Deep brain stimulation for Parkinson's disease works in some patients and not others. The electrodes are placed in motor regions — the subthalamic nucleus, the globus pallidus — because Parkinson's is classified as a movement disorder. The targets are chosen based on a model: dopaminergic neurons in the substantia nigra die, the motor circuit loses input, movement degrades. Stimulate the motor circuit. Some patients improve. Some don't. The response rate has been stubbornly variable for decades.
A Washington University team (Nature, 2026) imaged 863 patients across multiple intervention types — deep brain stimulation, transcranial magnetic stimulation, focused ultrasound, and medication — and found that every effective intervention target connects to the same structure: the somato-cognitive action network (SCAN). SCAN is not a motor circuit. It is a network that integrates bodily sensation with cognitive action planning. The substantia nigra connects to SCAN. Every DBS target that works connects to SCAN. In a clinical trial, SCAN-targeted stimulation produced a 56% response rate compared to 22% for stimulation of adjacent brain regions.
The 2.5-fold improvement comes not from a new technology but from a new target. The electrodes are the same. The stimulation parameters are similar. What changed is the understanding of what Parkinson's disease is. It is not a motor-circuit disease with cognitive symptoms. It is a network disease that manifests as movement dysfunction because the network's primary output is motor. The classification “movement disorder” selected the wrong treatment target for forty years.
The general principle: when a disease is classified by its most visible symptom, the classification can direct treatment to the wrong anatomical target. The symptom identifies the output. The mechanism identifies the process. These can be in different locations. The gap between where the symptom appears and where the mechanism lives is exactly the gap between 22% and 56%.