The anchoring effect in lightness perception in humans

1
The anchoring effect in lightness perception in
humans

• Alexander D. Logvinenko
• School of Psychology, Queens 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 17Volume 17, Issue 8, Pages R267-R269
• Lightnessthe 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
  Heringknown, 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.
• Failureto 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 theoriesthe 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 observers 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,
  Munsells system has outlasted its contemporary
  color models, and though it has been superseded
  by models such as CIE Lab and CIECAM02, it is
  still in wide use today.

19
Munsell color system

• HueRed, 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(??)?N1?N2...N10(??),N?Neutral???,???????
• Chromameasured radially from the center of each
  slice, represents the purity of a color, with
  lower chroma being less pure.
• ?0(???)??,??????????,??5R???14??,?5BG??6??,???
  ???????????,????? /1?/2????
• ????? ??/?? Hue Value/Chroma H V/C

21
Munsell color system

• ??? 10YR???????????

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 , 0001).
• Wilcoxon signed-rank test?????????

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 diamonds 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. 24 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. 24 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 diamonds 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.