09 Dec2020

A geometrical account to explain the fat face illusion

A geometrical account to explain the fat face illusion

Amit Rawal & Philip Tseng

In recent years, there have been multiple reports about a type of face illusions where among two vertically placed identical faces, yet the lower face looks larger (or fatter). This has been known as the “fat-face illusion” (Ming Zhong, Yu, Wang, & Sun, 2012; Sun et al., 2012, 2013). The conclusions about this type of illusion has been that this illusion requires the activation of face schema (deciding whether the object being looked at is a face or not ; Sun et al., 2013), to lead to a successful illusion, and that this is a face-specific illusion (not present for other objects; Sun et al., 2012). Is this really a face-specific illusion? We think this is a generalizable geometrical illusion that can occur in any non-face shapes, but previous studies just happen to have used exclusively faces of canonical geometry (wider upper-head and narrower/pointier lower-head). To demonstrate this, we proposed the AHA (axis of horizontal asymmetry) account and showed that the direction of this illusion may be reversed when comparing faces of opposite to canonical geometry (wide chin and pointier upper-head). Also, we show that non-face objects (clocks/building possessing horizontally asymmetric geometry) may also lead to this illusion. In addition, we show how upright or inverted faces or objects (clocks/building), left- or right-rotated faces, upright or inverted face contour/texture may also result in an illusion, with directions predicted by our AHA account. In conclusion, we propose that this illusion named ‘fat face illusion’ has neither to do with fatness of the object (given the illusory clocks and building) and may just be about largeness, nor anything in particular to do with faces (given the same illusory clocks and building), and that it may simply be a geometry-dependent illusion where we (erroneously) overestimate the size of one of the two adjacent identical objects, due to (perhaps) a comparison of their local features (such as a pointy chin vs. a wide forehead, leading us to perceive the one with the compared wide forehead as larger). 

Figure: This is the same picture pasted twice, but one appears slightly fatter than the other, can you guess which one?  While you are at it, flip the image upside-down to see if the illusion reverses.

If interested, also check out (just for fun) the “Thatcher Illusion” (Thompson, 2009) and “Fat face thin illusion” (Thompson, 2010) .

Published in i-Perception

Here is the link: in press