“I-wave” recruitment predicts response to tDCS in the upper limb, but only so far

Abstract

Anodal transcranial direct current stimulation (a-tDCS) of primary motor cortex (M1) facilitates motor evoked potentials (MEPs) in hand muscles. Recent evidence highlighted substantial inter-individual variability after tDCS based on the difference in MEP latency between latero-medial (LM) and anterior-posterior (AP) directed transcranial magnetic stimulation (TMS) (Hamada 2013; Wiethoff 2014). This difference in MEP latency is presumed to reflect the balance of early versus late I-wave recruitment to TMS. The present study examined whether AP-LM MEP latency difference could predict an individual’s MEP facilitation to a-tDCS or dual-hemisphere tDCS in the muscles of the forearm (Extensor Carpi Radialis; ECR) and proximal upper limb (Biceps Brachii; BB). We conducted a randomised double-blind study with 18 healthy adults. Each received anodal, dual-hemisphere, or sham tDCS in separate sessions (tDCS, 1 mA for 15 min). The anode was positioned over right M1 and the cathode over left supraorbit for a-tDCS or left M1 for dual-hemisphere tDCS (both 27 cm2). MEPs in BB and ECR were collected before and immediately after tDCS with posterior-anterior induced cortical current. tDCS effect on MEP size was measured (Post-Pre)/Pre. Linear regression analyses showed a-tDCS modulated MEP size dependent on AP-LM MEP latency for ECR only (p=0.01). Individuals with small MEP latency difference showed the expected facilitation of MEPs after a-tDCS, whereas those with large MEP latency difference had suppressed MEPs after a-tDCS. No association was found between AP-LM MEP latency difference and BB MEP modulation after a-tDCS. There was no observed association between AP-LM MEP latency difference after dual-hemisphere tDCS or sham in either muscle (p>0.1). These results support previous findings that a-tDCS modulates corticomotor excitability depending on I-wave recruitment, for more distal muscles of the hand, and the present results extend these finding to the forearm. These findings may inform how to individualise a-tDCS in future applications.