The Bulgarian ethnomusicologist, Bela Bartok, determined that the folk music originally thought to be Hungarian and used by many "classical" composers was actually of origin.

Eastern European- Russian origin

Explanation:

The Bulgarian ethnomusicologist- Béla Bartók determined that the Hungarian folk music used in the "classical" music of  Debussy and Stravinsky were actually of Russian origin.

Bartók writes: “We must attribute these, with doubtless certainty, to the influence of some Eastern European—probably Russian—folk music. Similar efforts can be found in the works of Igor Stravinsky".

Bartók's groundbreaking work in the field of music along with his deep scientific research paved for new developments in the field of music.