Finger-specific representations are sharpened during a fatiguing motor task

Abstract

Introduction
 Motor fatigability is a limiting factor in any kind of setting, in which physical performance is required. Motor fatigability can be quantified through the decrease in movement speed, when low-force movements are performed repeatedly with maximal speed. In this study, we measure motor fatigability in healthy participants through fast finger tapping for 30s. Previous research has shown that the decrease in movement speed, also called motor slowing, is associated with a rise in brain activity, a reduction in surround inhibition in the primary sensorimotor cortex (SM1), and an increase in co-activation of antagonistic muscle groups involved in the movement (Bächinger et al., 2019). However, it remains an open question of whether motor slowing and the associated release of inhibition causes a reduction of signal-to-noise ratio for movement-specific information. Here, we aim to answer this question by assessing finger representations using representational similarity analysis (RSA) when participants perform fatiguing tapping with either the index or middle finger. We hypothesized that a reduction of movement-specific information would be associated with the index and middle finger representations in SM1 getting “blurred” over time due to a gradual break-down of surround inhibition. Thus, if the signal-to-noise ratio of movement-specific information decreases in parallel with motor slowing, we would expect finger representations to become more overlapping. Vice-versa, if the signal-to-noise ratio of movement-specific information increases despite motor slowing, we would expect sharper finger representation.
 Methods
 26 healthy young participants performed a motor slowing task during functional MRI. The participants performed 30s of maximal speed finger tapping with the index and the middle finger, alternating between trials. To quantify motor slowing from the behaviour, we performed a linear regression analysis. For the first-level general linear model of the fMRI data, the fingers were regressed separately and the 30s of tapping were further split into 3 x 10s regressors (time bin 1, bin 2, bin 3). We performed RSA separately on each 10s regressor for the anatomically defined regions of interest M1 and S1 hand area (Diedrichsen et al., 2013; Walther et al., 2016) and therefore obtained a dissimilarity measure for each time bin. A mixed effects model with the factor time was used to test whether dissimilarity changed across time bins.
 Results
 On the behavioural level, motor slowing could be observed, as tapping speed significantly decreased in each finger over time (F(1, 76) >= 5.78, p <=.05). Concerning the dissimilarity measure, we found an increase over time for S1 as well as M1 (F(2, 50) >= 17.41, p <=.001).
 Discussion/Conclusion
 We conclude that the finger representations in the sensorimotor cortex become more distinct with motor slowing. This suggests that the signal-to-noise ratio of movement-specific information is increased, potentially to compensate for supraspinal changes caused by fatigability.
 References
 Bächinger, M., Lehner, R., Thomas, F., Hanimann, S., Balsters, J., & Wenderoth, N. (2019). Human motor fatigability as evoked by repetitive movements results from a gradual breakdown of surround inhibition. eLife, 8, Article e46750. https://doi.org/10.7554/eLife.46750
 Diedrichsen, J., Wiestler, T., & Krakauer, J. W. (2013). Two distinct ipsilateral cortical representations for individuated finger movements. Cerebral Cortex, 23(6), 1362–1377. https://doi.org/10.1093/cercor/bhs120
 Walther, A., Nili, H., Ejaz, N., Alink, A., Kriegeskorte, N., & Diedrichsen, J. (2016). Reliability of dissimilarity measures for multi-voxel pattern analysis. NeuroImage, 137, 188–200. https://doi.org/10.1016/j.neuroimage.2015.12.012