Both the top and bottom pairs are the same photos, but they look very different depending on whether they’re upright or inverted. In the top pair (the inverted ones), the face on the left is normal, and the face on the right has inverted eyes and mouth. Since they’re upside down, though, both look pretty normal. When you flip ‘em over, though, the face with the inverted eyes and mouth looks, well, gross.
Aside from being just plain cool, this illusion actually has some important theoretical implications. By the time this illusion was first published by Peter Thompson in 1980(2), researchers were well aware that people had trouble processing upside-down faces, but they weren’t sure why. By understanding the Margaret Thatcher illusion, they were able to figure out key pieces of the inverted-face puzzle, and thus the face-processing puzzle in general.
Basically, what the Margaret Thatcher illusion taught us was that we process faces using both local features (e.g., the nose or the eyes) and their configurations. When faces are inverted, the configural information is altered, so we just look at the local features. Because the inverted features don’t look all that different on the upside-down faces, we don’t really notice a problem. But when the faces are right-side-up, we can process the configural information, and so the inverted mouth and eyes look weird.
Another way to make face processing more difficult is to present the negatives of faces, as in this oh, let’s say randomly chosen example (from Antis, 2005, figure 1a 3):
You all probably recognize this face, because the guy’s name is in the post’s title, but it’s more difficult to recognize this than a normal face. Why? Probably because the negative makes it more difficult to process the local features. The shape’s are distorted, so things just don’t look quite right, even though the configural information is basically the same.
If that’s the case, then perhaps negative faces provide a nice analog to inverted faces. And if that’s the case, then maybe you can get an illusion analogous to the Thatcher illusion with negatives. And that’s just what Stuart Antis did to celebrate the 25th anniversary of the publication of the Thatcher illusion. Here’s what he got:
Again, these two pairs of faces are the same, except the top two are negatives. The one on the top-left (a) is the pure negative, which Antis describes as being “analogous to an upside-down face.” The one on the top-right (b) is negative except for the eyes and teeth, which are positive. This is analogous to the “Thatcherized” face (the one with the inverted mouth and eyes). The bottom two faces were created by reversing the contrast of the top two faces. Thus the bottom-left face (c) is normal, and the bottom-right face (d) is normal except for negative teeth and yes. Now the contrast between the positive face with two negative features makes for a hideous, zombie-like ex-PM (I keep waiting for lightning bolts to come out of his eyes), not unlike the upright Thatcherized face in its grotesqueness. And that’s the Tony Blair illusion.
I’m not exactly sure what causes this, though I suspect it has something to do with our old friends configural and local information. Perhaps exploring the Tony Blair illusion will provide insight into basic face processing, just as the Margaret Thatcher illusion did.
1Schwaninger, A., Carbon, C.C., & Leder, H. (2003). Expert face processing: Specialization and constraints. In G. Schwarzer & H. Leder (Eds.), Development of Face Processing, pp. 81-97. Göttingen: Hogrefe.
2Thompson, P. (1980). Margaret Thatcher: a new illusion. Perception, 9,483-484.
3Anstis, S.M. (2005). Homage to Pete Thompson: The Tony Blair illusion. Perception, 34, 1417.