June 1996
Roger N. Shepard was always a prankster. As a boy, he delighted in playing visual tricks -- like the time he surreptitiously moved all the furniture out of his sister's room.
He went on to a distinguished career at Bell Labs, Harvard, and Stanford, all the while continuing to play visual tricks on people. By recording and analyzing their reactions, he discovered astonishing truths about mental processes. Shepard showed, for example, that when people compare two objects rotated at different angles, they first reorient the objects in their mind's eye. The astonishing part: Everyone does this at approximately 60° per second.
In the 1950s, Shepard began to discover universal laws that govern how people and animals perceive similarities among sensory input like colors, sounds, or smells. He worked out a computer method for measuring the perceived difference between stimuli.
When Shepard applied this method to colors, the computer yielded a circle. The physical wavelengths of visible light fall along a straight line, moving from red to violet; but the circle more accurately depicts the psychological (as opposed to physical) truth. This is that people perceive red and violet as more similar to each other than either color is to an intermediate color such as green. Shepard also discovered that: