What if you duplicate some of the features of a portrait without overlapping them completely? It is relatively easy to create images in Photoshop in which the eyes and the mouth, but no other facial features, have been doubled. The results are little short of mind-bending: as the brain struggles (and fails) to fuse the doubled-up features, the photograph appears unstable and wobbly, and observers experience something akin to double vision.
The neural mechanisms for this illusion may lie within our visual system's specialized circuits for face perception. If you double up the eyes and mouths in a portrait, the neurons in the face-recognition areas of the brain may not be able to process this visual information correctly. Such failure could make the faces unsteady and difficult to perceive.