The cortical stimulator was a medical device used to revive neural activity in the nervous system of critical patients by delivering an electrical shock to induce brain activity. It was almost always attached to the temples or foreheads of humanoids. Cortical stimulators were standard equipment in the sickbays of Starfleet vessels in the 24th century.
On stardate 45944, Dr. Crusher attempted to revive Captain Jean-Luc Picard with a cortical stimulator following his exposure to the Kataan probe. The stimulator's interruption of the beam from the Kataan probe caused Picard to go into neural shock, which only stabilized when the cortical stimulator was removed. The probe later disengaged the beam safely, and Picard recovered without further intervention, though the probe made him live fifty years of another being's life in approximately twenty real-time minutes. (TNG: "The Inner Light")
In 2369, Dr. Crusher used it to revive Counselor Deanna Troi after temporarily inducing clinical death to break a telepathic bond that was sapping her life force to keep Federation mediator Ves Alkar artificially young. Also that year, Jean-Luc Picard again had to be revived with cortical stimulators after an energy blast in a sudden gunfight on an away mission caused his artificial heart to temporarily malfunction. (TNG: "Man of the People", "Tapestry")