In measuring eye position, for instance, both Fick and Wundt had adopted procedures which did not treat the co-ordinate frame of the primary position as fundamental. Rather, they related cyclorotations to the visual plane—which is variable—instead of to the horizontal in the primary position.
Source: wiktionary
The images of horizontal lines in the upper visual field acquire a gradient of cyclorotation with one sign, and those in the lower visual field acquire a gradient of cyclorotation with the opposite sign, as shown in Figure 7.8b.
Source: wiktionary
Also shown are the coefficients of the residual wavefronts assuming a partial correction of the original wavefront due to a cyclorotation of the eye. To estimate how much error may induce due to the cyclorotation of the eye, the residual RMS wavefront error as well as the residual high-order RMS wavefront error for each of the rotation angles is shown.
Source: wiktionary