tDCS is a non-invasive method to stimulate the brain using constant, low current. This technology has been used in patients with brain injuries or psychiatric conditions like major depressive disorder. In addition, tDCS has been explored to alleviate memory deficits in Parkinson’s disease and Alzheimer’s disease, schizophrenia, and pain.
tDCS has also been combined with Electroencephalogram (EEG) for motor rehabilitation and other clinical research applications. Therefore, we measured different complete EEG systems with a tDCS device from a UK company (focus, London) to explore simultaneous EEG and tDCS recordings.
Device and setup
Two types of amplifiers, the g.USBamp and g.Nautilus, are available for this measurement (see Table 1). Both active and passive g.LADYbird electrodes are available with g.USBamp, while only active g.LADYbird is with g.Nautilus. The tDCS electrodes are 37 mm square, and are soaked in fresh saline solution before measurement.
The following steps were followed to prepare each subject for a measurement:
- Place EEG and tDCS electrodes according to the specific application. In this example, three channels of EEG signals are recorded from C3, Cz and C4, while the tDCS electrodes are placed near ground (GND) and C3, respectively (see Figure 1).
- Prepare the scalp properly, and carefully place the conductive gel into the gap between EEG electrodes and tDCS electrodes. It is critical to avoid any conductive bridge in between, because this could potentially cause saturation.
- Switch the tDCS device on at the start of the measurement, and some time (e.g. 30 second) after it reaches the pre-set current (e.g. 1.0 mA, 1.5 mA and 2.0 mA), switch it off.
|tDCS Current (mA)||1.0||1.0||1.0|
Table 1. EEG devices and tDCS current.
Figure 1. Positioning of EEG and tDCS for this measurement. EEG signals are recorded from C3, Cz and C4, while the tDCS electrodes are placed next to GND and C3, respectively.
As expected, the EEG signals ramp up after switching on the tDCS device, and then remain fairly constant (for about 30 seconds), and ramp down after switching off (see Figure 2).
For both g.USBamp and g.Nautilus, there is no saturation with tDCS up to 2 mA (Figure 2). The magnitude of raw data is below 250 mV, and after band-pass filtering at 5-30 Hz, there is almost no visible difference between signals with and without stimulation. It is also clear that the magnitude of the DC component deceases when the distance between the tDCS and the EEG electrodes increases
Figure 2. Measurement results with g.USBamp and g.LADYbird active electrode. The top figures show the raw data, while the bottom figures show the filtered data (band-pass filtered at 5-30 Hz), from C3, Cz and C4, respectively.
In summary, the measurement shows that tDCS is compatible with both passive and active electrodes with g.USBamp and g.Nautilus. There is a strong DC component in the raw data with tDCS, but it could be removed with a proper filter.