Abstract
Research in lightness perception centers on understanding the prior assumptions and processing strategies the visual system uses to parse the retinal intensity distribution (the proximal stimulus) into the surface reflectance and illumination components of the scene (the distal stimulus—ground truth). It is agreed that the visual system must compare different regions of the visual image to solve this inverse problem; however, the nature of the comparisons and the mechanisms underlying them are topics of intense debate. Perceptual illusions are of value because they reveal important information about these visual processing mechanisms. We propose a framework for lightness research that resolves confusions and paradoxes in the literature, and provides insight into the mechanisms the visual system employs to tackle the inverse problem. The main idea is that much of the debate and confusion in the literature stems from the fact that lightness, defined as apparent reflectance, is underspecified and refers to three different types of judgments that are not comparable. Under stimulus conditions containing a visible illumination component, such as a shadow boundary, observers can distinguish and match three independent dimensions of achromatic experience: apparent intensity (brightness), apparent local intensity ratio (brightness-contrast), and apparent reflectance (lightness). In the absence of a visible illumination boundary, however, achromatic vision reduces to two dimensions and, depending on stimulus conditions and observer instructions, judgments of lightness are identical to judgments of brightness or brightness-contrast. Furthermore, because lightness judgments are based on different information under different conditions, they can differ greatly in their degree of difficulty and in their accuracy. This may, in part, explain the large variability in lightness constancy across studies.
Highlights
A central question in the study of human visual perception is how and under what circumstances the visual system is able to separate the physically invariant reflectance of a surface R(x, y) from its potentially changing illumination I(x, y)
We propose a framework for lightness research that resolves confusions and paradoxes in the literature, and provides insight into the mechanisms the visual system employs to tackle the inverse problem
The first is the idea that much of the debate and confusion in the brightness/lightness literature stems from the fact that the term lightness, defined as apparent reflectance, is underspecified with regard to illumination and due to the inverse problem, is often inadvertently used to refer to three very different and independent types of judgments that are not comparable (Blakeslee et al, 2008; Blakeslee and McCourt, 2012)
Summary
A central question in the study of human visual perception is how and under what circumstances the visual system is able to separate the physically invariant reflectance of a surface R(x, y) from its potentially changing illumination I(x, y). The first is the idea that much of the debate and confusion in the brightness/lightness literature stems from the fact that the term lightness, defined as apparent reflectance, is underspecified with regard to illumination and due to the inverse problem, is often inadvertently used to refer to three very different and independent types of judgments that are not comparable (Blakeslee et al, 2008; Blakeslee and McCourt, 2012) Experimental support for this idea is provided by the data of Arend and Spehar (1993a,b); and Blakeslee et al (2008). This idea has the potential to explain the large variability in the accuracy of lightness judgments (i.e., the degree of “lightness constancy”) observed across studies (Gilchrist, 2006; Kingdom, 2011, 2013)
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