Influence of tDCS of the right DLPFC on verbal working memory and non-noxious and noxious thermal stimulation

Objective: Experimentally induced pain can be decreased by 1-Hz repetitive transcranial magnetic stimulation (rTMS) of the right Dorsolateral Prefrontal Cortex (DLPFC), presumably by effects on the emotional-cognitive component of pain. Anodal transcranial Direct Current Stimulation (tDCS) ameliorated chronic pain when applied over the motor cortex and increased working memory (WM) function in a letter-based 3-back task when applied over the left DLPFC. Thus, we investigated whether (tDCS) of the right DLPFC modulates thermal sensation or thermal pain addition to WM function.

Methods: Warm and cold sensation thresholds (WSTs and CSTs) as well as heat and cold pain thresholds (HPTs and CPTs) were assessed by using contact thermodes on the contra lateral forearm before and following 20 minutes of 2 mA anodal, cathodal or sham tDCS of the right DLPFC (center F 4) in 18 healthy volunteers (balanced randomization). WM was assessed by using a two number-based 2-back task during the stimulation.

Results: The 2-back task revealed a prolongation of reaction time and an increase in omission rate for anodal tDCS compared to cathodal tDCS (T(16)=2.293, P=0.037; T(16)=-2.557, P=0.020). Further, for anodal tDCS there was a tendency to increase CSTs and CPTs compared to sham tDCS (T(17)=1.9; P=0.07; T(17)=1.7; P=0.09).

Conclusion: The increased cortical excitability – induced by anodal tDCS – may account for the prolonged reaction times and the higher omission rates. This slight deterioration of WM for the stimulation of the right DLPFC is in contrast to the effects on the left DLPFC and is only present compared to cathodal but not to sham tDCS. An amelioration of anticipation or a worsening of WM function may lead to increased thresholds for cold sensation and cold pain. Thus, the present data suggest an association of the perception of thermal sensation and thermal pain with higher cortical function.