Chapter 5: perceiving colour II Flashcards
Opponent-Process Theory
- red-green, blue-yellow, and black-white were organised in some opponent fashion so that the activation of one would supress the other
- On this basis it was possible to explain many colour phenomena
- Card sorting
- Colour afterimages
- Descriptions of colour mixtures
Physiology of Opponency
single-cell recordings found opponent neurons (1950s)
Located in the retina, LGN, V1
Respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other
Opponent Neurons
Responses of single-opponent neurons carry information about light wavelength within uniformly colored regions of the visual scene but do not provide much information about where adjacent regions are illuminated by different wavelengths.
Double-opponent neurons provide information about the location of edges between regions that differ in color, and complement neurons that provide information about edges between regions that differ in brightness.
Colour Afterimages
Single opponent cell normally responds with
excitation to long wavelength (“red)
inhibition to medium wavelength (“green”)
at baseline to equal amounts of red & green light (“yellow” or “white”).
Therefore,
During adaptation to a red patch, the cell (and/or the cones that send excitatory input to that cell) responds strongly and gets fatigued
After adaptation, the cell is fatigued and not responding as strongly as it normally would (i.e. responding with inhibition when white background presented)
Therefore, we experience a green afterimage in the area of the retina where the adaptation occurred.
Colour Constancy
Objects look the same colour in a broad range of illuminants - apple is red in store and at home
However, recall
light at eye = reflectance x illuminant
so how is it, that the colour of an object is constant in different illuminants?
3 Possible Causes of Color Constancy
-Chromatic adaptation
-the constant presence of the illuminant fatigues those cells most responsive and hence, they become less responsive - changes the pattern of activity
-Effect of surroundings
-Color constancy works best when an object is surrounded by many colors
-Memory and color
-Past knowledge of an object’s color can have an impact on color perception
Memory for color is not exact, so we don’t notice slight changes caused by illumination changes
Lightness Constancy
Lightness is a perceptual experience NOT a physical quality of an object Reflectance (albedo) is a physical surface quality proportion of light that is reflected independent of the source of light
Lightness Constancy Wallach Experiment
Adjust the center light in the second pair so center seems as light as in first pair
-Conclusion:
What is extracted from the scene is information about relative intensities rather than absolute intensities – ratio principle
If the ratio of light coming from two areas remains constant, then ganglion cells will give same neural response = same perception/lightness
Anchors
In any given scene, the region that reflects the most light is perceived as white (or as the lightest shade of gray in the scene), and the lightness of every other region is perceived in relation to that anchor point.
If the scene consists of regions under different amounts of illumination, the visual system applies the anchoring rule separately in each illumination zone.
Rod Monochromats
No colour vision
Poor acuity
Photophobic
Cone monochromats
No colour vision
Otherwise normal vision
Monochromacy
Very rare; often associated with other problems
May be considered as “true” colour blindness
Can match all wavelengths by adjusting intensity of any other single wavelength
two forms
Dichromacy
Can match all wavelengths by adjusting intensity of two other wavelengths Comes in three forms: -Protanopia -Deuteranopia -Tritanopia
Protanopia & Deuteranopia
(more common)They are missing the long and medium-wavelength pigment, respectively
Individuals see short-wavelengths as blue
Above neutral point, they see yellow
Occurs in more often in males than females (X-linked)
Tritanopia
They are missing the short-wavelength pigment
Individuals see short wavelengths as blue
(rare) Above neutral point, they see red
Equally common in males and females
