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• Research Paper Volume 12, Issue 5 pp 4617-4640

  Alterations of hand sensorimotor function and cortical motor representations over the adult lifespan

  Relevance score: 10.822299

  Melina Hehl, Stephan P. Swinnen, Koen Cuypers

  Keywords: Transcranial Magnetic Stimulation (TMS), cortical motor representation, aging, intracortical inhibition/facilitation, sensorimotor performance

  Published in Aging on March 11, 2020

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  Using a cross sectional design, we aimed to identify the effect of aging on sensorimotor function and cortical motor representations of two intrinsic hand muscles, as well as the course and timing of those changes. Furthermore, the link between cortical motor representations, sensorimotor function, and intracortical inhibition and facilitation was investigated. Seventy-seven participants over the full adult lifespan were enrolled. For the first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscle, cortical motor representations, GABA_A-mediated short-interval intracortical inhibition (SICI), and glutamate-mediated intracortical facilitation (ICF) were assessed using transcranial magnetic stimulation over the dominant primary motor cortex. Additionally, participants’ dexterity and force were measured. Linear, polynomial, and piecewise linear regression analyses were conducted to identify the course and timing of age-related differences. Our results demonstrated variation in sensorimotor function over the lifespan, with a marked decline starting around the mid-thirties. Furthermore, an age-related reduction in cortical motor representation volume and maximal MEP of the FDI, but not for ADM, was observed, occurring mainly until the mid-forties. Area of the cortical motor representation did not change with advancing age. Furthermore, cortical motor representations, sensorimotor function, and measures of intracortical inhibition and facilitation were not interrelated.

  

  Plots of best fitting regression models for changes in sensorimotor performance over the lifespan. For piecewise linear regressions the breakpoint (BP) is indicated. (A) Purdue Pegboard Test unimanual peg placement with the dominant hand (PPT_1), (B) Purdue Pegboard Test peg placement with both hands (PPT_2), (C) Purdue Pegboard Test assembly task (PPT_A), (D) maximal Grip Force, and (E) maximal Pinch Force. For piecewise linear regressions, data points below and above the breakpoint are represented by open and filled points, respectively. Below each plot the best fitting regression model is stated in a rectangle. Ribbons depict the 95% confidence interval of the fit. Significant p-values are indicated with asterisks (*** p < 0.001; ** p < 0.01; * p < 0.05) and printed in bold.

  
  
  

  

  Best regression fits for cortical motor map parameters area (AREA), volume (VOL), and maximal motor evoked potential (MAXMEP) of first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscle. Estimates of significant regression models are stated in a rectangle below the plot and the regression is indicated in the plot by a solid line. For non-significant relationships, the best fit, on which the R²adjusted and p-value are based, is stated; a dashed line represents a non-significant regression. Ribbons depict the 95% confidence interval of the fit. Significant p-values are indicated with asterisks (*** p < 0.001; ** p < 0.01; * p < 0.05) and printed in bold.

  
  
  

  

  Normalized volume (VOLnorm = volume (VOL)/maximal motor evoked potential (MAXMEP)) for dominant first dorsal interosseus (FDI) (left panel) and abductor digiti minimi (ADM) (right panel) by age. Below each plot the best fitting model is stated in a rectangle. Ribbons depict the 95% confidence interval of the fit. Both regressions were non-significant.

  
  
  

  

  Mapping Procedure performed with the Brainsight®2 software (Rogue Research Inc, Montreal, Quebec, Canada). (A) Grid used for searching the hotspot of 19x19, 1 cm-spaced predefined target locations and their trajectories around the vertex, visualized in black. The target representing the vertex is indicated by a cross, the hotspot by a circle. (B) Standardized mapping grid (11x11, 1 cm-spaced) around hotspot (circled target). Predefined target locations and trajectories are visualized in white. (C) Stimulated targets and their trajectories (in orange) are visualized on top of the mapping grid. Stimulation was performed in a 45° angle. (D) Similar to (C) without the mapping grid. (E) Cortical motor representation processed in Matlab with averaged motor evoked potential (MEP) values per point normalized to the individual maximal MEP.

  
  
  

  

  Visualization of the cortical motor representation parameters: area, volume, and maximal motor evoked potential (MAXMEP). Area was measured in cm² by calculating the area of a polygon expanding over all active points. Volume was measured in μV*cm² and defined as the sum of the mean motor evoked potential (MEP) peak-to-peak amplitudes of all active points, serving as an approximation of the three-dimensional integration of the map. The maximal mean MEP value of all active points was obtained as MAXMEP and measured in μV.

  
  
  

  

  Exemplary demonstration of locally weighted regression fit for the two-handed Purdue Pegboard Test (PPT_2) including the position of the calculated maximal and minimal value of the fit (triangles). The optimal fitting model, here a piecewise linear regression with the breakpoint (BP) at 32 years, was chosen based on significance of the model and its parameters, R²_adjusted, and Akaike Information Criterion (AIC). Ribbons depict the 95% confidence interval of the fit.