A new "fat face" illusion

Development of upper visual field bias for faces in infants

The spatial location of the face and body seen in daily life influences human perception and recognition. This contextual effect of spatial locations suggests that daily experience affects how humans visually process the face and body. However, it remains unclear whether this effect is caused by experience, or innate neural pathways. To address this issue, we examined the development of visual field asymmetry for face processing, in which faces in the upper visual field were processed preferentially compared to the lower visual field. We found that a developmental change occurred between 6 and 7 months. Older infants aged 7-8 months showed bias toward faces in the upper visual field, similar to adults, but younger infants of 5-6 months showed no such visual field bias. Furthermore, older infants preferentially memorized faces in the upper visual field, rather than in the lower visual field. These results suggest that visual field asymmetry is acquired through development, and might be caused by the learning of spatial location in daily experience.

The "Fat Face" illusion: A robust adaptation for processing pairs of faces

Converging evidence has demonstrated our remarkable capacities to process individual faces. However, in real-life contexts, we rarely see faces in isolation. It is largely unknown how our visual system processes a multitude of faces. The current study explored this question by using the "Fat Face" illusion: when two identical faces are vertically aligned, the bottom face appears bigger. In Experiment 1, we tested the robustness of this illusion by using faces varied by gender and race, by recruiting participants from different countries (Canadian, Chinese, and French), and by implementing different task requirements. We found that the illusion was stable and immune to variations in face gender or face race, perceptual familiarity, and task requirements. Experiment 2 further indicated that binocular vision was essential for this visual illusion. When participants performed the task with one eye covered, the previously robust illusion completely disappeared. Together, these findings revealed a visual adaptation for processing multiple faces in the environment: the face at the top is perceived as more distant from the viewer and appears smaller in size than the face at the bottom. More broadly, overestimating the size of the bottom face may represent a fundamental mechanism for social interactions, ensuring the deployment of attention to those closest to self.

A Geometrical Account to Explain the Fat-Face Illusion

Investigations of the "fat-face" illusion have unanimously agreed that the illusion is face-specific. Here, we offer several manipulations to highlight that the fat-face illusion is not restricted to the bottom image, isn't a property of internal features, facial contour/texture, and in general isn't even specific to faces. We propose the axis of horizontal asymmetry account to contextualize fat-face illusion as a geometry-led illusion.

Fat Face Illusion, or Jastrow Illusion with Faces, in Humans but not in Chimpanzees

When two identical faces are aligned vertically, humans readily perceive the face at the bottom to be fatter than the top one. This phenomenon is called the fat face illusion. Furthermore, an apparent similarity has been pointed out between the fat face illusion and the Jastrow illusion. Recent studies have suggested the importance of facial contours and the role of basic-level processing of faces. In the present study, we directly compared the typical Jastrow illusion and fat face illusion in humans and chimpanzees using the same task. Both humans and chimpanzees clearly showed the Jastrow illusion, but only humans perceived the face at the bottom as fatter than the top. Although further examination is necessary, these results might reflect different processing levels of faces between the two species.

Face contour is crucial to the fat face illusion

In 2010 Thompson reported a "fat face thin" illusion that, when next to an inverted face, an upright face looks "fatter". Sun et al (2012 Perception 41 117-120) observed that one of the faces need not be inverted for the illusion to emerge: when two identical faces are presented one above the other, the face at the bottom appears "fatter" than the top one. Neither inverted faces nor clocks induced the illusion. Here we conducted three experiments probing the role that face contour plays in producing the fat face illusion. In experiment 1 line drawing faces were found to induce the illusion, suggesting that face contour is important for producing the illusion. In experiment 2 line drawing faces with scrambled internal features and empty line drawing faces devoid of internal features were found to induce the illusion. In experiment 3 internal face features arranged in their canonical face layout, but not in a scrambled layout, were found to induce the illusion. However, the magnitude of the effect was significantly weaker than the effect obtained for empty face contour in experiment 2. Collectively, these results suggest that a fat face illusion is obtained when there is sufficient information in the stimulus to activate an internal face schema.