Shared target selection for combined version-vergence eye movements.

Primates frequently make rapid binocular eye movements to reorient gaze in both direction and depth. To explain the unequal movements made by the two eyes, it often is assumed that they result from the combined action of a conjugate saccadic system and a vergence contribution. Clearly such a scheme can only yield coordinated binocular movements if both systems are guided by a shared or coupled target selection mechanism. To investigate the degree of cooperation at this level, we studied binocular refixations to target-nontarget double-stimuli in three-dimensional (3-D) space. Binocular eye movements were recorded in seven subjects using the scleral coil technique. In the experiments, 20% of trials were composed of a green target and a red nontarget, presented at the same time, but at different locations in 3-D space. These were alternated randomly with single-target trials (80%) in which the green stimulus was presented randomly at one of eight possible positions in 3-D space. Instructions to the subject emphasized either the speed or accuracy of response. Our findings show that typical features of the saccadic response to double-stimuli (bistability, averaging, and a speed-accuracy trade-off), as found in earlier two-dimensional studies, are also prevalent for initial binocular refixations to double stimuli in 3-D space. When the first saccadic response is directed to one of the two stimuli, the vergence system almost invariably makes the same choice. Likewise, when the saccadic system makes a short-latency averaging response, the vergence system shows a similar compromise. Statistical analysis shows a high correlation between saccadic and vergence target selection, strongly suggesting that the amplitude computation process of both subsystems is due to a common target selection stage that has access to information about stimulus location in 3-D space.