This animation draws on Richard Gregory’s hollow mask experiment. Gregory rotated a mask of Charlie Chaplin so that its hollow back faced the viewer, and the brain refused to see it as hollow. A lifetime of looking at faces that curve outward overrules the evidence, and the face flips forward again.

Here the mask is a chimpanzee’s, rebuilt from photographs of his death mask. As it turns, the same error occurs. What feels like observation is partly expectation.