The two men who served as Wertheimer's subjects in the phi experiments were Köhler and Koffka. Köhler was an expert in physical acoustics, having studied under physicist Max Planck (1858–1947), but had taken his degree in psychology under Carl Stumpf (1848–1936). Koffka was also a student of Stumpf's, having studied movement phenomena and psychological aspects of rhythm. In 1917, Köhler (1917/1925) published the results of four years of research on learning in chimpanzees. Köhler showed, contrary to the claims of most other learning theorists, that animals can learn by "sudden insight" into the "structure" of a problem, over and above the associative and incremental manner of learning that Ivan Pavlov (1849–1936) and Edward Lee Thorndike (1874–1949) had demonstrated with dogs and cats, respectively.