The neural neutralizer, also known as the neural potentiator or neural damper, was a device that was designed by Doctor Tristan Adams and operated on the Tantalus Penal Colony in the mid-2260s. (TOS episode & novelization: Dagger of the Mind)
The neutralizer was stored in a theater which contained a single chair where the patient sat, and the neutralizer's beam emitter was positioned directly above the chair. A windowed alcove which was connected to the emitter room contained the neutralizer's controls, where the operator would control the device as well as observe the patient's reactions.
The neutralizer was designed to induce an increase in neural conductivity in a patient's brain, which in turn would see an increase in the number of cross-connections made in the brain. At a certain point, the increase cross-connections would result in the disappearance of information stored in the patient's brain. Initially, Dr. Adams and his team thought this treatment would help his patients cope with their most troublesome thoughts and desires, however, Adams soon realized that he could use this technique to implant thoughts into the minds of those who had been subjected to the neutralizer.
Once activated, the neutralizer would create a profound sense of loneliness in the patient. Anything that was said to the patient would become their own thoughts. The effectiveness varied upon a number of conditions such as the number of times the procedure was repeated, the volume of the beam, or the patient's strength of will. Long term or repeated exposure to the neutralizer could induce mental illness or synaptic damage.
In 2266, Dr. Adams began to use the neutralizer to conduct horrific experiments on both patients and staff and turned them into "zombies". When the USS Enterprise visited the colony on a routine supply mission, Dr. Simon Van Gelder, an assistant of Adams' who had been subjected to the neutralizer, managed to escape up to the starship. When Captain James T. Kirk and Dr. Helen Noel beamed down to the colony to investigate, Adams managed to use the neutralizer on Kirk, but long-term exposure was prevented when Dr. Noel managed to temporarily shut down the colony's power systems. The Enterprise crew managed to secure the colony, but not before Adams was killed from exposure to the neutralizer.