Visual discrimination thresholds for time to arrival.

In a seminal article, Todd (Journal of Experimental Psychology: Human Perception and Performance 7:795-810, 1981) reported a difference threshold of about 50 ms to discriminate the times of arrival of two differently sized objects that simultaneously approached head-on at constant but different velocities. Subsequent investigators, however, have often found much higher thresholds. We did one complete replication of Todd's experiment, and then modified his stimuli and experimental regime, which we hypothesized may have been responsible for some of the discrepancies reported in the literature. Unlike Todd and most other researchers, we exclusively used untrained observers. Several of our participants performed almost as well as the trained observers used by Todd and others, but the performance of most of our participants fell short of this standard. Furthermore, thresholds were affected by the experimental regimes, with large differences between objects' sizes and speeds compromising performance. Analyses of the response patterns revealed that the responses were driven mainly by the objects' relative apparent sizes.