Disparity-evoked vergence is driven by interocular correlation

Disparity-evoked vergence is studied in stereograms showing one or two depth planes which are defined by isolated dots of varying density and contrast. Vergence position immediately after stimulus presentation was measured using dichoptic nonius lines. Since the stimulus was not visible after the onset of the vergence movement, the experiment accesses the initiation of vergence rather than its eventual result. In the unequivocal stimuli (one depth plane), elicited vergence tends to reduce disparity. Disparities of 0.5-1 deg are most effective which is in accordance with earlier findings. If two depth planes are presented, elicited vergence lies between the two planes, approaching the plane with higher dot density and/or dot contrast. In quantitative measurements, we show that the depth-averaging mechanism uses signal power per depth plane as a weight. Therefore, the relative pulling strength of dot density compared with dot contrast follows a power law with exponent 2. We propose a population code for vergence control based on disparity-tuned pools of units.