Perceived Contrast Explains Asymmetries in Visual-Search Tasks with Shaded Stimuli

Abstract

Shaded stimuli have traditionally been used in the context of three-dimensional (3-D) shape perception. Many studies have shown a persistent asymmetry in that a circle filled with a shaded gradient that is dark at the top and bright at the bottom (top-dark circle) is much easier to locate among top-bright circles than in the opposite arrangement (a top-bright circle among top-dark circles). The immediate 3-D interpretation of top-dark and top-bright circles as hollows and protuberances, respectively, and the asymmetry just described have been explained in terms of 3-D percepts. The work described here challenges this view: the results of the first experiment show that top-dark circles are perceived as having 10% higher contrast than top-bright circles of the same physical contrast. Experiment 2 replicates classical visual-search experiments but adding a new condition where target and distractors were subjectively equated in contrast. For five of six subjects, the ubiquitous asymmetry disappears in this condition.

Lighting direction and visual field modulate perceived intensity of illumination

“…Our result that bottom-lit objects appear more intensely illuminated than their top-lit counterparts may also be related to the finding that discs with top-dark luminance gradients, seen as concavities due to the light-from-above assumption, are perceived to possess higher contrast than top-bright stimuli, which are perceived as convexities (Chacon, 2004). Despite appearing concave, disks with top-dark luminance gradients are nonetheless consistent with being convexities lit from below.…”

“…Another potentially related phenomenon is the finding that discs possessing top-dark luminance gradients, which are seen as concavities due to the light-from-above assumption, appear to possess higher contrast than identical top-bright discs which are seen as convexities (Chacon, 2004). Although the visual system exhibits a bias to assume convexity (Sun and Perona, 1997; Langer and Bulthoff, 2001; Champion and Adams, 2007), the light-from-above bias outweighs the convexity bias for simple disc stimuli.…”

Search for gross illumination discrepancies in images of natural objects

“…One potential criticism of Experiments 1 and 2 is that it is difficult to distinguish the results reported for cast shadows from those reported for stimuli featuring simple concave and convex bump and dimple stimuli (i.e., Adams, 2007;Chacón, 2004;Champion & Adams, 2007;Enns & Rensink, 1990;Kleffner & Ramachandran, 1992;Sun & Perona, 1996, 1997. This is an inevitable consequence of our desire to examine search for naturalistic objects and their shadows.…”

“…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 imageVwhich 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.…”

Reminder and 2AFC tasks provide similar estimates of the difference limen: A reanalysis of data from Lapid, Ulrich, and Rammsayer (2008) and a discussion of Ulrich and Vorberg (2009)

“…appear different because of the context in which they are embedded. Research on visual illusions (see, e.g., Armstrong & Marks, 1997;Chacón, 2004;Foley & McCourt, 1985;Jordan & Schiano, 1986) has often used discrimination tasks to provide objective data (i.e., beyond the initial consensus of introspections), indicating that the illusion actually occurs when the PSE is located at the point at which standard and comparison stimuli have the same magnitude along the dimension in which they are compared. Similar evidence that the PSE and the POE do not generally coincide comes also from other common studies involving nonillusory standard and comparison stimuli that differ along noncompared dimensions, like windowed sinusoidal gratings of different orientation, mean luminance, direction of motion, spatial frequency, size, or spatial phase whose contrast is to be compared (see, e.g., Brady & Field, 1995; García-Pérez know whether the functions S and T are identical even when test and standard stimuli are nominally identical in all respects (see our earlier discussion of temporary sensory impairment).…”