Egocentric orientation is influenced by trained voluntary cyclorotary eye movements

A CYCLOROTARY eye movement is a motor response of the eye made around the visual axis. Counter-rolling of the eye, for example, occurs during lateral head tilt1–5; conjugate rotary nystagmus can be induced by a large rotating field6–9; and disjunctive cyclotorsions can occur during ordinary convergence10,11. Because none of these torsional eye movements can be produced as an isolated voluntary response, eye torsion has always been classified as an involuntary response, a reflex. Using a visual-feedback procedure, however, we have trained humans to make conjugate voluntary cyclotorsional eye movements up to 30 degrees in magnitude12. We have also demonstrated that these large torsional movements are not visually induced and can be made in the absence of any visual stimulus. Accompanying the training and performance of these eye movements were a number of striking illusions related to one's own sense of body orientation. Because these newly trained eye movements are unprecedented, it is of interest to characterise accompanying illusions in detail, comparing them with other illusions of self rotation induced through vestibular13 and visual14,15 inputs. In this paper we compare the effects of trained cyclorotary eye movements with head and whole body tilts, showing a quantitatively similar change in egocentric orientation for each type of tilt. As such, our findings suggest the possibility of shared mechanisms affecting the stability of one's internal frame of reference, both for eye and body movements.