From a NY Times article on Pauline Oliveros:
A 1957 film-scoring engagement with the composer Terry Riley and Loren Rush, a bassist and koto player, introduced Ms. Oliveros to free improvisation. Invigorated by the experience, the trio experimented further and recorded the results.
This reminds me of the Keith Jarrett Spirit improvisations which were interesting but ultimately dry.
Kudos to Steve Smith for linking in his article to the stream on archive.org. The archive.org descritpion may be hyperbolic, though:
These early experiments with aleatoric and improvisatory music serves as a valuable historical record chronicling the emergence of a new musical aesthetic that was to dominate contemporary classical music for much of the latter half of the 20th century, and hints at the type of minimal and chance compositions for which many of the performers would later become famous. As such they should be of great interest and provide considerable enjoyment to any fan or student of 20th century, avant-garde, classical music.