The anchoring effect in lightness perception in humans

1. The anchoring effect in lightness perception in humans Alexander D. Logvinenko School of Psychology, Queen’s University of Belfast Neuroscience Letters 334(2002)

2. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

3. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

4. Introduction • Simultaneous lightness contrast is a classical visual illusion, which has been the focus of research for several generations of visual scientists. • Simultaneous lightness contrast is not specifically a lightness illusion, yet it is a particular case of a more general phenomenon known as the ‘anchoring effect’.

5. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

6. Lightness and brightness • Ref. form Alan L. Gilchrist, Current Biology, 2007 Apr 17；Volume 17, Issue 8, Pages R267-R269 • Lightness：the perceptual dimension that runs from black, through gray, to white. • Reflection of the surface： White－－about 90% of the light Black－－about 3% of the light • Lightness is perceived reflectance.

7. Lightness and brightness • Low-level theories stemming largely from Ewald Hering：known, relatively simple, peripheral neural mechanisms, such as lateral inhibition and spatial filtering. • These theories, of which there are many, are very concrete and consistent with the received wisdom of sensory physiology. • Failure：to predict what humans actually perceive when viewing typical scenes from our everyday world • Scenes that contain multiple depth planes or projected illumination boundaries（such as the edge of a shadow）

8. Lightness and brightness • High-level theories：the perceptual process as a thought-like process • Helmholtz through an unconscious process of inference let us taking into account the level of illumination in different parts of the scene. • The computerage spawned more modern high- level theories which argue that reflectance can be recovered by decomposing the retinal image into those physical properties, like reflectance and illumination, that became entangled in the formation of the image.

9. Lightness and brightness • Not found support in the empirical data. • High- level theories resonate with our visual experience of surfaces, especially in complex scenes, but are criticized as abstract and physiologically implausible.

10. Lightness and brightness • Mid-level theories are more sophisticated than the low-level theories, yet more concrete than the high- level theories. • using simple properties of the image, like fuzzy boundaries or depth edges, to parse the image into separate frameworks of illumination and then apply a simple computation, using a different standard of white within each framework.

11. Lightness and brightness • Brightness is the perceptual dimension that runs from dim to bright. • Luminance－－the absolute intensity of light reflected in the direction of the observer’s eye by a surface • Brightness is perceived luminance.

12. Lightness and brightness • The reflectance of an object is a relatively permanent property, whereas its luminance is transient. • Lightness concerns the objective side of visual experience while brightness concerns the subjective side. • When we track a golf ball in flight we attend to the path of the ball, ignoring that the image of the ball remains stationary in our field of view.

13. Lightness and brightness • The special visual attitude, typically adopted by an artist painting a scene, is known as the proximal mode of perception. • Brightness perception then, falls within this mode. • Brightness is our experience of the raw energy emanating from a surface, and the impact of that raw energy on our sensorysystem.

14. Lightness and brightness • That energy includes the combined effects of the surface reflectance and the intensity with which it is lit. • A white surface appears brighter than a gray surface in the same illumination, but a sunlit white surface also appears brighter than a shadowed white surface.

15. Lightness and brightness • All the properties of the world that we perceive through vision — size, motion and reflectance — come to the eye via light. • Prior to the computer revolution the distinction between energy and information was less clear, and the term brightness was used to refer to both lightness and brightness, without distinction.

16. Lightness and brightness • There are more theories of brightness than of lightness. • Brightness theorists make the tacit assumption that lightness is based on brightness.

17. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

18. Munsell color system • Professor Munsell first started work on the system in 1898 and published it in full form in Color Notation in 1905. • Some deficiencies as a physical representation of the theoretical system were improved significantly in the 1929 Munsell Book of Color and through an extensive series of experiments carried out by the Optical Society of America in the 1940s resulting in the notations (sample definitions) for the modern Munsell Book of Color. • Because of this basis in human visual perception, Munsell’s system has outlasted its contemporary color models, and though it has been superseded by models such as CIE L*a*b* and CIECAM02, it is still in wide use today.

19. Munsell color system • Hue：Red, Yellow, Green, Blue, and Purple, along with 5 intermediate hues halfway between adjacent principal hues. Each of these 10 steps is then broken into 10 sub-steps, so that 100 hues are given integer values.

20. Munsell color system • Value（lightness ）：from black (value 0) at the bottom, to white (value 10) at the top N0（黑色）、N1、N2...N10（白色），N是Neutral的縮寫，是灰色的意思。 • Chroma：measured radially from the center of each slice, represents the “purity” of a color, with lower chroma being less pure. 從0（無彩色）開始，也因各純色而長短不同，例如5R純紅有14階段，而5BG只有6階段，其表色樹狀體也因而呈不規則狀，其表示法為 /1、/2、、等。 標色法：色相 明度／彩度= Hue Value/Chroma = H V/C

21. Munsell color system • 色相環 10YR色相面之彩度及明度色樣

22. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

23. Anchoring effect • Munsell 31-point neutral scale • The difference (0.5 Munsell units) was statistically significant (Wilcoxon signed-rank test, P , 0:001). • Wilcoxon signed-rank test：統計學的一種檢測法

24. Anchoring effect • an even larger difference (0.75 Munsell unit) between the perceived lightness of the diamonds and squares lying on the main diagonal

25. Anchoring effect

26. Anchoring effect • The illusion is even stronger (1.25 Munsell units) than in the previous figures.

27. Anchoring effect • While we cannot point out any sensory factors, which could possibly make the diamond’s lightness change in the presence of another grey patch, there is a remarkable resemblance between this lightness shift and another famous visual illusion.

28. Anchoring effect • The presence of a particular stimulus arbitrarily selected as an ‘anchor’ affects the shape of the psychophysical scale, namely, it becomes steeper in the neighbourhood of the anchor. In lightness perception, the anchoring effect is produced by the background on which the patches to be scaled are presented.

29. Anchoring effect • First, the anchoring effect in Figs. 2–4 is caused by relatively small anchors (squares off the main diagonal in Figs. 2 and 3; and hoops in Fig. 4) rather than the background. • Second, there is no common border – thus no local luminance contrast – between the anchors and the targets. • In this theory anchoring is a hypothetical process converting luminance into lightness.

30. Anchoring effect • The anchoring effect observed in Figs. 2–4 is an effect (not a process), which is experienced as a lightness shift induced by introducing a patch called an anchor. • When presented alone the diamond’s median lightness was found to be 5.75 Munsell units. Introducing the lighter anchor (Fig. 2a) shifts the diamond lightness towards the darker end of the lightness scale by one small Munsell step (0.25). At the same time, the darker anchor (Fig. 2b) shifts the lightness of the same diamond towards the lighter end of the lightness scale by two small Munsell steps (0.50).

31. Outline • Introduction • Lightness and brightness • Munsell color system • Anchoring effect • Conclusion

32. Conclusion • The anchoring effect observed in forward figures is an error of judgement that probably takes place at the decision-making rather than the sensory level. • A satisfactory explanation of this effect is to be sought within the broader context of psychophysical scaling rather than lightness perception per se.