A New Illusion of Movement Dependent on Eye Movement during Head Rotations

A previously undocumented illusion of movement is reported which occurs when an observer, in an otherwise dark room, attempts to maintain fixation on a stationary light point located straight ahead of him while rotating his head from left to right. The light appears essentially stationary during head rotations up to about 25°, but with more extreme rotations the point suddenly appears to sail off in the direction opposite to the head turn. This illusion was investigated and data are reported which indicate that it results from unmonitored movements of the eye (i.e. eye movements which are not efferently controlled and about which there is no proprioceptive information), in the direction of the head turn, which occur when the eye is no longer able to maintain fixation by counterrotating.