Cyclovergence: a comparison of objective and psychophysical measurements

Several psychophysical procedures have been used to measure cyclovergence but none has been adequately validated with respect to an objective measure. Scleral search coils were used to measure cyclovergence induced by dichoptic textured patterns which cyclorotated in antiphase through 6 degrees at frequencies between 0.05 and 2 Hz. In one psychophysical procedure subjects nulled the apparent oscillatory motion of radial nonius lines superimposed on a small black disc at the center of a large cyclorotating display. In the second, subjects nulled the apparent inclination in depth of a vertical line displayed in the same way. The amplitude of cyclovergence measured objectively and the magnitude of oscillation of the nonius lines were similar at low stimulus frequencies. At higher frequencies of cyclorotation, both amplitudes declined but the nonius amplitude exceeded that of cyclovergence. The apparent inclination of the vertical test line, as indicated by the nulling procedure, also declined with increasing frequency of cyclorotation, but not at the same rate. At low frequencies, the apparent inclination of the test line was greater than that corresponding to the cyclodisparity induced into the line and almost as great as that corresponding to the relative disparity between the oscillating pattern and the line. The implications of these results for measuring cyclovergence and for an understanding of stimuli driving cyclovergence and perceived inclination in depth are discussed.