Estimates of time to contact based on changing size and changing target vergence

Apparent constant-velocity approaches of an outline circle were generated by changing the size and/or the lateral separation of dichoptically presented circles at a physically constant distance from subjects. The lateral separation of the circles defines target vergence which corresponds proximally to combinations of ocular vergence and (absolute) retinal disparity. In the first experiment, estimates of time to contact were found to depend both on changing size and on changing target vergence; in the case of conflicting information, changing size turned out to be the more powerful stimulus. In the second experiment, the size of the stimulus object was varied. The relatively stronger influence of changing size on estimates of time to contact was again found for the larger stimulus objects, but for the smaller stimulus objects changing target vergence became more powerful. The results of both experiments show that estimates of time to contact are not uniquely determined by changing size, as seems to be implied by some tests of the so-called "tau hypothesis', but that they are based on multiple sources of information that have situationally specific effects on the estimates.