The Synapse Switcher is a mind-switching machine invented by Iggy Koopa in the twelfth Nintendo Adventure Book, Brain Drain. The device, which has its name printed on one of the sides, resembles a large box with various tubes and wires attached to it; it also has a built-in cassette player, a self-destruct mechanism, and a Fungus Up cola dispenser. In the book, Iggy, from his base in Dinosaur Land, uses the Synapse Switcher to cause havoc throughout the Mushroom Kingdom by projecting the machine's "synapse ray" at the WMUSH radio station, causing everyone who listens to the radio without a protective helmet to switch minds with another random person or creature.
In the book's good ending, Iggy (who had taken over WMUSH) tries to use the Synapse Switcher on Mario and his friends at close range, but the group protect themselves with some Fungus Up (which can counteract the Synapse Switcher) that Luigi had stolen from Iggy's personal stash. Having neglected to put on a protective helmet before activating the Synapse Switcher, Iggy ends up in the body of a Wiggler and is left behind when his minions evacuate WMUSH. Those affected by the Synapse Switcher (including Yoshi, who ended up in Iggy's body) were ultimately cured when Princess Toadstool had a large amount of Fungus Up dumped in the Mushroom Kingdom water supplies.
In one bad ending, Mario destroys the Synapse Switcher with a barrage of fireballs, while in another, Luigi unintentionally activates the self-destruct mechanism when he presses the orange button on the machine, instead of the one that activates the Fungus Up dispenser.