Short-term predictive changes in the dynamics of disparity vergence eye movements

Repetitive stimulation of the disparity vergence system to large convergent step stimuli has been shown to increase the dynamics of subsequent responses to smaller step stimuli. Here we show that decreases in the dynamics of both disparity convergence and divergence eye movements can be induced using a frequently occurring small amplitude conditioning stimulus to modify responses to a larger, occasionally presented test stimulus. In one experiment, a simple conditioning stimulus consisting of repetitive 1 degrees step stimuli was used to modify the dynamic vergence response to an occasional 4 degrees step test stimulus. An experimental trial consisted of three phases: baseline, conditioning, and recovery. The baseline and recovery phases used only the 4 degrees test stimuli. The dynamic characteristics of the responses to test stimuli were quantified by measuring the magnitude of the peak velocity. A statistically significant change was observed between the dynamics of conditioned responses compared to baseline and recovery responses indicting modification by the conditioning stimuli. During recovery, the response dynamics returned to levels near baseline levels showing that the decrease in response dynamics was caused by the conditioning stimulus, not fatigue. Another experiment showed that the response dynamics to large stimuli could be decreased whereas the dynamics of small stimuli could be increased by the same intermediate conditioning stimulus. Other experiments suggest that the modifications are due to a predictive mechanism. The results indicate that the dynamics of disparity vergence eye movements are malleable and depend to some extent on the amplitude of preceding stimuli.