Estimation of ocular torsion with dynamic changes in pupil size

The accuracy of estimating ocular torsion by the cross-correlation technique was evaluated in five healthy subjects. During the study, the subjects fixated a stationary target while their pupil size was manipulated by increasing/decreasing the amount of light falling on their retinas. For all subjects, ocular torsion estimates had an overall mean value of 0.22° and a standard deviation of 0.89°. We suggest that the reasons for the relatively large standard deviations are: (a) physiological torsional oscillations and drift during fixation; and (b) non-radial movements of the iris structure during pupil constriction and dilation. The non-radial movement of the iris structures resulted in different ocular torsion estimates from different regions of the iris. The range of ocular torsion estimates from different regions varied from 0.26° for a 15% increase in pupil radius to as much as 3.0° for a 90% increase in pupil radius