Fictional science fiction musical instrument from short story "Gotta Sing, Gotta Dance" by John Varley and first published in Galaxy, July 1976. It's a high-tech sort of full body theremin that translates dance into music.
“What’s a synapticon?”
She stared at him, not believing what she had heard. Then her expression changed to one of delight.
“You really don’t know? Then you have something to learn.” And she bounced over to her desk, grabbed something with her peds, and hopped back to the synthesizer. It was a small black box with a strap and a wire with an input jack at one end. She turned her back to him and parted her hair at the base of her skull.
“Will you plug me in?” she asked.
Barnum saw the tiny gold socket buried in her hair, the kind that enabled one to interface directly with a computer. He inserted the plug into it and she strapped the box around her neck. It was severely funtional, and had an improvised, breadboarded look about it, scarred with tool marks and chipped paint. It gave the impression of having been tinkered’ with almost daily.
“It’s still in the development process,” she said. “Myers—he’s the guy who invented it—has been playing with it, adding things. When we get it right we’ll market it as a necklace. The circuitry can be compacted quite a bit. The ﬁrst one had a wire that connected it to the speaker, which hampered my style considerably. But this one has a transmitter. You’ll see what I mean. Come on, there isn’t room in here.”
She led the way back to the outer office and turned on a big speaker against the wall.
“What it does,” she said, standing in the middle of the room with her hands at her sides, “is translate body motion into music. It measures the tensions in the body nerve network, amplifies them, and…well, I’ll show you what I mean. This position is null; no sound is produced.” She was standing straight, but relaxed, peds together, hands at her sides, head slightly lowered.
She brought her arm up in front of her, reaching with her hand, and the speaker behind her made a swooping sound up the scale, breaking into a chord as her fingers closed on the invisible tone in the air. She bent her knee forward and a soft bass note crept in, strengthening as she tensed the muscles in her thighs.
She added more harmonics with her other hand, then abruptly cocked her body to one side, exploding the sound into a cascade of chords. Barnum sat up straight, the hairs on his arms and spine sitting up with him.
Tympani couldn’t see him. She was lost in a world that existed slightly out of phase with the real one, a world where dance was music and her body was the instrument. Her eyeblinks became stacatto punctuating phrases and her breathing provided a solid rhythmic base for the nets of sound her arms and legs and fingers were weaving.
The beauty of it to Barnum and Bailey was the perfect fitting together of movement to sound. He had thought it would be just a novelty: sweating to twist her body into shapes that were awkward and unnatural to reach the notes she was after. But it wasn’t like that. Each element shaped the other. Both the music and the dance were improvised as she went along and were subordinate to no rules but her own internal ones.