Explicit versus implicit gaze processing assessed by ERPs

Gaze processing was investigated using event-related potentials in two different tasks in which front-view and 3/4-view faces were presented, with eyes gazing straight ahead or averted. Task alternated between an explicit gaze direction judgment and a judgment on head orientation where gaze was irrelevant. Accuracy and reaction times were affected by the congruency of gaze and head directions in both tasks suggesting gaze was processed implicitly in the head orientation task. In both tasks, larger P1 and N170 were found for 3/4-view faces compared to front-view faces that were not due to the luminance or contrast of the pictures. The N170 was also larger for averted than straight gaze but for front-view faces only. In contrast, larger amplitudes for straight than averted gaze were reliably measured around 400-600 ms regardless of head orientation or task demands, and likely reflected the outcome of gaze processing. The results suggest that head orientation and gaze direction discrimination occur regardless of task demands and interact at the decision making level. Neural markers of head orientation occur before those for gaze direction and the early structural encoding stages of face processing are view-dependant.