EVIDENCE FOR SEPARATE CONTROL OF SLOW VERSION AND VERGENCE EYE-MOVEMENTS - SUPPORT FOR HERINGS LAW

When a visual stimulus changes direction and distance simultaneously, Hering's Law argues that the resulting eye movements are the result of combined version and vergence control processes. Recently, it has bee n suggested that slow asymmetrical eye movements might be guided by mo nocular control processes wherein each eye is driven by its own retina l image. Experimental results presented here show behavioral differenc es between slow version and slow vergence eye movements, indicating th at different control processes drive the two ''pure'' responses. Speci fically, version tracking of constant velocity stimuli (i.e., smooth p ursuit) is more precise, showing less variation in tracking velocity t han movements of equal velocity produced by vergence stimuli. When the two stimuli are combined, the variability in tracking is consistent w ith the addition of the two components in proportion to their respecti ve stimuli. These results provide support for Hering's Law, at least f or low velocity, smooth tracking movements (i.e., slow version and slo w vergence). (C) 1998 Elsevier Science Ltd. All rights reserved.