Investigating Inhibitory Control Networks in Parkinson's Disease

Movement disorders are a prominent cause of disability worldwide. In the United States, it is estimated that more than 4 million people suffer from Parkinson's disease (PD), essential tremor (ET), and dystonia, some of the most prevalent of neurologic disorders. Of these, PD is the most common, and is primarily characterized by tremor, rigidity, and bradykinesia. However, many patients also have prominent non-motor features, including depression and cognitive impairment, with deficiencies in processing speed, memory, attention, and learning. One of the most debilitating cognitive deficiencies is in response inhibition (RI), or the inability to suppress a habitual action. PD patients have significant difficulty with RI, and report its substantial contribution in limiting their quality of life. While some studies show that dopamine can improve this aspect of cognitive function, many patients remain considerably impaired.

RI manifests clinically in many different and important ways, with reduced mental flexibility, task-switching, and concentration. RI may also contribute to motor impairment, with gait dysfunction, falls, and freezing of gait. Unfortunately, these features of PD and RI are less well-studied and lack effective treatment options, necessitating that new treatments be investigated. Deep brain stimulation (DBS), while a highly effective treatment for motor manifestations, is essentially ineffective for, and can even worsen cognition, with few studies currently investigating how different parameters may improve NMS. In an effort to begin addressing these debilitating features of PD, the investigators propose to study RI in patients with movement disorders, and to correlate movement and cognition with underlying neural electrophysiology before and during tasks of motion and response inhibition.

During routine DBS surgery, the stimulating electrode is implanted with the aid of intraoperative recordings in the awake state. These routine recordings enable neurologists and neurosurgeons to directly observe neuronal firing in the brain, identifying characteristic patterns to delineate anatomic structures. Once in place, the DBS electrode is tested using stimulation parameters known to be clinically efficacious for motor impairment. This allows acute, intraoperative testing for therapeutic benefit and side effects, and give information for how a patient will respond to the therapy once the cranial electrode is connected to the battery and turned on.

In addition to this routine recording and stimulation, this setting also provides a unique opportunity to study neural electrophysiology, with minimal increased risk. By measuring brain activity in the outer layers (cortex) as well as from the DBS electrode itself, while patients perform various tasks, it is possible to correlate behavioral function and neural activity. Our center, and several others, already have research paradigms in place to achieve these goals, by placing a subdural strip electrode over cortex prior to placing the DBS lead. These strip electrodes lie along the surface of the brain, and have historically been used for several decades to perform seizure mapping, typically as an array of electrodes placed via a burr hole. Their use has only more recently been implemented for investigation of neural circuits during DBS surgery, however, their safety in this specific setting is now well-established, and their temporary placement is currently being performed in similar studies at this institution. However, though previous studies have placed these strips over prefrontal areas, the vast majority of research in this area is focused on motor circuits, with placement over sensorimotor cortex. In order to study NMS, strips will be placed over prefrontal cortex, with recordings made during various motor and cognitive tasks and during different stimulation patterns.