"Magnetic Sand" : Illusions of Interactivity

Abstract

We found a series of new illusions, in which actions performed near a random white-noise display lead to the perception that the display is altered interactively with the observers actions. The perceptions resemble interactions with a box of magnetic sand, where the hand can leave traces, or attract and repulse grains in its vicinity. 1) the observer puts a finger very close to a dynamic noise display, slowly moving as though drawing a letter or a shape. A trace appears left in the fingers path, decaying within 500 ms or so. 2) When the observer moves their palm toward and away from the display, opening and closing their fingers as if grabbing and releasing grains of sand, the random dots appear as though they were magnetically attracted to or repelled by the fingers. 3) When an open hand close to the display is slowly moved back and forth laterally, the nearby dots appear to get attracted to or captured by the fingers and thus appear to move with them. 4) The same kind of action capture occurs even when the hand is not visible, moving behind the display. These illusions are robust across a wide range of parameters, including frame rate, luminance contrast, dot size (spatial frequency), and finger movement factors. Inter-subject variability is not correlated across illusion types, and the illusions also diverge in behavior across dynamic and static noise conditions. This indicates that multiple mechanisms are involved to different extents across illusions. Several known visual motion detectors and other low-level mechanisms may be involved in seeding the perceptual phenomena. However, a complete explanation would require mechanisms of action capture, whereby the internal model of the persons actions and their predicted consequences organizes visual attention and processing of the random stimulus components.