The aim of our studies is to explore specific aspects of (i) neurovascular coupling and to (ii) explore the larger functional connectivity of the sensory-motor system. This research is based on the exploitation of the unique combination of simultaneous transcranial electrical stimulation (TES), electromyographic recordings (EMG) and functional magnetic resonance imaging (fMRI).

Figure 1: Comparison of the stimulation effects of paired-pulse TES p-a and TMS p-a as indicated by the small icons: Mean time courses of EMG-responses are expressed as a percentage of the size of the mean test-pulse amplitude (y-axis) at different inter-stimulus intervals (x-axis)(Brocke et al. 2005).

In previous work we showed that:

1. Solitary single-pulse supra-threshold stimulation induces a positive BOLD fMRI response in ipsilateral primary motor cortex.
2. Solitary single-pulse supra-threshold stimulation also induces positive BOLD fMRI responses in contralateral primary motor cortex, although the activity of the pyramidal cells is being suppressed. This response is smaller than the ipsilateral response and is due to transcallosal inhibition (see below).
3. Subtraction analysis of paired-pulse bilateral supra-threshold stimulation shows that interactions due to contralateral stimulation attenuate the BOLD response in ipsilateral motor cortex.

Figure 2: Local and distant BOLD responses induced by TES over primary motor cortex (left panel) and visual areas (right panel).

In a current study we compare transcallosalnhibition with intracortical inhibition, supra- with subthreshold stimulation and initiate the investigation of the functional connectivity of the sensory-motor system as identified by iterative stimulation at respective sights of distant effects. The aim is (i) to identify differential contribution to the BOLD signal from inhibitory and excitatory synaptic processes and further (ii) to define node and network parameters supporting the sensory-motor system in the human brain.

Presently, we ask:

1. Whether transcallosal versus intracortical inhibition utilize some identical interneurons?
2. Can we develop a model of TES induced BOLD fMRI response that subsumes differential parametric weighting of inhibitory and excitatory processes?
3. If the stimulation of M1, and subsequently at the sites of distant effects, will define a wide-spread network of cortical nodes possibly related to sensory-motor integration?
4. How are the feed-forward o feed-back interactions between the various nodes unique?
5. Will TES in multimodal areas will involve a much larger spread of distant effects as compared to TES in unimodal areas (e.g. M2 versus M1)

Selected Publications

Brocke J, Irlbacher K, Hauptmann B, Voss M, Brandt SA (2005) Transcranial magnetic and electrical stimulation compared: Does TES activate intracortical neuronal circuits? Clin Neurophysiol Clin Neurophysiol, 116(12):2748-56.

Brocke JH, Schmidt S, Irlbacher K, Kraft A, Naito A, Brandt SA (2004) Excitation and inhibition during motor cortex stimulation: A combined transcranial brain stimulation and event-related fMRI study. Klin Neurophysiol Abst 26, 132.

Kühn AA, Brandt SA, Kupsch A, Trottenberg T, Brocke J, Irlbacher K, Schneider GH & Meyer B-U (2004). Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation. Exp Brain Res 155: 48-55.

Gothe J+, Brandt SA+, Sabel B, Roericht S, Kasten E & Meyer B-U (2002). Changes of visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation. Brain 125: 479-490 +(equal contribution).

Irlbacher K, Meyer B-U, Voss M, Brandt SA & Roericht S (2002). Spatial reorganization of motor output maps of stump muscles in human upper-limb amputees. Neursci Lett 321: 129-132.

Irlbacher K, Brandt SA & Meyer B-U (2002). In vivo study indicating loss of intracortical inhibition in tumour-associated epilepsy. Ann Neurol 52: 95-99.

Brandt SA, Brocke J, Roricht S, Ploner CJ, Villringer A, Meyer BU (2001) In vivo assessment of human visual system connectivity with transcranial electrical stimulation during functional magnetic resonance imaging. Neuroimage 14: 366-375.

Brandt SA, Davis TL, Obrig H, Meyer BU, Belliveau JW, Rosen BR, Villringer A (1996) Functional magnetic resonance imaging shows localized brain activation during serial transcranial stimulation in man. Neuroreport 7: 734-736.