Modulation of slow cortical potentials by transcranial magnetic stimulation in humans

We studied the effects of transcranial magnetic stimulation (TMS) on slow cortical potentials (SCPs) of the brain elicited during performance of a feedback and reward task. Ten healthy participants were trained to self-regulate their SCP amplitude using visual feedback and reward for increased or decreased amplitudes. Subjects participated in 27 runs (each comprising 70 trials) under three different conditions: single-pulse TMS delivered with the coil centered over Cz (vertex), over a lateral scalp position (LSP), which increased task difficulty, and in the absence of stimulation. Cz stimulation led to a non-significant enhancement of negative SCPs, while LSP stimulation led to a significant increase of positive SCPs. These results are consistent with the idea that enhanced task difficulty, as in LSP stimulation, enhances cognitive processing load leading to an increase of positive SCPs. Additionally, the data raise the hypothesis that TMS delivered to bilateral midcentral regions could modulate the amplitude of negative SCPs.