour in young male drivers aged 18-21 (N=30) and whether this relationship is mediated by driver vigilance and moderated by executive control capacity. Mind wandering was measured with the Sustained Attention to Response Task (SART) and the Daydreaming Frequency Scale (DDFS). Risky driving was assessed by mean speed in a driving simulator and driver vigilance was quantified by horizontal eye movements measured with eye tracking. Results showed that greater mind wandering tendency based on SART performance significantly predicts faster mean speed, confirming the main hypothesis. Neither driver vigilance mediated nor executive control capacity moderated this relationship as hypothesized. These findings speak to the complexity of individual differences in mind wandering. Overall, mind wandering tendency is a significant marker of risky driving in young drivers, which could guide the development of targeted interventions.