The effect of stimulus contrast and interocular correlation on disparity vergence

In previous reports, we developed a metric for describing the signal strength of a dynamic random-dot stereogram (DRDS) stimulus at binocular (cyclopean) levels of the human visual system, which takes both contrast and interocular correlation into account. In this study we tested the generality of that metric in relation to the control of horizontal vergence eye movements. Signal strength was assessed by measuring the extent to which a DRDS stimulus could elicit involuntary vergence responses from a subject who was attempting to fixate steadily. Results for both step and sinusoidal disparity modulation paradigms showed that vergence velocity increased when either interocular correlation (IOC) or contrast was increased. Furthermore, IOC and contrast were found to contribute to signal strength for vergence in the same proportion as was found psychophysically. In general, the results indicate that the signals that drive this passive form of vergence are derived according to the same binocular combination rules as the signals that give rise to the perception of surfaces in DRDS stimuli.