Obligatory integration of face features in expression discrimination

Previous composite face studies have shown that an unattended face half that differs in identity or in expression from the attended face half distracts face perception. These studies have typically not controlled for the amount of information in different face halves. We investigated feature integration while participants discriminated angry and happy expressions. The stimuli were scaled using individual thresholds to control the expression strength in face halves. In the first experiment, we varied the relative amount of information in upper and lower parts. In the second experiment, participants tried to ignore one half of the face, which was either congruent or incongruent with the attended half. We found both beneficial and obligatory integration of face halves. A robust face composite effect was found both when attending eyes or mouth, and both for congruent and incongruent expressions, suggesting similar processing of face halves when the amount of information is controlled for.