The Simon effect and visual motion.

S-R compatibility and Simon effects were studied for real visual motion. In Experiment 1, two small stimulus lights were constantly visible, 5 degrees to the left and right of fixation; after a random delay, one began to move at 2 degrees/s. In Experiment 2, a single stimulus light moving at 2 degrees/s suddenly appeared 5 degrees to the left or right of fixation, i.e., motion onset and stimulus onset coincided. In both experiments, subjects responded by a key press with their left or right index finger as soon as they detected motion. In Condition A responses were made to the position (left or right) from which the motion started, irrespective of its direction (position compatibility); in Condition B responses were made to the direction of motion (leftward or rightward) irrespective of whether motion started to the left or to the right of fixation (direction compatibility). The results show strong compatibility effects for both position and direction of motion in both experiments. A Simon effect, however, occurred only when position was task irrelevant in Experiment 1; no Simon effect was found in Experiment 2. The data only partly confirm previous results obtained with apparent motion. The selective lack of a Simon effect supports the integrated model of Umiltà and Nicoletti (1992), which requires orienting of attention for the Simon effect to occur. It is specifically assumed that this attention-orienting is triggered only by the saccade program and does not extend to the pursuit program that is initiated by smooth stimulus motion.