Why are faces hard to recognize in photographic negative?

Faces may be difficult to recognize in photographic negative simply because they contain a large range of grays, while printed words and geometric shapes, which contain no grays, are easy to recognize in negative. This explanation was partly tested in an experiment where Ss had to recognize positive and negative pictures of well-known people, both using normal monochrome photographs and using lith photographs in which all areas of gray were removed. Lith photographs were harder to recognize than normal photographs, but the difference between positive and negative was the same for lith pictures as for normal ones. This does not rule out an explanation in terms of grays, but it does put a major constraint on it.