Speed of response initiation in a time-to-contact discrimination task reflects the use of η

Avoiding collisions and making interceptions seem to require an organism to estimate the time that will elapse before an object will arrive to the point of observation (time-to-contact). The most outstanding account for precise timing has been the tau hypothesis. However, recent studies demonstrate that tau is not the only source of information in judging time-to-contact. By measuring reaction time in a time-to-contact discrimination task, we show that the η function, which is a specific combination of optical size and rate of expansion, explains both accuracy and the observed RT pattern. The results conform to the hypothesis that the observers initiate the response when η reaches a response threshold value. © 2002 Elsevier Science Ltd. All rights reserved.