White Matter Tract Disruptions Predict Less Affected Hand Impairment Following Stroke: A Longitudinal Diffusion MRI Study

Although less-affected hand (LAH) deficits following unilateral stroke are well documented, many aspects of LAH impairment mechanisms remain unresolved. To provide a better understanding of these mechanisms, we used diffusion MRI to examine the disruptions of white matter structural connections. Based on the redundancy theory, we hypothesized that a summation of motor-related tract disruptions would characterize LAH impairment. We assessed LAH impairment and fractional anisotropy (FA) in 28 patients at one-month post-stroke (baseline), and 6 and 24 months later. LAH impairment was assessed with the Purdue Pegboard Test (PPT), handgrip strength, and movement time. FA was estimated in the CST, Anterior- Corona Radiata (ACR), and Limb of Internal Capsule (ALIC), Superior Longitudinal Fasciculus (SLF), and corpus callosum (CC). We used Linear Mixed Models to determine the tracts associated with LAH impairment over time. Baseline PPT, grip, and movement time were impaired in 43%, 61%, and 25%, respectively. PPT was modeled by baseline ipsilesional-CST (t=3.75; p<0.001), ipsilesional-SLF (t=3.19; p=0.002), contralesional-ALIC (t=-4.89; p<0.001), and lesion volume (t=-3.18; p=0.004); handgrip by baseline ipsilesional-CST (t=3.39; p=0.001), contralesional-ALIC (t=-3.91; p<0.001) and sex (t=-1.43; p=0.007); movement time by baseline ipsilesional-SLF (t=-3.64; p=0.001), CC (t=4.00; p=<0.001), and lesion volume (t=3.03; p=0.006). In conclusion, white matter tract disruptions determine the LAH impairment profile, with ipsilesional-CST related to motor and ipsilesional-SLF to visuomotor processing. LAH impairment was associated with the summation of several tract disruptions, supporting the concept of cerebral redundancy. These results provide a theoretical basis for integrating LAH in rehabilitation programs and for treatment interventions such as neuromodulation.

Fate of turbid glacial inflows in a hydroelectric reservoir

Turbidity from glacial meltwater limits light penetration with potential ecological consequences. Using profiles of temperature, conductivity, and turbidity, we examine the physical processes driving changes in the epilimnetic turbidity of Carpenter Reservoir, a long and narrow, glacier-fed reservoir in southwest British Columbia, Canada. Following the onset of permanent summer stratification, the relatively dense inflows plunged into the hypolimnion, and despite the high glacial load entering the reservoir, the epilimnion cleared due to particle settling. Using a one-dimensional (longitudinal) diffusion equation for a decaying substance to describe the variation in epilimnetic turbidity, we obtain two nondimensional parameters: the epilimnetic inflow parameter, $$\mathcal {I}$$ I , a measure of the turbidity flux into the epilimnion; and the dispersion parameter, $${\mathcal {D}}$$ D , a measure of longitudinal dispersion. In the case of Carpenter Reservoir: $$\mathcal {I}\ll 1$$ I ≪ 1 , indicating that turbidity declines over the summer; and $${\mathcal {D}}\ll 1$$ D ≪ 1 , indicating a strong gradient in turbidity along the epilimnion. Using our theoretical formulation of epilimnetic turbidity variations in conjunction with monthly field surveys, we compute the particle settling velocity ($${\sim}{0.25}\,{\hbox {m}\,\hbox {d}^{-1}}$$ ∼ 0.25 m d - 1 ), the longitudinal dispersion coefficient (50–70 $${\hbox {m}^{2}\,\hbox {s}^{-1}}$$ m 2 s - 1 ), and the flux of turbid water into the epilimnion ($${\sim }1{\%}$$ ∼ 1 % of the total inflow). Our approach is applicable to other reservoirs and can be used to investigate changes in turbidity in response to changes in $$\mathcal {I}$$ I and $${\mathcal {D}}$$ D .