Photographic negation is thought to interfere more with the perception of faces than other classes of object (Galper, 1970). Two accounts have been offered for this effect - negation may disrupt observers' ability to use shading cues to infer facial structure. Alternatively, representation of surface reflectance cues, such as patterns of pigmentation and colouration, may be disrupted.
While the top right and bottom right images are in fact identical, the image manipulation (inversion of the mouth and eyes) is more salient when the face context is upright. This is an example of the so-called 'Thatcher illusion' having first been demonstrated with an image of Margret Thatcher (Thompson, 1980). This is frequently cited as evidence for the holistic representation of upright faces, the logic being that quick detection of feature-context disparities can only be achieved if a configural process is recruited.
Participants are faster at making imitative responses to action onset than non-imitative respones - an effect known as automatic imitation. For example, when asked to close their hand whenever the above stimulus moves, reaction times are faster when the hand closes than when it opens. However, when required to make hand-open responses, most people respond faster to the sight of hand opening.