Abstract: PURPOSE: Overweight has become very common all over the world. Being overweight has an implication of not just physical health problems but also psychosocial consequences. Previous studies have revealed a closely association between weight judgments and facial expression judgments. However, these studies, mainly basing upon explicit judgment tasks, demonstrated inconsistent results regarding the effects of facial overweight on facial expression judgments. Here we combined a cross-adaptation paradigm with event-related potential (ERP) technology to examine whether facial overweight could exert a positive or negative influence on facial expression judgements in an implicit manner, and whether they could share partly common neural substrates.
METHODS: The constant stimulus method and top-up adaptation were employed in a psychophysical test, and EEG signals were recorded during the test. The adapting stimuli included four different types of overweight faces, normal-weight faces, and two phase-scrambled stimuli corresponding to overweight and normal-weight faces. The test stimuli included five emotional levels morphed from 100% happy to 100% angry (i.e., 83/17%, 67/33%, 50/50%, 33/67%, and 17% happy /83% angry). In each trial, an adaptor was presented at the center of the screen for 4500 msec, followed by a random interval of 400 - 600 msec. Then a test face was presented for 200 msec. Participants (N = 23) were instructed to judge whether the test stimulus could be perceived as happy or angry by pressing the left or right arrow on the keyboard.
RESULTS: The behavioral results showed that participants were less likely to perceive the ambiguous morphed test faces as angry after adapting the overweight faces compared to the normal-weight faces, indicating that the cross-adaptation between facial overweight and emotional expressions emerged. Furthermore, the ERP results revealed potential neural correlates of the behavioral perceptual bias. The amplitude of N170 difference wave elicited by test faces under the condition of the overweight adaptor was significantly smaller (less negative) than that under the condition of the normal-weight adaptor, indicating that previous prolonged exposure to overweight faces modulated the amplitude of N170 difference wave for subsequent emotional test faces.
CONCLUSIONS: Our results provided direct evidence for the cross-adaptation between facial overweight and emotional expressions and its neural correlates, demonstrating that the perception of facial overweight and facial emotional expressions share at least partly common neural substrates. We conclude that facial overweight implicitly exerts a negative influence on facial expression judgements.

Key words: adaptation, emotional expressions, overweight, N170