A neural caliper in use on Jean-Luc Picard
The neural caliper was a small device often used as part of the preparation for surgery. It was placed on the patient's forehead and, when activated, it caused the patient to lose consciousness.
In 2365, a neural caliper was used on Captain Jean-Luc Picard during surgery to have his artificial heart replaced on Starbase 515. (TNG: "Samaritan Snare")