Computational protocol: Direction of Movement Is Encoded in the Human Primary Motor Cortex

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Protocol publication

[…] Processing of images and statistical analyses were conducted with Statistical Parametric Mapping (SPM) version 5 (2005, Wellcome Department of Cognitive Neurology, London, UK; Pre-processing included standard slice time correction, realignment and co-registration of functional and anatomical scans. Images were normalized to the template of the Montreal Neurological Institute (MNI) and smoothed using a Gaussian filter of 8 mm full width at half maximum (FWHM). Data was detrended using the standard SPM high-pass filtering with a cut-off at of 128 Hz. Analyses were time-locked to the eight different stimuli to enable event-related analysis. Brain activations were computed according to the standard statistical procedures in SPM. Statistical parametric maps per subject (first level analysis) were derived using a linear multiple regression model with 8 event-related regressors (one for each direction) and movement parameters as regressors of no interest to account for head movement-related effects . Activation maps (for each movement direction) were entered in an ANOVA (flexible factorial design) with eight levels (corresponding with the eight directions) to investigate cerebral activation patterns related to direction on a group level.We used whole brain analysis to identify activation patterns related to direction in M1. In order to prevent filtering out subtle activation differences related to directions that might be dominated by overlap in the representation of the hand in M1, we did not use direct comparisons between the different movement directions. Instead, we assessed maximally activated voxels per individual direction. We used false discovery rate (FDR) correction at a threshold of p = 0.01 to limit the number of false positive voxels. To gain insight in segregated/overlapping activations of directions, results were further (visually) assessed using overlays in MRICron at a threshold of T = 3.4 (equivalent to p = 0.01, FDR corrected).Since the present study did not vary between hand positions during the task, M1 activations related to the various directions resulted from the activity of distinct muscle couples. Therefore, activations in M1 might be attributed to the representation of these muscles instead of being direction-related . In order to dissociate between direction-related and muscle-related activations in M1, we determined activations related to movements along the horizontal and vertical axes. By pooling the two opposite directions for each axis, contributions by individual muscles are ‘averaged out’. Therefore, a difference between these axes in M1 will provide an argument against our findings being muscle representations. Here, activations related to movement along the vertical axis were obtained by pooling directions 90° and 270° and to obtain activation related to movement along the horizontal axes directions 0° and 180° were pooled. We used exclusive masks (masking threshold: p = 0.001) to compare the two main axes at thresholds of p = 0.01 (FDR corrected). Note that exclusive masks remove all voxels reaching significance in one contrast that overlap with the significant voxels in the other contrast. Results were visually compared in MRICron, at thresholds of T = 3.1 (equivalent to p = 0.01, FDR corrected). […]

Pipeline specifications

Software tools SPM, MRIcron
Application Magnetic resonance imaging
Organisms Homo sapiens