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An Empirical Explanation: Simultaneous Brightness Contrast

Insofar as a stimulus is consistent with the experience of the visual system with differently reflective objects in different levels of illumination, targets and their contexts will tend to appear differently light or bright.
It has long been apparent that the perceived brightness of objects does not correspond in any simple way to their luminance (i.e., to the measured intensity of light corrected for the spectral sensitivity of the human visual system) (Figure 1A). In particular, two surfaces returning the same amount of light to the eye can look differently bright if the surfaces are observed in different contexts, a phenomenon called simultaneous lightness or brightness contrast (most psychologists refer to 'lightness' contrast to distinguish the appearance of surfaces that reflect light from the appearance of endogenous sources of light; for present purposes, this distinction is not critical).
The explanation of this remarkable effect found in many textbooks is predicated on lateral interactions among retinal ganglion cells or other lower order visual neurons, which demonstrably cause distorted rates of neuronal firing at contrast boundaries, presumably to enhance the detection of edges (Figure 1B). This interpretation implies that the relative intensities perceived in response to such stimuli are, in effect, 'readouts' of the relative firing rate of neurons at the input stages of the visual system. On this basis, any target predominantly surrounded by an area of higher luminance should look darker than the same target predominantly surrounded by an area of lower luminance.
Figure 1

Figure 1 / Standard demonstration of simultaneous brightness contrast, and the conventional explanation of this effect. A) A target (the diamond) on a less luminant background (left) is perceived as being brighter than the same target on a more luminant background (right), even though the two targets are physical identical, and appear so if both are presented on the same background (as shown above). B) Diagram of the usual explanation of this phenomenon, based on the center-surround receptive field properties of retinal ganglion cells. the center-surround receptive field organization of input level neurons will, as illustrated here, cause less lateral inibition, and therefore more signal passed centrally from high contrast boundaries than from lower ones.

Despite the apparent concordance of perception and retinal physiology in this instance, a number of observations indicate that identical targets embedded in scenes that have exactly the same local contrast relationships with their surrounds can nonetheless look differently bright (Figure 2). Indeed it is even possible to construct stimuli in which a target in a predominantly higher luminance surround looks brighter than an identical target in a predominantly lower luminance surround (Figure 2B). How, then, can these seeming contradictions in the relationship of luminance and brightness be explained?
Figure 2

Figure 2 / Evidence that distorted neuronal responses to local contrast (Figure 1B) cannot explain simultaneous brightness contrast. A) In the Wertheimer-Benary stimulus, two equiluminant targets (the gray triangles) elicit different sensations of brightness despite having the same local contrast relationships (for most observers the upper triangle looks slightly brighter/lighter than the lower one). B) White's illusion is particularly interesting because it generates a perception of relative brightness that is similar to the sensations elicited in Figure 1A, despite the fact that the local contrast of the patches (set inset left) is more or less opposite the standard brightness contrast stimulus shown in Figure 1A. Thus, the targets that appear brighter (the patches on the left) are mainly surrounded by areas of higher luminance, whereas the targets that appear darker are surrounded mainly by areas of lower luminance. C) Differences in lightness/brightness of equiluminant targets in the absence of any differences at all in local luminance contrast. Top panel - Light and dark surrounds with equiluminant test diamonds on the adjacent faces of a cube. Middle panel - The same cube rotated 180°. Bottom panel - Graph showing the average adjustment made by observers to equalize the brightness of the two test targets in the upper and middle panels.

In terms of a wholly empirical strategy of vision, the explanation of the difference in perceived brightness of the two equiluminant targets in Figure 1A and in this Demonstration is summarized in Figure 3 (Williams et al, 1998a and b; see also Lotto and Purves, 1999). Since the amount of light returned to the eye from any portion of a scene depends on the illumination and reflectance of the relevant surfaces (among other factors), the equiluminant returns from the targets are inherently ambiguous. Such stimuli will often have been generated by similarly reflective surfaces on differently reflective surrounds under the same illuminant; the same luminance profiles, however, will often have signified differently reflective target surfaces under different amounts of illumination.
Figure 3

Figure 3 / A probabilistic explanation of simultaneous brightness contrast effects. A) A standard simultaneous brightness contrast stimulus. B and C) Cartoons illustrating the two major categorical sources of the stimulus in (A). The different lightness/brightness of the two identical targets in (A) is seen because the response to the stimulus incorporates all its possible sources in proportion to their past frequency of occurrence, which differs in natural scenes.

Since dealing successfully with this or any stimulus depends on responding appropriately to the sources of the retinal stimulus rather than the stimulus as such, the visual system can only solve this problem on the basis of past experience. If this idea is correct, then to the extent that the stimulus is consistent with similarly reflective target surfaces under the same illuminant, the targets will tend to appear similarly bright. However, in so far as the stimulus is consistent with the past experience of the visual system with differently reflective objects in different levels of illumination, the targets will tend to appear differently light or bright. Because the standard simultaneous brightness contrast stimulus is consistent with either of these possible sources, the pattern of neural activity elicited - that is, the percept experienced when looking at the stimulus in Figure 1A or Figure 3A (or the related demonstrations) - is a manifestation of both possibilities (and indeed all of the many other possibilities not illustrated) in proportion to their relative frequency of occurrence in past experience with stimuli of this general sort.
In support of this explanation, crafting the stimulus in this Demonstration to be more consistent with differently reflective surfaces in different illuminants increases the 'illusion' of simultaneous brightness contrast (see Demonstration, for example), whereas making the stimulus less consistent with this possibility, and more consistent with the source being similar reflective objects under similar illuminants causes the targets to appear more similar, even if all the luminance relationships in the scene are preserved. Other more complex examples that support this interpretation of how lightness/brightness percepts are generated are found in this Demonstration.


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