Spatial Simon effects and compatibility effects induced by observed gaze direction

Preconditions for the processing of observed gaze direction were studied by spatial Simon and compatibility effects. In five experiments, responses in the direction of a gaze were faster than responses in the opposite direction. Gaze direction influenced response speed although its processing was not required (Experiments 1, 2, 4, and 5). The result supports the notion of an inclination to process observed gaze direction in humans. However, the processes underlying the Simon effect of observed gaze direction were relatively time-consuming: Simon effects induced by gaze direction increased with response speed. Focusing of attention seemed to be part of the necessary processes: Simon effects were eliminated if attention was focused away from the eyes (Experiment 2). Further, if the processing of observed gaze direction was required, Simon effects by faces' observer-relative screen positions were absent (Experiment 3). Control conditions revealed that Simon effects by gaze direction can be produced with faces tilted by 90°, but that corresponding Simon effects do not result from numerical stimuli with an analogue spatial structure (Experiment 4), and that Simon effects result from gaze directions of scrambled faces (Experiment 5).