Functional resting-state connectivity of the human motor network: Differences between right- and left-handers

Background: Handedness, i.e., the preference to use one hand over the other, has been shown to be associated with differences in activation levels in various motor tasks performed with the dominant or non-dominant hand (Gilbert & Wysocki, 1992; White et al., 1994). We were interested whether also in the absence of an overt motor task, i.e., during “resting-state”, spontaneous fluctuations of primary motor cortex activity shows differential functional connectivity in the left or right hemisphere depending on whether subjects report left- or right-handedness.

Methods: We, therefore, scanned 24 volunteers (12 right-handers and 12 left-handers) with resting-state functional magnetic resonance imaging (rs-fMRI). Handedness was assessed by the Edinburgh-Handedness-Inventory (EHI) that ranges from -100 (strongly left-handed) to +100 (strongly right-handed). We computed two explorative whole-brain group analyses where the time course within a sphere of 8 mm-diameters centered on the M1 seed voxel (“hand knob” formation) was correlated with every other voxel in the brain. To test for differences in functional connectivity between right- and left-handers, we used M1 (left/right) as seed region to consider both the “dominant” (left) M1 of right-handers and the “dominant” (right) M1 of the left-handers. We, then, computed regression analyses including resting-state connectivity and EHI as covariates.

Results: We found a positive correlation between EHI and resting-state functional connectivity of left M1 with the left supplementary motor area (SMA). That is, the stronger the M1-SMA functional connectivity within the left hemisphere during rest, the stronger the preference to use the right hand for manual skills (P< 0.05 FWE-corrected on the voxel-level). The reverse contrast did not yield significant voxels. Likewise, multiple regression analyses between EHI and resting-state connectivity of right M1 not reveal differences between right- and left-handers.

Conclusion: Our results show that functional connectivity of the human motor cortex differs between right- and left-handers, even in the absence of overt motor performance. Right-handedness is predicted by a stronger functional connectivity between left SMA and left M1. An intrinsically higher connectivity of the dominant motor cortex with premotor areas in right-handers might explain the behavioural notion that right-handedness is usually strongly lateralised while left-handers are more flexible in also using their non-dominant hand in activities of daily living.

References:

Gilbert AN, Wysocki CJ. (1992): Hand preference and age in the United States. Neuropsychologia 30(7):601 – 8.

White LE, Lucas G, Richards A, Purves D. (1994): Cerebral asymmetry and handedness. Nature 368(6468):197 – 8.