As the two sides of the rectangle are pulled apart, one side of the center ring appears to be darker than the other. See how this effect changes with varying distance and gray levels.
Max Wertheimer and his two assistants, Wolfgang Köhler and Kurt Koffka, studied perception at the Psychological Institute of Frankfurt University in the early 1900s. Their work lead to the development of the field of Gestalt psychology.
K. Koffka, Principles of Gestalt Psychology, London: Lund Humphries, 1935.