The neural neutralizer was a device at the Tantalus V penal colony that could wipe thoughts from a subject's mind, misused by Dr. Tristan Adams to control his staff and inmates.
Dr. Adams presented the neutralizer as a therapeutic tool for treating criminally insane patients, but he used it to empty minds and implant suggestions. One of its victims, Dr. Simon van Gelder, escaped to the Enterprise in 2266 in a desperate state.
Kirk investigated and was himself subjected to the device, which left him painfully susceptible to Adams's commands. With help from Dr. Helen Noel he broke free, and Adams died after being left alone in the beam of his own machine.