A synaptic stimulator was a small device capable of directly accessing an individual's nervous system.
The Akritirian culture of the Delta Quadrant utilized synaptic stimulators as a form of control over their prison population. Known as "the clamp," it operated by stimulating the production of acetylcholine in the victim's hypothalamus, essentially helping to stimulate one's aggressive tendencies. This ensured that prisoners constantly fought amongst themselves, instead of organizing and potentially posing a threat to order. In 2373, USS Voyager officers Tom Paris and Harry Kim were outfitted with these stimulators, after being falsely convicted of supporting an Akritirian separatist movement. (VOY: "The Chute")
Later that year, The Doctor devised a plan whereby a synaptic stimulator would be used to tap into a cortical implant in Kes's body, removing an alien neural pattern which had taken control of her, and allowing her own consciousness to take over. Voyager's tactical officer, Lieutenant Tuvok, attempted to carry out the plan, but the alien in control of Kes was able to retain control of her body. (VOY: "Warlord")
The Delta Quadrant race known as the Ponea utilized synaptic stimulators as neural interfaces placed just behind one's ear, which operated by tapping into one's visual cortex and generating images of various alien landscapes inside the user's mind. The crew of the USS Equinox acquired one of these devices in 2376, referred to as "a poor man's holodeck" by Ensign Marla Gilmore. Captain Ransom would often use the device to experience the Tenkaran coast, a peaceful alien beach vista. (VOY: "Equinox", "Equinox, Part II")