Finger somatotopy is preserved after tetraplegia but deteriorates over time

Previous studies showed reorganised and/or altered activity in the primary sensorimotor cortex after a spinal cord injury (SCI), suggested to reflect abnormal processing. However, little is known about whether somatotopically specific representations can be activated despite reduced or absent afferent hand inputs. In this observational study, we used functional MRI and a (attempted) finger movement task in tetraplegic patients to characterise the somatotopic hand layout in primary somatosensory cortex. We further used structural MRI to assess spared spinal tissue bridges. We found that somatotopic hand representations can be activated through attempted finger movements in the absence of sensory and motor hand functioning, and no spared spinal tissue bridges. Such preserved hand somatotopy could be exploited by rehabilitation approaches that aim to establish new hand-brain functional connections after SCI (e.g. neuroprosthetics). However, over years since SCI the hand representation somatotopy deteriorated, suggesting that somatotopic hand representations are more easily targeted within the first years after SCI.


Sample-size estimation
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Replicates
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For the representational similarity analysis, we acquired data in 4 independent runs. The reported inter-finger distances are crossvalidated using these 4 runs as independent crossvalidation folds, as detailed in the Materials and Methods "Representational similarity analysis" section.
The structural MRI cross-sectional spinal cord area was calculated semiautomatically for every slice and averaged over all 10 slices to obtain a single score for cross-sectional spinal cord area. This is described in detail in the Materials and Methods, section "Structural MRI analysis", subsection "Cervical cross-sectional spinal cord area analysis".
The structural MRI midsagittal tissue bridges analysis was conducted once per subject and care was taken to reduce potential biases. The manual segmentation for the structural midsagittal tissue bridges analysis was conducted by an experimenter who was blinded to patient identity. Jim 7.0 software (Xinapse Systems, Aldwincle, UK) was used for this manual lesion segmentation at the lesion level, for which high intra-and interobserver reliability has previously been reported (Huber et al., 2017;Pfyffer et al., 2019). This is described in detail in the Materials and Methods, section "Structural MRI analysis", subsection "Midsaggital tissue bridges analysis".
Inclusion and exclusion criteria are reported in the Materials and Methods "Participants" section.

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Group allocation
• Indicate how samples were allocated into experimental groups (in the case of clinical studies, please specify allocation to treatment method); if randomization was used, please also state if restricted randomization was applied • Indicate if masking was used during group allocation, data collection and/or data analysis Please outline where this information can be found within the submission (e.g., sections or figure legends), or explain why this information doesn't apply to your submission: Additional data files ("source data") • We encourage you to upload relevant additional data files, such as numerical data that are represented as a graph in a figure, or as a summary table • Where provided, these should be in the most useful format, and they can be uploaded as "Source data" files linked to a main figure or table • Include model definition files including the full list of parameters used • Include code used for data analysis (e.g., R, MatLab) • Avoid stating that data files are "available upon request" Please indicate the figures or tables for which source data files have been provided: Information regarding statistical tests used, values of N, methods of multiple test correction etc. can be found in the relevant analysis sub-analysis header in the Materials and Methods section. Statistical analysis methods are described in the Materials and Methods, "Statistical analysis" section. Both frequentist and Bayesian statistical analysis was performed for the main statistical analyses.
Group allocation is described in the Materials and Methods, "Participants" section.
The structural MRI analysis required manual intervention. To reduce potential biases, the experimenter performing this analysis was blinded to patient identity.
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