Towards Bidirectional Brain-computer Interfaces that Use fNIRS and tDCS

We envision a future user interface that measures its user’s mental state and responds not only through a display but also by sending output directly to the brain, leading to a primitive bidirectional brain-computer interface. Previous interactive systems have measured brain state with functional near-infrared spectroscopy (fNIRS) for communication from user to computer; we now explore transcranial direct-current stimulation (tDCS) as a channel in the opposite direction. Our goal is to integrate this with brain measurements from fNIRS, so that the stimulation parameters governing tDCS may be set dynamically to enhance user cognition based on current mental state and task demands. To do this, the first step is to determine how long it takes for tDCS to register cognitive effects and how long these effects last. We present an experiment that investigates the temporal dimension of tDCS for this purpose. The findings suggest a long lag-time between the onset of stimulation and any measurable cognitive effect, which may prohibit the effectiveness of tDCS in a brainadaptive application.