Initial control component in disparity vergence eye movements.

Recent experimental evidence indicates that a portion of the oculomotor response to disparity stimulation is functionally open-loop; that is, the response occurs without the aid of visual feedback. To investigate the stimulus features that elicit or influence this dynamic movement, convergence responses to a step, a step followed by target disappearance, and a pulse followed by target disappearance were obtained from four subjects using infrared oculography. The target was a thin vertical line (0.25 degrees) either 2 or 10 degrees in height. Stimuli having different amplitudes (1, 2, 4 and 8 degrees) and disappearance times (50, 100 and 200 ms) were selected randomly along with occasional divergent stimuli to minimize prediction and voluntary vergence. Experiments showed that the dynamic characteristics of the initial portion of the response were essentially the same, even when the target disappeared before the movement took place. The magnitude of the initial response depended on the stimulus amplitude, but was not influenced by either stimulus duration or target height. For example, stimulus durations as short as 50 ms elicited responses similar to those caused by standard steps. The initial response was shown to be active over a well-defined time period of about 200 ms, after which the response appears to be mediated by a visually-guided control component. These results support the recently developed dual-mode theory of vergence control in which an initial preprogrammed (open-loop) control component is followed by a feedback (closed-loop) controlled component which reduces any remaining disparity.