A single red, plus-shaped LED was positioned on a movable platform, driven by a high precision linear motion actuator (Zaber Technologies, Inc., Vancouver, BC, Canada). The direction of motion was aligned with the animal's midsagittal plane, which elicited smooth, symmetric vergence tracking. The target moved at a constant velocity, following a triangle wave pattern. The frequency ranged from 0.05 to 0.2 Hz. Some experiments were conducted with the recording booth dimly illuminated, with the target surrounded on three sides (top, left, and right) by a 15 × 15-cm black and white checkerboard pattern. This was done to provide monkey CI1 with a more robust set of disparity and accommodation cues. Without this background, monkey CI1 was unable to bring both eyes to the target when more than ∼6° of convergence was required.