Biology subjects

Lundell, H.

Publications and source records attributed to Lundell, H..

4 recordsLinked to original sources

bioRxiv2025

No Evidence for Stronger Brain-Behavior Associations in the Bimanual Motor Network of Older Adults

Bimanual motor performance declines with age, is accompanied by cortical thinning and alterations in white matter microstructure within motor control networks. Aging is also associated with increased interindividual variability in behavioural and structural markers, and some studies report stronger brain-behaviour associations in older compared to younger adults. However, evidence for this remains mixed, and it is unclear whether young adults express similar relationships under comparable task demands. We tested whether bimanual performance is related differently to structural brain properties in younger and older adults. Twenty-three younger (22-27 years) and twenty-three older adults (65-70 years) performed a visually guided bimanual pinch-force task and underwent whole-brain structural magnetic resonance imaging (MRI). Fractional anisotropy (FA) and cortical thickness (CTh) were extracted from a pre-defined bilateral visuo-motor network and compared between groups. In regions showing significant age-related differences, we tested whether and how FA and CTh values correlates with bimanual performance within each age group. Older adults showed lower FA in the anterior portions of the bilateral superior longitudinal fasciculus (SLF) III and right inferior fronto-occipital fasciculus (IFOF), and lower CTh across most visuomotor regions, along with lower bimanual performance compared to young adults. Structure-function analyses in these areas revealed that bimanual performance correlated positively with FA in the anterior segment of the right SLF III in younger but not older adults. Furthermore, larger hemispheric asymmetries in cortical thickness between the dominant and non-dominant SMA correlated positively with larger intermanual performance differences between the dominant and non-dominant hand in younger but not older adults. These findings suggest that intrahemispheric white-matter integrity and interhemispheric cortical balance support efficient bimanual control in early adulthood, but they provide no evidence that structure-function relationships increase with age.

neuroscience↗

bioRxiv2023

Susceptibility-induced internal gradients reveal axon morphology and cause anisotropic effects in the dMRI signal

Diffusion-weighted MRI is our most promising method for estimating microscopic tissue morphology in vivo. The signal acquisition is based on scanner-generated external magnetic gradients. However, it will also be affected by susceptibility-induced internal magnetic gradients caused by interaction between the tissue and the static magnetic field of the scanner. With 3D in silico experiments, we show how internal gradients cause morphology-, compartment-, and orientation-dependence of spin-echo and pulsed-gradient spin-echo experiments in myelinated axons. These effects are unseen in previous 2D modelling. For an ex vivo monkey brain, we observe the orientation-dependency generated only when including non-circular cross-sections in the in silico morphological configurations, and find orientation-dependent deviation of up to 17% for diffusion tensor metrics. Our findings underline the importance of accounting for realistic 3D axon morphology in modelling. Interestingly, the morphology-specific orientation-dependency trends show potential for a novel sensitivity to morphology, which is not attainable by the theoretical diffusion-weighted MRI signal itself.

neuroscience↗

bioRxiv2021

Does powder averaging remove dispersion bias in diffusion MRI diameter estimates within real 3D axonal architectures?

Noninvasive estimation of axon diameter with diffusion MRI holds potential to investigate the dynamic properties of the brain network and pathology of neurodegenerative diseases. Recent methods use powder averaging to account for complex white matter architectures, such as fibre crossing regions, but these have not been validated for real axonal geometries. Here, we present 120 - 313 m long segmented axons from X-ray nano-holotomography volumes of a splenium and crossing fibre region of a vervet monkey brain. We show that the axons in the complex crossing fibre region, which contains callosal, association, and corticospinal connections, are larger and exhibit a wider distribution than those of the splenium region. To accurately estimate the axon diameter in these regions, therefore, sensitivity to a wide range of diameters is required. We demonstrate how the q-value, b-value, signal-to-noise ratio and the assumed intra-axonal parallel diffusivity influence the range of measurable diameters with powder average approaches. Furthermore, we show how Gaussian distributed noise results in a wider range of measurable diameter at high b-values than Rician distributed noise, even at high signal-to-noise ratios of 100. The number of gradient directions is also shown to impose a lower bound on measurable diameter. Our results indicate that axon diameter estimation can be performed with only few b-shells, and that additional shells do not improve the accuracy of the estimate. Through Monte Carlo simulations of diffusion, we show that powder averaging techniques succeed in providing accurate estimates of axon diameter across a range of sequence parameters and diffusion times, even in complex white matter architectures. At sufficiently low b-values, the acquisition becomes sensitive to axonal microdispersion and the intra-axonal parallel diffusivity shows time dependency at both in vivo and ex vivo intrinsic diffusivities.

neuroscience↗

bioRxiv2018

Uncovering a role for the dorsal hippocampal commissure in episodic memory

The dorsal hippocampal commissure (DHC) is a white matter tract that provides inter-hemispheric connections between temporal lobe brain regions. Despite the importance of these regions for learning and memory, there is scant evidence of a role for the DHC in successful memory performance. We used diffusion-weighted MRI (DW-MRI) and white matter tractography to reconstruct the DHC across both humans (in vivo) and nonhuman primates (ex vivo). Across species, our findings demonstrate close consistency between the known anatomy and tract reconstructions of the DHC. Anterograde tract-tracer techniques also highlighted the parahippocampal origins of DHC fibers in nonhuman primates. Finally, we derived Diffusion Tensor MRI (DT-MRI) metrics from the DHC in a large sample of human subjects to investigate whether inter-individual variation in DHC microstructure is predictive of memory performance. The mean diffusivity of the DHC was correlated with performance in a standardised episodic memory task; an effect that was not reproduced in a comparison commissure tract - the anterior commissure. These findings highlight a role for the DHC in episodic memory, and our tract reconstruction approach has the potential to generate further novel insights into the role of this previously understudied white matter tract in both health and disease.

neuroscience↗