Perception and recognition of normal and negative faces: the role of shape from shading and pigmentation cues

A face is surprisingly difficult to recognise when presented in photographic negative, and negation has also been shown to affect simple perceptual judgments about a face. Two possible explanations for this effect are examined. In the shape-from-shading explanation it is argued that negating an image results in an impossible pattern of shading, and that this disrupts the formation of a three-dimensional representation of the surface geometry of the face. In an alternative account for this effect it is suggested that identification errors occur as a consequence of changes to the apparent pigmentation of the face caused by negating the image. Three experiments are reported which are designed to test these explanations by using novel colour-image transformations in which the hue and luminance components of images are independently manipulated. The results of these studies suggest that although changes to the apparent pigmentation of a face might result in identification errors in some situations, the loss of shape-from-shading cues is a more important cause of the negation effect. The role of these two sources of information in the recognition of normal faces is also discussed.