It can be traced to way earlier than that, even as far as late 19th century — just check out Wikipedia. Futurist artist and composer Luigi Russolo was pretty influential with his manifesto ‘The Art of Noises’ and the intonarumori instruments, which were partially electric. Theremin came about in the 1920s and is still used to this day. The 50s is just when Stockhausen popped up with fully electronically synthesized sound. (Fun fact: Stockhausen had a little spat with Aphex Twin, wherein he called Aphex's music too repetitive in some interview, and Aphex said that Stockhausen's music is undanceable.)

Also of note, synthesizers were instruments of avantgardists until Wendy Carlos and Rachel Elkind released the album ‘Switched-On Bach’ in 1968, which greatly popularized the Moog synth. Carlos is most famous for the soundtrack of ‘A Clockwork Orange’.

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