Effect of sustained cyclovergence on eye alignment: Rapid torsional phoriaadaptation

PURPOSE. To describe adaptive changes in torsional alignment that follow su stained cyclovergence in healthy humans. METHODS. Eye movements were recorded binocularly from four healthy subjects using dual-coil scleral annuli. Cyclovergence movements were evoked over p eriods of 30 to 150 seconds using a stereoscopic display, presenting gratin gs of lines arranged horizontally, vertically, or at 45 degrees, subtending angles of up to 48 degrees. In- and excyclodisparities of 5 degrees were i ntroduced and removed in a single-step fashion. After stimulation, the time course and magnitude of the decay in cyclovergence was compared with the s ubject either in darkness or viewing a baseline stimulus of zero cyclodispa rity. RESULTS. As reported previously, the cyclovergence response to incyclodispa rities was greater than to excyclodisparities. After sustained excycloverge nce, however, in all subjects and in response to all orientations of the gr atings, the decay in darkness was incomplete, implying an adaptive change i n torsional alignment. In response to the horizontal gratings, for incyclov ergence there was also an incomplete decay in darkness but to a lesser degr ee than in response to excyclovergence. and in only three of four subjects. The incyclovergence evoked by the oblique and vertical gratings was of sma ll 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 v isual stimulus may be incomplete. The residual component ma) be interpreted , by analogy with horizontal and vertical vergence, as reflecting so-called phoria adaptation for torsional alignment.