Towards Closed-Loop Transcranial Alternating Current Stimulation

Introduction: Transcranial alternating current stimulation (tACS) is a noninvasive tool for modulating brain oscillations. Simultaneous recording of endogenous cortical oscillations such as electroencephalography (EEG) enables frequency and phase selective stimulation, presumably optimizing the modulation of brain activity. Nevertheless, measuring EEG while stimulating the brain is a challenging problem. Often, different devices are used for signal acquisition, signal processing, and signal generation, each with specific latencies and distortions leading to bad overall signal quality. Furthermore, stimulation artifacts commonly exceed the endogenous EEG signals. Here, a unified approach for an EEG-driven tACS system with guaranteed undistorted input-to-output coupling is proposed as a first step towards closed-loop tACS. Methods: A device based on custom input/output peripherals and a FPGA (field-programmable gate array) is presented. In contrast to conventional computers, FPGAs are able to rapidly execute complex concurrent processes. So, signal acquisition, signal processing (e.g. fast Fourier transform), and signal generation can take place at the same time on the same device. Synchronous timestamps for measuring and stimulating guarantee direct coupling of input and output. The signal-to-noise ratio (SNR) and the total harmonic distortion (THD) are determined to confirm the quality of the device. Its functionality and efficiency are shown in first animal experiments. Results: A first prototype of the proposed system was built for stimulating small animals, primarily mice. The SNR and THD of the whole implementation were determined in a closed-loop setup, i.e. the generated output was fed back to the input. The measured SNR and THD are both -80 dB. In preliminary neuroplasticity animal experiments, the prototype was able to detect desired brain oscillations and stimulate in-phase with endogenous parameters. Conclusion: The current prototype will be used in more animal experiments, and thus provide relevant information for the further development of the whole system. In the future, a new device will be implemented for human stimulation. The next important step to achieve full closed-loop capabilities is the development of an appropriate artifact removal. This is current work in progress. In the long run, neuroplasticity experiments will benefit from the improved stimulation procedure. With increased quality, new fields of application of tACS may also arise.