Effect of sustained cyclovergence on eye alignment: rapid torsional phoria adaptation.

PURPOSE To describe adaptive changes in torsional alignment that follow sustained cyclovergence in healthy humans. METHODS Eye movements were recorded binocularly from four healthy subjects using dual-coil scleral annuli. Cyclovergence movements were evoked over periods of 30 to 150 seconds using a stereoscopic display, presenting gratings of lines arranged horizontally, vertically, or at 45 degrees, subtending angles of up to 48 degrees. In- and excyclodisparities of 5 degrees were introduced and removed in a single-step fashion. After stimulation, the time course and magnitude of the decay in cyclovergence was compared with the subject either in darkness or viewing a baseline stimulus of zero cyclodisparity. RESULTS As reported previously, the cyclovergence response to incyclodisparities was greater than to excyclodisparities. After sustained excyclovergence, however, in all subjects and in response to all orientations of the gratings, the decay in darkness was incomplete, implying an adaptive change in torsional alignment. In response to the horizontal gratings, for incyclovergence there was also an incomplete decay in darkness but to a lesser degree than in response to excyclovergence, and in only three of four subjects. The incyclovergence evoked by the oblique and vertical gratings was of small magnitude, and its decay was unaffected by the presence or absence of a visual stimulus. CONCLUSIONS After sustained cyclovergence, its decay in the absence of a visual stimulus may be incomplete. The residual component may be interpreted, by analogy with horizontal and vertical vergence, as reflecting so-called phoria adaptation for torsional alignment.