Oculomotor consequences of feeble image size inequality at near reading distance

Reading and working with a computer screen are activities of everyday life that take place at near vision. This study examines whether at such a near distance, normal subjects are capable of modifying the natural conjugacy of their saccades when exposed to a feeble image size inequality of 2%; similar inequality exists for persons who wear spectacles of slightly different power for the two eyes. Subjects were seated at 40 cm in front of a screen where a random dot pattern was projected. They were asked to make saccades of 7.5 degrees and 15 degrees along the horizontal and vertical principal meridians and horizontal saccades between secondary and tertiary positions in the upper and lower field. Five subjects performed the experiment with a 2% overall reduction lens inserted over their dominant eye; three subjects participated in a second experiment with an overall magnification lens of 2% also inserted in front of the dominant eye. The results showed a persistent decrease or increase in the size of the saccade of the dominant eye that was subject dependent for horizontal saccades, but consistent over subjects for vertical saccades. Persistent disconjugacy, at least for vertical saccades, is interpreted as evidence of learning.