Directed functional connectivity of the sensorimotor system in young and older individuals

Abstract

IntroductionStudies in the sensorimotor system of older versus young individuals have shown alterations in functional connectivity and organization. Our objective was to explore the implications of these differences in terms of local organizations, and to identify processes that correlate with neuropsychological parameters.MethodsUsing a novel multivariate analysis method on resting-state functional MRI data obtained from 50 young and 31 older healthy individuals, we identified directed 4-node functional pathways within the sensorimotor system and examined their correlations with neuropsychological assessments.ResultsIn young individuals, the functional pathways were unidirectional, flowing from the primary motor and sensory cortices to higher motor and visual regions. In older individuals, the functional pathways were more complex. They originated either from the calcarine sulcus or the insula and passed through mutually coupled high-order motor areas before reaching the primary sensory and motor cortices. Additionally, the pathways in older individuals that resembled those found in young individuals exhibited a positive correlation with years of education.DiscussionThe flow pattern of young individuals suggests efficient and fast information transfer. In contrast, the mutual coupling of high-order motor regions in older individuals suggests an inefficient and slow transfer, a less segregated and a more integrated organization. The differences in the number of sensorimotor pathways and of their directionality suggests reduced efferent degenerated pathways and increased afferent compensated pathways. Furthermore, the positive effect of years of education may be associated with the Cognitive Reserve Hypothesis, implying that cognitive reserve could be maintained through specific information transfer pathways.

Directed functional connectivity of the default-mode-network of young and older healthy subjects

“…For neuroimaging macroscopic observations, we followed the line of the network degeneration hypothesis that claims that the topography of atrophy/hypometabolism follows specific brain connectivity networks 30 . We applied it to the DMN to identify degenerated or compensatory processes in older adults, using a novel method of directed connectivity previously developed by us 18 – 23 . By comparing directed pathways between healthy young and older subjects, we identified network alterations, and correlations of pathways with a battery of neuropsychological domains tests suggested their behavioral associations.…”

“…Two male subjects were excluded by these criteria, yielding a final sample of 20 men (age:23.9 ± 2.9 years) and 30 women (23.9 ± 2.4 years). Part of these data was used in a previous study 23 , 31 .…”

“…Psychomotor speed and working memory, executive function, language, long-term memory and visuospatial function were measured (see below). Part of these data was used in previous studies 21 , 23 , 40 .…”

“…A detailed description of the analysis was presented in our previous publications 18 – 23 , therefore, only the main points are summarized below. For a group of four weakly coupled rs-fMRI BOLD temporal signals, with each signal corresponding to a different anatomical location, the analysis assumed that the phases contained information of the temporal order of their mutual coupling.…”

“…This innovative approach facilitates the identification of interactions and directionality among four distinct anatomical regions using phase coherence. Specifically, it allows for the delineation of functional directed couplings and pathways, enabling the identification of directed interactions among these regions to define anterograde information flow pathways 18 – 23 . Computer simulations of the Kuramoto model have tested the accuracy of this analysis 18 , and its applications to the human brain were demonstrated in the above sited studies.…”