Klinische Neurophysiologie 2004; 35 - 26
DOI: 10.1055/s-2004-831938

Excitation and inhibition during motor cortex stimulation: A combined transcranial brain stimulation and event-related fMRI study

JH Brocke 1, S Schmidt 2, K Irlbacher 3, A Kraft 4, A Naito 5, SA Brandt 6
  • 1Berlin
  • 2Berlin
  • 3Berlin
  • 4Berlin
  • 5Berlin
  • 6Berlin

What does the BOLD response tell us about excitation and inhibition? Current approaches of fMRI are limited by an ambiguity in correlating BOLD effects with either a predominant inhibitory or excitatory neuronal activity. Transcranial electrical cortex stimulation (TES) was combined with simultaneous fMRI to study the relationship between the activity of inhibitory and excitatory neuronal circuits and the thereby induced BOLD signal changes. In a previous contribution (DGKN 2003) we were able to show that TES will induce similar effects on the motor cortex to TMS if a particular direction of the current flow is applied. Transcallosal inhibition was induced with TES by applying a single stimulus to one cortex. This stimulation lead to an inhibition on the contralateral cortex. We used both single pulse and bilateral stimulation to study the inhibitory effects on resting and activated contralateral cortex and analysed the induced BOLD signal changes.

Methodology included event-related fMRI which triggered transcranial stimulation and online recording of the electromyographic responses monitoring the inhibitory influence on discharging corticospinal neurons. Test stimulus and conditioning stimulus had an intensity of 120% of resting motor threshold to evoke a potential of 0.7 mV±0.45 mV. FMRI was performed at 3 Tesla (GE) using an EPI sequence (TR=3s, TE=35ms, FA=90°, voxel-size=3.75×3.75×3mm, slices=15). Data was analysed with BrainVoyager 4.9 using ROI based event-related averaging.

Results revealed that the test stimulus alone induce a positive BOLD response both in the ipsilateral (0.3 to 0.7% signal change) and contralateral (0.2 to 0.5% signal change) primary motor cortex. Interestingly, contralateral co-activation exactly matched to the previously identified location of the hand area. If a conditioning stimulus preceded the test stimulus then the resulting ipsilateral BOLD signal was significantly reduced compared to the test stimulus alone. Thus, we conclude that both excitatory and inhibitory effects will generally lead to an increased BOLD signal, but that increased inhibitory neuronal activity and reduced cortico-spinal output results in smaller increases in local signal intensity. Sponsored by the BMBF, Berlin NeuroImaging Centre