A black and white spiral slowly spins. Visitors stare at the center of the spiral for twenty seconds or more and then suddenly look at other objects. The objects appear to swell up or to shrink. Visitors control the direction of rotation of the spiral with a switch and its speed with a knob.
As the spiral turns it appears to expand or contract depending on its direction of rotation. There are two channels in the part of the optic pathway in the brain which detects expanding and shrinking sizes. Nerves in one channel fire when it detects objects which expand nerves in the other fire when they detect shrinkage. If an object does not change size both of these channels fire in balance. However, if an expanding object is viewed for many seconds, the firing rate of the expansion-detecting channel decreases. The decrease in firing rate continues for a while even once the visitor has looked away from the spiral so that when a stationary object is viewed and the shrink detecting channel continues to fire at an undiminished rate, the stationary object is then seen to shrink.
An object usually appears to expand or shrink because it is moving toward or away from a viewer. An expanding object is seen as approaching, a shrinking one is seen as receding.