Torsion and stereopsis.

The perceptual correlates of sensory cyclofusion have received little attention. However, they are easily elicited by asking patients with torsion about perceived slant of the visual world, and by watching how they hold the Titmus or Randot book. Sensory cyclofusion induces two types of perceptual aberrations. First, it rotates the plane of maximal stereoscopic volume in the pitch plane. As such, patients with large degrees of disconjugate binocular torsion can overcome a cyclodisparity by slanting the Titmus book to align the vertical images of the object with those of the retinas. Second, patients may report a perceived slant of vertical images and of the ground plane in three-dimensional space, so that the ground and the visual world appear to be slanted toward or away from them. Patients undergoing oblique muscle surgery should be examined preand postoperatively for corollary perceptual shifts in their stereoscopic perception of three-dimensional space.