He plays a bit of synth on that track, but chiefly what you hear him playing is the Clavinet: (Hohner Clavinet D6) This is the same instrument made famous in "Superstition" and "Higher Ground" several years earlier. read more
Though it was originally intended as a kind of "electric clavichord" for home use in playing Baroque and Renaissance keyboard music, the Clavinet was soon discovered by funk and rock musicians -- Stevie Wonder did more than anyone else to popularize the instrument and its distinctive nasal sound, and today perhaps no single instrument is so squarely identified with funk. read more
Vocals, synthesizer, piano, harmonica, drums, bass guitar, congas, bongos, clavinet, melodica 10 according to the Stevie Wonder page on Wikipedia. read more