Divergence between oculomotor and perceptual causality.

When two objects such as billiard balls collide, observers perceive that the action of one caused the motion of the other. We have previously shown (Badler, Lefèvre, & Missal, 2010) that this extends to the oculomotor domain: subjects make more predictive movements in the expected direction of causal motion than in a noncausal direction. However, predictive oculomotor and reactive psychophysical responses have never been directly compared. They should be correlated if they tap into the same mental processes. To test this, we recorded oculomotor responses to launching stimuli, then asked subjects to manually classify those stimuli as causal or noncausal. Overall the psychophysical classifications matched the oculomotor biases, although correlations across subjects were mostly absent. In subsequent experiments, 50% of the trials had a 300-millisecond delay after the collision to impede the perception of causality. Subjects maintained their causal oculomotor bias but used different classification strategies, usually grouping the stimuli either by delay or by direction. In addition, there was no evidence that the two response types were correlated on a trial-by-trial basis. The results suggest divergent processes underlying oculomotor responses to and judgments of causal stimuli.