Anticipating the three-dimensional consequences of eye movements.

Rapid eye movements called saccades give rise to sudden, enormous changes in optic information arriving at the eye; how the world nonetheless appears stable is known as the problem of spatial constancy. One consequence of saccades is that the directions of all visible points shift uniformly; directional or 2D constancy, the fact that we do not perceive this change, has received extensive study for over a century. The problems raised by 3D consequences of saccades, on the other hand, have been neglected. When the eye rotates in space, the 3D orientation of all stationary surfaces undergoes an equal-and-opposite rotation with respect to the eye. When presented with a an optic simulation of a saccade but with the eyes still, observers readily perceive this depth rotation of surfaces; when simultaneously performing the corresponding saccade, the 3D orientations of surfaces are perceived as stable, a phenomenon I propose calling 3D spatial constancy. In experiments presented here, observers viewed ambiguous 3D rotations immediately before, during, or after a saccade. The results show that before the eyes begin to move the brain anticipates the 3D consequences of saccades, preferring to perceive the rotation opposite to the impending eye movement. Further, the anticipation is absent when observers fixate while experiencing optically simulated saccades, and therefore must be evoked by extraretinal signals. Such anticipation could provide a mechanism for 3D spatial constancy and transsaccadic integration of depth information.