AT&T Tech Channel

About this video

A Pair of Paradoxes utilizes two sensory illusions in one — the visual illusion called the Penrose Steps (popularized by MC Escher) of the endlessly climbing staircase, and the lesser-known audio correlation — the endlessly ascending tone, called a Shepard tone. Roger Shepard and Edward Zajac made this film in 1967. Zajac, a Bell scientist, is also credited with the very first computer-animated film.

The Penrose Steps were a visual trick discovered in 1958 by a family team, Lionel Penrose and his son Roger. The artist MC Escher used the technique to create his artwork Kimmen en Dalen (Ascending and Descending) in 1960. He followed it up with a similar work on the same theme in 1961, Waterval (Waterfall).

The Shepard Tone is a sound consisting of groups of sine waves separated by octaves, fading from one to the other and back again. It's an auditory illusion—and can be made in an ascending or descending version. It's been described as a "sonic barber's pole." This illusion is the 1964 invention of psychologist Roger Shepard, a cognitive specialist who worked at Bell Labs until 1966, when he went to Stanford as a psychology professor.

Footage courtesy of AT&T Archives and History Center, Warren, NJ

Release date: 10/10/2011