Fourth Purkinje image signals reveal eye-lens deviations and retinal image distortions during saccades

Saccadic eye movements of various sizes and directions were registered simultaneously with the scleral search coil and a fifth-generation Dual Purkinje image eyetracker. Comparison of the search coil and the Purkinje image tracker records reveal considerable dynamic deviations during and immediately after the saccade, which we ascribe to the movements of the eye lens relative to the optical axis of the eye. Due to the increased stiffness of the tissues these deviations are smaller in older subjects. Also, they are larger at near accommodation. The size of the retinal image displacement which results from the lens movement proper can be as large as 0.5 deg, which may not be negligible in a number of visual tasks.