Eye torsion in response to a tilted visual stimulus

When the head is tilted laterally the eyes rotate in the opposite direction and come to rest at a less tilted position than the head with respect to the gravitational vertical (e.g. Howard and Templeton, 1966). This tonic countertorsion response occurs in the absence of visual stimulation and appears to be controlled primarily by the vestibular otolith (e.g. Graybiel, 1974). A visual stimulus is also capable of inducing eye torsion in a stationary observer, if it rotates around the observer’s line of sight (e.g. Howard and Templeton, 1964). It has generally been accepted, however, that eye torsion does not occur in response to a stationary visual field which is tilted with respect to the gravitational vertical (Howard and Templeton, 1964). Recent evidence suggests that this conclusion may not be correct under all conditions. Evidence of eye torsion in the direction of a tilted visual stimulus was reported in early studies using subjective afterimage techniques (Greenberg, 1960; Mesker, 1953). The subjective method of recording ‘torsion was criticized by Woward and Templeton (19643, however, who could find no objective, photographic evidence of visually induced torsion. The work of Howard and Templeton was carefully done and is often cited to support the conclusion that a stationary visual stimulus induces no eye torsion. Some positive photographic evidence has been reported in several recent studies, however. In an unpublished dissertation study, Hughes (1973) found torsional responses averaging about 0.5”. and Crone (1975) found up to I” of torsion in the direction of a tilted visual field. The question of whether eye torsion may occur in response to a stationary visual stimulus is of interest for several reasons. Recent evidence indicates that a variety of vestibular responses can be induced by visual stimuli (e.g. Dichgans and Brandt, 1974; Ebenholtz and Benzschawel, 1977; Henn er al., 1974). The most pertinent of these studies for the question at issue have shown that a stationary tilted visual display is capable of inducing an illusion of self-tilt in the opposite direction. It seems reasonable to suppose that this self-tilt illusion is due to a visual driving of the vestibular otolith system. If so, then torsional responses might also be expected, and would provide an objective method for the study of visual effects on the otolith system. Visually induced torsion would also be of some interest in the attempt to account for a variety of illusions. For example, the well-known rod-and-frame illusion could be partly due to torsional responses that are uncompensated perceptually. In this illusion an objectively vertical line (rod) looks tilted away from a surrounding tilted frame {e.g. Witkin et al., 1962/1974). Hughes (1973) found that individual differences in degree of torsion were highly related to differences in susceptibility to this illusion. Hughes concluded that part of the frame effect on apparent rod orientation can be attributed to torsional effects. In view of the discrepancies in results reported so far, and the interesting implications involved, it seemed useful to re-examine the question of visually induced torsion. In the present study, eye position was recorded photographically while observers viewed the standard rod-and-frame display (e.g. Witkin er al., 1962/1974).