Dynamics of cover test eye movements.

The dynamics of unilateral cover test eye movements, never before systematically investigated with an objective recording system, are shown to be more complex than textbook accounts of them. We administered the cover test to nine heterophoric subjects by means of electromechanical occluders. Eye movements were recorded using the infrared photoelectric technique. Saccadic and vergence movements of the fixating eye were observed in almost all records when the occluded eye was uncovered. These movements were found in esophores and exophores and in both large and small phoria cases. Such movements were previously described in other asymmetric vergence tasks and appear to obey Hering's law of equal innervation. Uncovering the dominant eye, in cases of clear dominancy, resulted in shorter latency and larger amplitude saccades than did uncovering the nondominant eye. These large saccades were frequently of unequal amplitude in each eye. Trained subjects appear to use dynamic overshoots to increase this saccadic inequality and thereby attain vergence during saccades. Movements after the application of a cover to one eye, while grossly similar to textbook descriptions of them, are found to contain small vergence drifts and refixation (correcting) saccades in the nonoccluded eye.