A general axiom in vision science is that our visual system attempts to recover the properties of objects in the visual environment, while ignoring the properties of the illumination of the scene, since the objects are likely to be important while the illumination is subject to large and capricious changes in quantity, spectral quality, and direction. Thus, research on lightness and color constancy shows evidence of processes, which “discount” the properties of the illuminant (Gilchrist,
2006). In a sense, shadows are a property of the illumination and are therefore possible candidates for such discounting. Two distinct types of shadows occur. Attached shadows are changes in the brightness of part of the surface of an object, which reflects that surface's orientation in relation to the prevailing illuminant. Cast shadows are changes in the illumination of a surface that results from occlusion of the illuminant by an intervening object. In natural images, both types of shadow cooccur and their presence is highly correlated. However, it is well known that both attached shadows (Adams,
2007; Chacón,
2004; Champion & Adams,
2007; Enns & Rensink,
1990; Gregory,
1966; Kleffner & Ramachandran,
1992; Ramachandran,
1988; Sun & Perona,
1996,
1997) and cast shadows (Kersten, Mamassian, & Knill,
1991) provide strong cues to the geometrical structure of the objects in the image—which suggests that shadows may not be universally ignored by vision because they can be a rich source of information about object properties. However, the idea that something about the encoding of shadows is subject to suppression processes remains attractive and has been the subject of experiments using a visual search paradigm. Gross manipulations of the directions of cast shadows are often not seen by observers (Jacobson & Werner,
2004). On the other hand, visual exploration of the possible interpretation of shadow figure/ground effects that results in multistable, ambiguous figures suggests that shadows provide a strong input to form perception and are certainly not universally discounted (Leonards & Troscianko,
2004).
Apparent evidence for the discounting of shadows comes from visual search experiments by Rensink and Cavanagh (
2004). They examined search efficiency when participants searched for a rotated shadow and found that search times were inefficient when images were upright but were efficient, and faster, when images were inverted. This suggests that shadows are discounted, but only when the shadow is consistent with the normal lighting direction. Because their stimuli were artificial 2D gray rectangles, they only had cast shadows (dark quadrilaterals) and the target shadow was rotated by only 30° (see
Figure 1). When stimuli were presented in the ‘upright’ condition the percept was of a plane viewed from above with a number of upright pillars casting shadows. When viewed in the ‘inverted’ condition the percept was of rectangular objects hanging from a ceiling plane. The inversion manipulation relies upon the ubiquitous assumption that light comes from above (Berbaum, Bever, & Chung,
1983; Gibson,
1950; Ramachandran,
1988) or at least within 30° of above (Mamassian & Goutcher,
2001) and consequently that shadows should be below objects. When observers view the search images in an upright orientation they were searching for an odd shadow among shadows, whereas when the images were inverted, the targets and distracters were not treated as shadows. The relatively slow search times and low efficiency for images presented ‘upright’ were interpreted as evidence that shadows were at least partially discounted, i.e., they were less directly accessible to perception than non-shadows—with similar visual characteristics, i.e., the inverted shadows. However, when the images were inverted, both the shadows and the implied ground plane were rotated, so the possibility remains that the effect of inversion was caused not by the processing of a shadow mechanism and a non-shadow mechanism, but instead by changing the ‘normal’ viewed-from-above scene into an unusual scene. Torralba, Oliva, Castelhano, and Henderson (
2006) suggest that, when scenes are upright, the ‘gist’ is grasped but access to individual scene objects and their shadows is weakened, thereby delaying search processes.
In the present study, we examine the question of shadow detection in more detail, using a greater variety of shadow manipulations and using photographs of natural objects with or without cast shadows rather than the schematic objects used previously. We employ stimuli that share characteristics of the original concave and convex “bumps” (Gregory,
1966; Ramachandran,
1988), i.e., using a fronto-parallel presentation. However the current stimuli feature natural objects with natural shadows that have both a cast and an attached component. By using natural objects (Cunningham, Beck, & Mingolla,
1996) and their shadows in a visual search paradigm, we can manipulate the heterogeneity of the shadow casters. Where heterogeneity is increased, search becomes less efficient and search slopes increase (Duncan & Humphreys,
1989). If the mechanism underlying shadow processing is coarsely scaled and presumes overhead lighting, then we should expect a smaller influence of heterogeneity upon search slope for upright images than for inverted images, despite the fact that the contents of the scenes are identical—except for the inversion. It has been argued that shadow processing operates at a coarse visual scale (Mamassian,
2004). Put simply, the purpose of our experiments was to be able to dissociate explanations of shadow perception based on “suppression” and “coarse processing,” while using natural stimuli and controlling both cast and attached shadows.