“Magnetic sand”: Illusions of interactivity

We discovered a series of new illusions in which hand or finger movements near a random white noise display lead to the perception that the display is interactively altered contingent upon the observer's actions. These perceptions resemble interactions with a surface of magnetic sand, where the hand can leave traces or attract and repel grains in its vicinity. 1) When an observer places a finger very close to a noise display, moving slowly as though drawing a letter or shape, a visible trace appears and decays within approximately 500 ms; importantly, this iconic trace illusion can occur not only with dynamic noise but also with static noise stimuli (especially with small cell sizes). 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 finding indicates that multiple mechanisms are involved to different extents across illusions. We propose that these illusions arise from a hierarchy of mechanisms - from low-level contrast adaptation and dynamic occlusion cues to attentional selection and action capture - and that they may serve as a direct phenomenological tool for visualizing the spatial extent and dynamics of the observer's attentional field.