Exogenous brain stimulation pulses are carried along the motor pathways from pallidum to distant targets: Results of a transfer function analysis

Deep brain stimulation (DBS) is a neuromodulation method for treatment of various neurological disorders. It is often assumed that the primary effect of DBS occurs at the site of stimulation, similar to the effects of lesions. However, recent work has shown that DBS can lead to robust evoked potentials (EP) not only at the stimulation site, but also in multiple distant brain regions. While the significance of these EPs for therapeutic outcomes is not known, it appears that the electrical effects of DBS have at least a partial modulatory impact on downstream targets. Nonetheless, it remains unclear how DBS pulses travel to the distant targets. The possible scenarios include, but are not limited to, orthodromic or antidromic pathways, accessory pathways, normally inhibited pathways, and direct electromagnetic activation of distant sites. The ability to record signals from brain regions with DBS on and off provides an opportunity to determine the mechanism of DBS. This study aims to examine how electrical stimulation from DBS pulses is carried along neural pathways. We hypothesize that the pathways that transmit DBS pulses include the pathways that transmit endogenous neural signals, and are more likely to travel in the same direction as endogenous signals. To test this hypothesis, we performed a transfer function analysis on deep brain recordings when DBS is on and off to compare the transmission of signals from electrical stimulation with the transmission of endogenous signals. Our results support our claim that the electrical pulses travel at least in part along the normal motor pathways.