Temporal Interference Stimulation (TIS)
TIS, unlike the previously explained DBS, is a novel noninvasive approach that is able to target deep brain structures without activating the overlying cortex. It consists on the placement of two electrode pairs at the skull connected to a current controlled stimulator. In this case instead of square biphasic pulses, sinusoidal waves are used to interact with neurons. Specifically, out of each electrode pair a high frequency (kHz) sinusoidal wave is outputted. The waves are outputted at a slightly different frequency from each other. Although these waves by themselves cannot interact with neural activity because of their fast nature, when they interfere and add constructively a new amplitude modulated waveform is created that has a beat envelope exactly at the difference of frequency between both high-frequency sine waves. Since the beat envelope is of much lower frequency neurons are capable of sensing it, and through this beat envelope is how neural activity can be modulated. By adjusting electrode placement and current amplitudes it is possible to non-invasively target different brain regions without having to go through an invasive procedure. Just like in DBS, the interaction between the electrical stimulation and the neuronal activity is also parameter dependent. In TIS, there are two main parameters that can be adjusted. Carrier waves frequency, which corresponds to the frequency of both sinusoidal waves and also determine the beat envelope frequency. Amplitude, which corresponds to the injected current through the sine waves and determines the envelope amplitude. By modifying these parameters it is possible to research the parameter-dependent effects of this therapy and its potential to treat neurological disorders.
