Parkinson's disease and essential tremor are chronic movement disorders for which there is no cure. When medication is no longer effective, deep brain stimulation (DBS) is recommended. Standard DBS is a neuromodulation method that uses a simple monophasic pulse, delivered from an electrode to stimulate neurons in a target brain area. This monophasic pulse spreads out from the electrode creating a broad, electric field that stimulates a large neural population. This can often effectively reduce motor symptoms. However, many DBS patients experience side effects - caused by stimulation of non-target neurons - and suboptimal symptom control - caused by inadequate stimulation of the correct neural target. The ability to carefully manipulate the stimulating electric field to target specific neural subpopulations could solve these problems and improve patient outcomes.

It has been shown that modifying the electrical waveform (e.g. pulse duration, pulse polarity, etc.) determine the spatial selectivity in functional electrical stimulation. Also, a recent clinical study examined for the first time the acute effects of anodic compared to cathodic neurostimulation in 10 PD patients. They found that thresholds for anodic stimulation were significantly higher than thresholds for cathodic stimulation, which is in agreement with previous research in animal studies and model calculations. However, they also reported a better clinical effect of anodic compared to cathodic stimulation. Furthermore, a modeling study from Anderson et al. (2018) found that fiber orientations can be selectively targeted depending on the stimulus waveform (i.e. cathodic or anodic). Another recent study examined the effect of an active symmetric biphasic pulse in 8 PD and 3 ET patients. They found that this pulse shapes produced significant clinical improvements compared to the standard clinical pulse shape.

Besides the symmetric biphasic pulse shape, the asymmetric pre-pulse shows great potential for the refinement of DBS therapy. If the pre-pulse is anodic, it has a hyperpolarizing effect and is therefore referred to as a hyperpolarizing pre-pulse. If it is cathodic, it has a depolarizing effect near the electrode and is therefore referred to as a depolarizing pre-pulse. Clinical studies focused on the use of asymmetric pulse shapes to improve the spatial selectivity by selectively exciting fibers in cochlear implant listeners13-16. Modeling studies indicate that a hyperpolarizing pre-pulse can actually decrease the threshold for axons and that the threshold is decreased more for axons close to the electrode than axons further away. This indicates that a hyperpolarizing pre-pulse may help focus the effects of stimulation to axons near the electrode, thus leading to an increase in the therapeutic window and potentially more efficient symptom control.

Evidence suggests that temporal manipulations (i.e. the use of complex pulse shapes, specifically biphasic pulses and asymmetric pre-pulses) of the stimulation field can exploit biophysical differences in neurons to target specific subpopulations. Ultimately, this may lead to an increase in the therapeutic window and/or more efficient symptom control. In this study, we aim to understand the neural mechanism underpinning the clinical effects observed by manipulating the pulse shapes, by comparing neurophysiological responses to the standard clinical pulse shapes to the responses to the complex pulse shapes. This will be achieved using two approaches. The first approach will study neural responses to different pulse shapes using electroencephalography (EEG) recordings. The second approach will study neural responses to different pulse shapes using intra-operative local field potential (LFP) recordings. This study and research protocol relates only to the collection of EEG and LFP recordings in DBS patients. The protocol does not cover any surgical procedures, which will already take place as part of the patient's normal clinical care.