A neural neutralizer was a type of device used at the Tantalus Penal Colony in 2266. It was an outgrowth of experimental beam neutralization technologies developed on Earth. The neural neutralizer was essentially a beam emitter, a round orange lens that was located in a treatment room, above a chair in which the patient sat. A windowed alcove, connected to the room, housed a control panel, which was used for manually operating the device, and space for the therapist, who could view the patient through the window.
Operation of the device was straightforward. One control turned the device on and off, and another selected the intensity of the beam. The therapist could communicate with the patient through a microphone. The remainder of the control panel consisted of status indicator lamps.
The neural neutralizer was somewhat effective as a means of implanting suggestions. It created in the patient a profound sense of loneliness, also described as an "emptiness". Anything said to the patient while he was in this state would become his own thoughts. The effectiveness varied according to the complexity of the suggestion, the number of times it had been repeated, the volume of the beam, the strength of the patient's will, and the degree to which the suggestion was opposed by his own desires. Long term or repeated exposure could produce profound mental illness and synaptic damage. Additionally, the "emptying" effect could be immediately lethal; Dr. Tristan Adams was killed by accidental, unsupervised exposure to the beam.
In 2266, Dr. Adams began to conduct unauthorized and unethical experiments upon inmates and staff alike. When the USS Enterprise visited the Tantalus colony in that year, Adams' assistant Simon Van Gelder had been made severely ill by exposure. Adams later used the device on Kirk, and might have injured him severely had not Dr. Helen Noel managed to interrupt the power long enough for Enterprise crewmen to beam down and secure the colony. Dr. Van Gelder subsequently dismantled and destroyed the neural neutralizer. (TOS: "Dagger of the Mind")