Neural target engagement

Target engagement is direct evidence that a stimulation actually reached and modulated its intended neural substrate, the neuromodulation analog of receptor occupancy in pharmacology. It matters because a negative clinical trial is uninterpretable if one cannot show the intervention hit the target: the null could mean the therapy is ineffective, or simply that it never engaged the circuit. Demonstrating engagement separates a mechanism failure from an aiming failure.

Engagement is shown through a measurable physiological response to stimulation that is independent of slow clinical endpoints, for example an evoked resonant response in the target region, a stimulation-evoked potential, a task-independent fMRI or PET change, or a motor evoked potential for cortical stimulation. A recurring principle in modern neuromodulation trial design is to require and optimize a target-engagement readout for dose-finding before committing to a large, expensive trial powered on clinical outcome.