Anchording Theory of Lightness Perception

Abstract

Anchoring theory of lightness perception is widely accepted to help researchers understand the light perception ability of human. By applying this theory, researchers are able to explore the relationship between luminance, reflectance, and illumination. It can also help people understand the reason behind some popular visual illusion. This essay will review the theoretical base, Gelb effect, behind anchoring theory. Meanwhile, this essay will explain how this theory can apply to solve the central problem of human light perception to both simple surface and frameworks. Also, this essay will discuss the strengths and weakness of this theory. Finally, this essay will also explore the future possibility of this theory. (108 words)

Introduction

The world we perceived might not be same as the physical world. Why? This is because the value of luminance did not equal to the value of reflectance, but largely affected by the value of illumination (Gilchrist et al.,1999). According to Adelson (2000), luminance represents the amount of light we could perceive; Illumination means the amount of light hit the surface of the object; Finally, reflectance means the amount of light actually reflected from an object surface. We could only compare our perception to Munsell scale and get a rough result. This is known as the key problem in human light perception since we rarely know the value of illumination; we could not calculate the value of reflectance of an object surface. To solve this problem, researcher’s raised different theories in order to figure out the relationship between luminance, reflectance, and illumination. Among all these theories, the anchoring theory of lightness perception (ATLP) proposed by Gilchrist et al. (1999) is widely accepted. This essay will explain the principle behind ATLP and discussed the strength and weakness of this theory.

Simple Surface

Experiments indicated that human brain will spontaneously perceive the object with the highest luminance as white. Study held by Gelb (1929) asked participants to view a black paper while the light hit it. The result showed that people will consider this paper as white if there is no white object beside it. This is called Gelb effect. Later researchers provided evidence to support this finding. Results of three experiments (Li & Gilchrist, 1999; Gilchrist & Cataliotti, 1994; Cataliotti & Gilchrist, 1995) were showed in Figure 1. [pic 1]

Figure 1. ‘Reproduced from “An Anchoring Theory of Lightness Perception,” by Gilchrist et al, 1999, Psychological review 106(4), p. 801. Copyright 1999 by the American Psychological Association, lnc. Reproduced with permission

In both three studies, despite the actual reflectance, participants perceived the object as white since it is the highest luminance object they can observe. After human believed the highest luminance surface is white, they would compare other surfaces to the original one. Gilchrist et al (1999) raised a formula according to this perception feature

PR= L1/Lh × 90%

PR represent the perceived reflectance, L1 as the luminance of the object we want to test, and Lh is the surface with highest luminance (Gilchrist et al.,1999). By using this formula, we could calculate the luminance of any simple surface under the same illumination with the highest luminance surface. However, the real-life question is more complicated than that.

Framework

Framework is the collection of several surfaces (Gilchrist et al.,1999). According to the content, a framework can be divided into two kinds. The local framework only including the surface around target surface, yet global framework including every surfaces that can be perceived at the same time (Adelson, 2000).

Perceived reflectance would be affected by both local framework and global framework so the formula needs to be adjusted when calculated (Gilchrist et al.,1999). According to Gilchrist et al (1999), both the size and the number of different pieces with the framework will influence the strength or framework. The final formula is

PR = WI(L,ILhl X 90%) + (W - I)(L,ILhg X 90%)

In this formula, WI and W – I represent weight of local framework and weight of global framework (Gilchrist et al.,1999).

A framework can be influenced by several factors. Gilchrist (1977) found that by changing the depth and orientation of the target, the framework it belonged will actually change and participants will compare target with a different reference. Also, human have the ability to identify the shadow, even it did not have clear distinct. Adelson (2000) found that human could spontaneous recognized the exists of shadow and treated it as a different framework. Also, this is the reason human get illusion sometimes. Finally, the segments that target grouped with would affect perception since human spontaneously compared target with background components.

Apply ATLP

By using ATLP, some question can be further explained. Remembering the Gelb effect, which indicated that people will perceive a black paper as white because it has highest luminance. When putting a white or lighter paper beside black paper, the illusion would disappear because the black paper did not have the highest luminance now. However, is this results from anchoring effect or other theory could explain the result? Some argued that the result was caused by contrast effect, yet Cataliotti and Gilchrist (1995) found evidence to support the idea that anchoring effect was the reason behind the Gelb effect. Based on contrast theory, the distance between black paper and white paper will have influence yet anchoring theory deny it because all the paper already formed a framework. Cataliotti and Gilchrist (1995) found that, when gradually adding papers between black paper and white paper, the influence of distance did not affect the power of highest luminance surface. The result was showed in Figure 2.[pic 2]

Figure 2. ‘Reproduced from “An Anchoring Theory of Lightness Perception,” by Gilchrist et al, 1999, Psychological review 106(4), p. 824. Copyright 1999 by the American Psychological Association, lnc. Reproduced with permission

Cataliotti and Gilchrist (1995) suggested that if the Gelb effect can be explained by contrast effect, the graph would be a slope, not a straight line. They also mixed the squares so the white paper appeared beside black paper. Again the result was not affected.

Limitation

No theory can be perfect. Just like the strength, there are also some questions cannot be explained by ATLP. According to ATLP, when a low luminance surface grouping with a high luminance surface, it will be perceived darker than the same one grouping with low luminance surface. However, Howe (2001) raised an evidence which rejected this theory in some ways.[pic 3]