Effect of Brain Stimulation on Effort Is Task Dependent: Evidence From an HD-tDCS Study on Cardiovascular Responses

This study tested the effect of frontal hemispheric asymmetry (FHA) on mental effort in tasks of fixed and unfixed difficulty. To manipulate FHA, we applied high-definition transcranial direct current stimulation (HD-tDCS) to the dorsolateral prefrontal cortex (dlPFC) and assessed its impact on cardiovascular responses. Forty-three participants performed two task conditions: (a) a fixed, easy task with a 50% accuracy criterion for success and (b) an unfixed, self-paced task that delivered monetary rewards for each correct response. Before completing the tasks, participants received left cathodal versus right cathodal stimulation. Prior to each task, participants were told they could earn a moderate monetary incentive. We recorded cardiovascular responses, including pre-ejection period (PEP), systolic blood pressure (SBP), heart rate (HR), and diastolic blood pressure (DBP). We expected right cathodal stimulation to induce left FHA. This should lead to higher effort when task difficulty is unfixed, due to increased participants' approach motivation and success importance. By contrast, no effect of stimulation was expected in the fixed and easy condition. As predicted, PEP reactivity-where more negative values indicate higher effort-was stronger in the unfixed task demand following right cathodal stimulation compared to the left cathodal stimulation and both stimulation conditions when the task was fixed and easy. These findings suggest that neuromodulation of the dlPFC alters effort intensity by shifting FHA, thereby increasing the perceived importance of success, which in turn determines higher effort under unfixed task demands.