Identifying cortical and subcortical contributions to anticipatory postural adjustments preceding arm reaching...

Anticipatory postural adjustments (APAs) are generally understood to be cortically driven, however there is some evidence to suggest subcortical regulation and contribution. We tested the hypothesis that subcortical pathways, if contributing, would be evidenced in increased late portions of the motor evoked potential (MEP) elicited by transcranial magnetic stimulation during different time intervals before and during the APA. Recordings from both the tibialis anterior and soleus muscles showed little evidence for this throughout the course of the APA, pointing instead to a generally cortical source for the APA. Despite this, the brainstem did not appear inactive, with MEPs elicited during voluntary contraction of the soleus being significantly smaller than those elicited prior to executing an APA- specifically during the later portion of the MEP, thought reflective of subcortical contribution. Further work to isolate specific brainstem upregulation and contribution is needed.