Abstract
In the chopstick illusion (Anstis 1990) both sticks translate along a circular path. Consider now the following modification of the original display. Both sticks physically oscillate along a rectilinear path, one vertically and the other horizontally, with a phase difference such that any stick reverses its motion when the other crosses the midpoint of its trajectory. For most observers, this display generates a strong induced motion effect: one stick is perceived to move along a more or less compressed elliptical trajectory while the other oscillates along a rectilinear trajectory. Most observers perceive the physically-horizontal motion of the horizontal stick veridically and spontaneously perceive the vertically-oscillating vertical stick as moving along an elliptical path. The effect can be attributed to an asymmetry among cardinal axes: since the position along the vertical axis is more salient than the position along the horizontal axis, a vertical oscillation is more easily misperceived as a kind of induced motion along an elliptical trajectory than a horizontal oscillation (provided that conditions for induced motion are present in the relative displacements of the two sticks). This hypothesis was tested by comparing elliptical paths perceived in vertical/horizontal chopstick displays with those perceived in displays containing diagonally-oriented sticks. The relative role of egocentric vs environmental frames of reference was assessed by asking observers to evaluate vertical/horizontal chopstick displays with their heads tilted 90 deg. Finally, the magnitude of the effect was measured in conditions in which the salience of one cardinal axis over the other was balanced by the modification of the relative length of the two sticks.
Grant MIUR-COFIN 2003115470 to Walter Gerbino