Integration of speed and time for estimating time to contact.

Abstract

To coordinate movements with events in a dynamic environment the brain has to anticipate when those events occur. A classic example is the estimation of time to contact (<i>TTC</i>), that is, when an object reaches a target. It is thought that <i>TTC</i> is estimated from kinematic variables. For example, a tennis player might use an estimate of distance (<i>d</i>) and speed (<i>v</i>) to estimate <i>TTC</i> (<i>TTC</i> = <i>d</i>/<i>v</i>). However, the tennis player may instead estimate <i>TTC</i> as twice the time it takes for the ball to move from the serve line to the net line. This latter strategy does not rely on kinematics and instead computes <i>TTC</i> solely from temporal cues. Which of these two strategies do humans use to estimate <i>TTC</i>? Considering that both speed and time estimates are inherently uncertain and the ability of the human brain to combine different sources of information, we hypothesized that humans estimate <i>TTC</i> by integrating speed information with temporal cues. We evaluated this hypothesis systematically using psychophysics and Bayesian modeling. Results indicated that humans rely on both speed information and temporal cues and integrate them to optimize their <i>TTC</i> estimates when both cues are present. These findings suggest that the brain's timing mechanisms are actively engaged when interacting with dynamic stimuli.