Simple curves can influence whether we see happy or sad faces.

Here is an interesting bit of work from Xu et al. showing that adaptation to simple stimuli (like the shape of a mouth) that are processed early in the visual hierarchy can influence our perception of higher level perceptions (i.e., of faces) that are analyzed at higher levels of the visual hierarchy. Thus adaptation to a concave (sad) cartoon mouth shape makes subsequent perception more likely to report a happy face, and vice versa. Their abstract:

Adaptation is ubiquitous in sensory processing. Although sensory processing is hierarchical, with neurons at higher levels exhibiting greater degrees of tuning complexity and invariance than those at lower levels, few experimental or theoretical studies address how adaptation at one hierarchical level affects processing at others. Nevertheless, this issue is critical for understanding cortical coding and computation. Therefore, we examined whether perception of high-level facial expressions can be affected by adaptation to low-level curves (i.e., the shape of a mouth). After adapting to a concave curve, subjects more frequently perceived faces as happy, and after adapting to a convex curve, subjects more frequently perceived faces as sad. We observed this multilevel aftereffect with both cartoon and real test faces when the adapting curve and the mouths of the test faces had the same location. However, when we placed the adapting curve 0.2° below the test faces, the effect disappeared. Surprisingly, this positional specificity held even when real faces, instead of curves, were the adapting stimuli, suggesting that it is a general property for facial-expression aftereffects. We also studied the converse question of whether face adaptation affects curvature judgments, and found such effects after adapting to a cartoon face, but not a real face. Our results suggest that there is a local component in facial-expression representation, in addition to holistic representations emphasized in previous studies. By showing that adaptation can propagate up the cortical hierarchy, our findings also challenge existing functional accounts of adaptation.

Here are some examples of face stimuli used in the studies, in which subjects were experiments as well as naive subjects:


Figure - Examples of the face stimuli used in this study. a, Cartoon faces used in experiment 1, generated with our anti-aliasing program. The mouth curvature varied from concave to convex to produce sad to happy expressions. b, Ekman faces used in experiment 2. The first (sad) and last (happy) images were taken from the Ekman PoFA database, and the intermediate ones were generated with MorphMan 4.0. c, MMI faces used in experiments 3 and 4. The first (sad), middle (neutral), and last (happy) images were taken from the MMI face database, and the other images were generated with MorphMan 4.0.