Increased sensitivity to age-related differences in brain functional connectivity during continuous multiple object tracking compared to resting-state
Introduction
Individual life-span trajectories in brain and cognition are shaped by a dynamic interplay between genetic and environmental factors (Lindenberger, 2014). Aging confers a notable increase in inter-individual variability in cognitive functions (Buckner, 2004, Singh-Manoux et al., 2012), and developing sensitive and specific in vivo biomarkers for identification of individuals at risk for cognitive impairment and dementia is a key challenge for clinical neuroscience.
The brain is intrinsically organized into neuronal networks or nodes of which continuous cross-talk enables cognition and complex behaviors (Sporns et al., 2004). Functional connectivity (FC) measured by functional magnetic resonance imaging (fMRI) refers to the synchronization between nodes, reflecting an organizational principle of the brain (Van Den Heuvel and Pol, 2010). In line with this system-level view of brain function, aging studies converge on less specialized and segregated brain networks, supporting the theory of cognitive and neuronal dedifferentiation in aging (Baltes and Lindenberger, 1997; Chan et al., 2014; Daselaar et al., 2005; Lindenberger, 2014; Park et al., 2012).
Age-related FC differences have been extensively studied, implicating a range of networks including the executive control (ECN), dorsal attention (DAN), default mode (DMN), frontoparietal (FPN) and hippocampal networks (Salami et al., 2014, Voss et al., 2010, Zhang et al., 2014), and links between DMN connectivity and cognitive aging (Andrews-Hanna et al., 2007, Sambataro et al., 2010) corroborate the notion of network-level vulnerability to advancing age (Mowinckel et al., 2012, Salami et al., 2012).
In line with the selective vulnerability of brain networks, age-related cognitive decline is not uniform across domains (Glisky, 2007). Whereas crystallized functions including vocabulary and general knowledge remain relatively unaffected, a commonly reported impairment relates to deficient selective attention (Quigley and Müller, 2014) which manifests as reduced ability to selectively attend to and ignore information based on relevance (Kensinger and Corkin, 2009, Murray and Kensinger, 2012). Whereas resting-state fMRI has provided an opportunity to test specific hypotheses regarding the underlying brain connectivity alterations, the selective cognitive vulnerability suggests that attention-demanding tasks may comprise a more sensitive and informative context for the study of age-related brain network alterations. Thus, in order to characterize the brain functional connectomic signature of cognitive aging in different cognitive contexts, we compare fMRI brain connectivity indices obtained during rest and during two load levels of a continuous multiple object tracking (MOT) task (Pylyshyn and Storm, 1988) between young and older healthy adults.
MOT requires the subject to attend to multiple target items as they move among distractor items, which is a basic feature of sustained multifocal visual attention (Cavanagh and Alvarez, 2005). Alnæs et al. (2015) reported widespread connectivity modulations during MOT in young adults, and machine learning revealed high classification accuracy when discriminating between task and rest, demonstrating successful decoding of cognitive load using FC features. Here, we utilize a similar decoding approach to describe the neuronal characteristics of cognitive aging by comparing two groups of healthy adults at both ends of the adult lifespan. We hypothesized that (1) multivariate classification would yield robust discrimination between resting state and data collected during MOT as well as between younger and older adults, reflecting age and cognitive context related changes in brain network configuration. Higher task classification accuracy in young compared to older adults would support the notion of cognitive and neuronal dedifferentiation in aging. Additionally, we anticipated stronger group differences with higher load, resulting in improved group classification with increasing load. On edge-level, we hypothesized (2) that the most discriminative edges for rest and task are found in connections between attention networks and somatosensory and motor nodes (Alnæs et al., 2015, Tomasi et al., 2014). With regards to age-related edgewise effects in response to attentional demand and keeping with the theory of dedifferentiation, we hypothesized a system-level loss in network segregation, reflected in decreased FC within modular networks and in particular within the DMN as well as increased connectivity between large-scale brain networks, in particular during continuous tracking, which would support a relative inability to selectively inhibit competing neural processes during task engagement.
Finally, since any effects on edgewise functional connectivity may be partly driven by nodal increases or decreases in activity, we probe temporal variability on node level by computing the standard deviation of signal amplitude (SDSA) (Garrett et al., 2010). Brain signal variability has been shown to reveal distinct patterns not captured by mean-based analyses (Garrett et al., 2011), and increased signal variability is thought to be indicative of a more sophisticated and complex neural machinery (McIntosh et al., 2008), offering greater dynamic range and facilitating the brain's ability to explore different network states. When applied to aging studies, reduced SDSA has proved to be a better predictor of advancing age and poorer task performance than the mean measure (Grady and Garrett, 2014). Additionally, in a study investigating age-related variability changes in response to cognitive demand, Garrett et al. (2012) observed a broad increase in SDSA on task compared to fixation for both age groups, the magnitude and spatial extent of this variability increase was comparatively reduced for the older group. Based on this, we hypothesize (3) task related modulations of signal variability that is network selective, reflected in increased and decreased SDSA in task-positive and task-negative/irrelevant networks, respectively. Further, reflecting reduced dynamic repertoire in response to cognitive state-transition, we anticipate group differences in signal variability manifested as diminished task-related SDSA changes for older compared to younger adults.
Section snippets
Sample
We recruited 26 young and 26 older adults through a newspaper ad and social media. The sample is overlapping with a previous publication (Dørum et al., 2016) reporting results from the blocked MOT runs (see below). All subjects provided an informed consent and underwent neuropsychological screening (details below). Participants reported normal or corrected-to-normal vision. Exclusion criteria included estimated IQ<70, previous history of alcohol- and substance abuse, history of neurologic or
Neuropsychology and demographics
Table 1 summarizes key demographics and WASI and MOT performance. Briefly, the younger group performed significantly better on the matrix reasoning subtest of the WASI. Repeated measures ANOVA revealed significant main effects of load (F=17.58, p<.001) and group (F=4.11, p=.049) on MOT accuracy, indicating higher accuracy during L1 compared to L2 and for young compared to old, but no group by load interaction (F=.35, p=.56). No other significant differences were found.
Classification analysis
Fig. 1 shows the confusion
Discussion
The heterogeneity in cognitive aging across cognitive domains is mirrored in a differential vulnerability of brain network connectivity, which has been proposed as a candidate imaging biomarker for age-related cognitive decline and risk and progression of dementia. Since previous studies have primarily utilized resting-state fMRI, we assessed the generalizability of the predictive FC patterns across the resting-state and constrained contexts characterized by higher cognitive load. In line with
Conclusion
Our main findings demonstrate that FC during a constrained continuous tracking task strongly increases sensitivity to age compared to the conventional resting-state paradigm, and secondly, that data obtained during rest and task is more distinguishable in young compared to older individuals, providing further neuronal support of the conjecture of cognitive and neuronal dedifferentiation in aging. Complementary connectivity analyses on node and edge level, as well as nodal signal variability
Acknowledgements
This work was funded by the Research Council of Norway (204966/F20), the South-Eastern Norway Regional Health Authority (2013054, 2014097, 2015073), and the Norwegian Extra Foundation for Health and Rehabilitation (2015/FO5146).
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2018, NeuroImageCitation Excerpt :We chose an ICA, because it is a data-driven approach that results in spatially independent components well suited for FC analyses. We used a group ICA and imposed the group components onto each subject (Dørum et al., 2017; Miller et al., 2016), because it is important to strive for the same parcellation in each subject in order to compare connectomes across subjects (Smith et al., 2013). We used the ICA components directly as nodes for the FC analysis.
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