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Recently it has been demonstrated that saccades become different in size in the two eyes if a subject is adapted to anisometropic spectacles, which provide visual images of different magnitude to the two eyes. These nonconjugate adaptations adequately meet the requirements of those spectacles and, once acquired, they persist (with some reduction) even during monocular viewing. We now demonstrate that such nonconjugate adaptations of saccades can be meridian-specific, if there is a pressure for such meridian-specificity. This pressure was provided by means of a cylindrical spectacle-lens. Adaptations along a vertical, horizontal or oblique meridian did not transfer to the orthogonal meridian. These results demonstrate a capability of saccadic adaptation to deal with calibration problems restricted not only to one eye, but even to one specific plane of muscular action. Our results also suggest that the meridian-specific adaptations of oblique saccades take place at a stage before the decomposition of motor commands into separate horizontal and vertical components. The meridian-specific nonconjugacies were also expressed in smooth-pursuit eye movements. Post-saccadic drift adapted only along the horizontal meridian.

It is generally believed that saccades follow Hering's law in the sense that they are equally large in the two eyes. We demonstrated that saccades are different in size in the two eyes in 8 habitual wearers of anisometropic spectacles, which have lenses of different refractive powers, and therefore supply each eye with a differently sized visual image. The eye provided with the larger visual image made larger saccades than its fellow eye. This nonconjugate adaptation was almost complete for both horizontal and vertical saccades. Post-saccadic drift was also asymmetrically adapted: it reduced any fixation-disparity present at saccadic offset. The nonconjugate adaptation was also expressed in smooth-pursuit eye movements. In addition, these nonconjugate adaptations were present during monocular viewing, which shows that they were hard-programmed.

It has been demonstrated before that the long-term wearing of anisometropic spectacles may induce nonconjugate adaptations of saccades. Saccades then become different in size in the two eyes. We examined the time-course and the limits of such adaptations of horizontal and vertical saccades during the short-term (1-6 hr) wearing of anisometropic spectacles. After only 1 hr of conditioning to 2 D of anisometropia, the nonconjugate size-adaptations were almost complete along the horizontal meridian. For progressively larger anisometropias (up to 8 D) the adaptative nonconjugacies after 1 hr became also systematically larger. An anisometropia larger than 6 D did not further increase the rate of adaptation during the first 6 hr of conditioning, which suggests that about 6 D of difference in spectacle correction, causing size differences of about 12%, may be the upper limit of the nonconjugate adaptive range of the saccadic subsystem. Post-saccadic drift of horizontal saccades was also adequately changed. In addition, nonconjugate adaptations had developed in smooth-pursuit eye movements. All of these plastic changes persisted during monocular viewing, indicating that the basic programming of these eye movements was changed.

Saccades have traditionally been studied in response to suddenly changing visual stimuli, such as jumping targets. In every-day life, however, most targets are stationary. We studied saccades made in either target condition. Saccadic accuracy was two- to five-fold better with stationary targets than with jumping targets. In addition, both the number of secondary saccades and the total time required to foveate the target were decreased with stationary targets in comparison to jumping targets. We also found that with an illuminated background the total time required to foveate the target was shorter than when the background was dark.