Relation of Brain Stimulation Induced Changes in MEP Amplitude and BOLD Signal

Background Non-invasive human brain stimulation can induce long-term plasticity reflected by changes in putative markers of synaptic activation, such as the motor evoked potential (MEP) amplitude elicited by transcranial magnetic stimulation or the task-dependent blood oxygenation level-dependent (BOLD) signal measured by functional magnetic resonance imaging. Objective To study the relationship between brain stimulation induced changes in MEP amplitude and BOLD signal. Methods Paired associative stimulation of the hand area of the left primary somatosensory cortex (S1-PAS) was applied in 15 healthy subjects to induce excitability change in the adjacent primary motor cortex (M1) [Krivanekova et al. 2011, Eur J Neurosci 34:1292-1300]. Before and after S1-PAS, MEP amplitude in a right hand muscle, and the BOLD signal during a right hand motor or somatosensory activation task were measured. Results S1-PAS resulted in substantial individual MEP and BOLD signal changes, but these changes did not correlate in M1 or S1. Conclusions Findings indicate that MEP amplitude and BOLD signal within the tested M1 reflect physiologically distinct aspects of synaptic excitability change. Therefore, it is suggested that MEP amplitude and BOLD signal are complementary rather than interchangeable markers of synaptic excitability.