Disparity minimisation, cyclovergence, and the validity of nonius lines asa technique for measuring torsional alignment

Frisby et al (1993 Perception 22 Supplement, 115) proposed that the visual system might make cyclovergent eye movements in order to minimise the overa ll pattern of both vertical and horizontal disparities when an observer vie ws an inclined stereoscopic surface. Their measurements of cyclovergence, w hich used vertically oriented nonius lines, were found to be consistent wit h that proposal. In our experiment 1, we measured torsional eye movements o bjectively, using scleral coils, and found no evidence of a cyclovergent re sponse to either a real inclined surface or to a simulated inclined surface in which the two stereoscopic images were related by a horizontal shear tr ansformation. These results are inconsistent with the disparity minimisatio n hypothesis. In order to account for the discrepant findings of the two st udies, we propose that vertically oriented nonius lines may not be a valid method for assessing cyclovergence because the lines can be seen as lying ' within' the inclined surface. In experiment 2, we tested the predictions of the cyclovergence hypothesis of Frisby et al against our own 'within surfa ce' explanation, using both horizontally and vertically oriented nonius lin es and dichoptic images related by either a horizontal or a vertical shear. If cyclovergence were the cause of the misalignment, both horizontal and v ertical nonius lines should appear misaligned to the same extent. This was not found to be the case. We conclude that vertical nonius lines may not be a valid technique for measuring cyclovergence when the lines are seen agai nst a background of an inclined surface.