Explaining the Aperture Illusion Looking through a peephole can
change the direction an object appears to move—a tilted rod
going left to right seems to move downward at an angle when
viewed through a hole (see video clip below). Dale Purves and
his colleagues at Duke University think they know why. They
asked volunteers to describe how they perceived the motion of
moving lines seen through apertures. They also developed
computer simulations of a virtual rod moving in three-
dimensional space in which information regarding its direction
was stripped out (via projection onto a two-dimensional
surface). How the volunteers saw the movement nearly perfectly
matched those generated by the flattened-out simulation,
suggesting that images formed on our basically two-dimensional
retinas do not convey aspects of three-dimensional motion.
Hence, our perceptions of the directions of moving objects are
mental constructs based on past experience. Scrutinize the
analysis in the January 6 Proceedings of the National Academy