Lock at the cross in the center between the flashing faces, and with your “inner eye” compare the features of the faces over time. Do they, occasionally, look grotesquely distorted? Looking directly at the faces reveals that none is distorted.
First: sometimes, you may not believe that face was not really distorted. If so, stop the run by hitting the space bar, and then you can go backwards with the index counter or, simpler, by using the ← → cursor keys. [It can be very effective to rapidly move to-and-fro with the cursor keys.]
There are a number of parameters which you can explore for their effect on the effect: distance with the horizontal slider, speed of presentation, and the orientation of the faces with the rotary slider (left of distance).
[There are altogether 40 face images, which appear in random sequences; after all were shown, the sequences are randomized anew. They were extracted (with minor variation) from the original movie by Tangen et al.]
This phenomenon was discovered and published in 2011 by Jason Tangen, Sean Murphy, and Matthew Thompson, and earned second place in the illusion contest 2012. The authors modestly mention that it was a chance discovery – but then one must notice this chance and follow through. Initially, it was thought that upside-down faces have less distortion, but Balas & Pearson (2019) quantified (among other things) that inverted faces have a very similar effect.
That is not really clear at this time. It is highly likely that high-level adaptation (e.g. Webster et al., 2004) plays a role. Tangen et al. observe, “The distortion comes from the many differences between each face and the one that follows. A particularly tanned face, for example, will make the next face seem pale, and squinty eyes will make normal eyes bulge.” The current state can be summarized like this: “The Flashed Face Distortion effect is a non-face-specific visual illusion that depends primarily on the increased perceived blur associated with peripheral vision, and also requires sufficient presentation time per image for some amount of rapid mid-level adaptation to face appearance to take place.” Gao et al. conclude from their findings that the site of the mechanism is cortical.
Perhaps you would like to read more at Wikipedia: Flashed face distortion effect on it.
Tangen JM, Murphy SC, Thompson MB (2011) Flashed Face Distortion Effect: Grotesque Faces from Relative Spaces. Perception 40:628–630
Balas B, Pearson H (2019) The Flashed Face Distortion Effect Does Not Depend on Face-Specific Mechanisms. Sci Rep 9:1612
Webster MA, Kaping D, Mizokami Y, Duhamel P (2004) Adaptation to natural facial categories
Gao Y, Miller KN, Webster MA, et al (2024) Time course and neural locus of the Flashed Face Distortion Effect. Vision Research 224:108492
Wikipedia: “Flashed face distortion effect”
