Cerebellar transcranial direct current stimulation does not alter motor surround inhibition

Motor surround inhibition (mSI) is one mechanism by which the central nervous system individuates finger movements, and yet the neuroanatomical substrate of this phenomenon is currently unknown. In this study, we examined the role of the cerebellum in the generation of mSI, using transcranial direct current stimulation of the cerebellum (cDC). We also examined intrasubject and intersubject variability of mSI. Twelve subjects completed a three session cross over study in which mSI was measured before and after (0 and 20 minutes) sham, anodal and cathodal cDC. mSI of the surround muscle (adductor digiti minimi) at the onset of flexion of the index finger was consistently observed. Anodal and cathodal cDC did not modulate the magnitude of mSI. For individual subjects (across the three sessions), the intrasubject coefficient of variation was 27%. Between subjects, the intersubject coefficient of variation was 47%. mSI was a stable effect in individual subjects across multiple sessions. This is an important observation and contrasts with other neurophysiological paradigms such as paired associative stimulation response, which exhibit great variability. In addition, we have quantified intrasubject variability of mSI, which will allow future therapeutic studies that attempt to modulate mSI to be adequately powered. We have not found evidence that the cerebellum contributes to the neuroanatomical network needed for the generation of mSI. Understanding the mechanisms of mSI remains a challenge but is important for disorders in which it is deficient such as Parkinson's disease and focal hand dystonia.