Transcranial electrical stimulation (TES) covers methods for the noninvasive electrical stimulation of the brain. One differs between the transcranial direct current stimulation (tDCS) and the transcranial alternating current stimulation (tACS). For tDCS, constant current is applied between two electrodes placed at the head's surface. Depending on the direction of the current flow, the excitability of the stimulated neurons is increased or decreased. In contrast, for tACS, an alternating current is applied between the two electrodes. Amplitude, frequency and the relative phase of the current can be controlled. Thus, the rhythmic activity of the brain can be directly modified.

An innovative system for dynamic, multichannel tACS has been developed at our institute. A closed control loop with minimal latency enables the specific analysis of recorded EEG signals and the application of an adapted stimulation in real-time. Next to the evaluation in experiments with small animals for the investigation of memory processes we transfer our system to a commercial, human stimulation system.

In general, ordinary tES electrodes for human stimulation are of big size, so that large parts of the brain are stimulated. Thus, the focal stimulation of small areas in the human brain are a new challenge for us. Therefore, we develop a new approach for the placement of many small electrodes which can be separately and dynamically addressed. Due to addition and cancellation of many, small electrical fields, currents can be applied tightly focused in a small area. With the help of this procedure, the current can also be transported deeper into the brain than it is possible at the moment.