Blended facial expressions with a smiling mouth but non-happy eyes (neutral, sad, etc.) are often (incorrectly) judged as “happy”. We investigated the time course of this phenomenon, both forward and backward. To do this, we varied the order of presentation of a prime stimulus (upper half of a face) and a probe (lower half of a face) stimulus, and their display durations. The forward and the backward influence of the smile was assessed when the mouth was seen before or after the eyes. Participants categorised the eye expression when the mouth and the eyes were congruent or incongruent. Results showed that, as a forward prime, a smiling mouth biased the recognition of incongruent (non-happy) eyes as if they were happy. The effect started as early as 100 ms and dissipated by 1000 ms. As a backward prime, the smile also biased recognition of non-happy eye expressions as happy for at least the first 300 ms. These results suggest, respectively, that the presence of a smiling mouth impairs the accurate encoding and memory for non-happy eyes. Angry eyes are the least susceptible to this effect, probably due to their distinctiveness. An alternative response (rather than sensitivity) bias was partially ruled out.
