Some varieties of shadow illusions: split shadows, occluded shadows, stolen shadows, and shadows of shadows. Perception, 41, 357-360.

Shadows are visual objects and as such are subject to preference rules for segmenting the visual scene (such as Gestalt laws). These rules govern shadows along two dimensions: their general unity and individuation (e.g., they describe a certain area as a unitary shadow as opposed to a set of two distinct shadows) and their being the shadow of a given object, as opposed to being the shadow of any other object in the scene (the Shadow Correspondence Problem, Mamassian 2004). We describe a few phenomena that indicate the interplay of preferences in determining the final visual aspect of scenes in which shadows are present.