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Chapter 5

PSYB57H3 Chapter Notes - Chapter 5: Retinal Ganglion Cell, Lateral Geniculate Nucleus, Trichromacy


Department
Psychology
Course Code
PSYB57H3
Professor
George Cree
Chapter
5

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Chapter 5: The Perception of Color
Basic Principles of Color Perception
Three Steps to Color Perception
Several problems must be solved in order to go from physical wavelengths to
the perception of color. We will organize our discussion around three steps:
1. Detection: Wavelength must be detected
2. Discrimination: we must be able to tell the difference between one
wavelength (or mixture of wavelengths) and another.
3. Appearance: We want to assign perceived color to lights and surfaces in
the world. Moreover, we want those perceived colour to go with the
object (blood is red) and not to change dramatically as the viewing
condition change (blood should remain red in a sun and shadow, for
example)
Step 1: Colour Detection:
The cones that have a peak at about 420nm are known as short-wavelength
cones (S-cones for short). The middle wave-length cones (M-cones) peak at
about 535nm, and long wavelength cones (L-cones) peak at about 565nm.
Remember also that cones work at daylight (photopic) light levels. We have
one type of rod photoreceptor; it works in dimmer (scotopic) light and has a
somewhat different sensitivity profile, peaking at about 500nm
S-cone: A cone that is preferentially sensitive to short wavelengths; known as
a blue cones
M-cone: A cone that is preferentially sensitive to middle wavelength known
as a green cone
L-cone: A cone that is preferentially sensitive to long wavelengths, known as
a red cone
Photopic: Referring to light intensities that are bright enough to stimulate the
cone receptors and bright enough to saturate the rod receptors.
Scotopic: Referring to light intensities that are bright enough to stimulate the
rob receptors but too dim to stimulate the cone receptors.
Step 2: Colour Discrimination:
We know that different wavelengths of light give rise to different experiences
of colour, and the varying responses of this photoreceptor to different
wavelengths could provide a basis or colour vision. But there is a problem.
Suppose we change the wavelength to 450nm. An equal amount of 450nm
light will produce the same response from this photoreceptor that 625nm
light does. If we were looking at the output of the photoreceptor, we would
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have no way of distinguishing between the two lights. But when we look with
a normal human colour vision system, the 625nm light looks orange and the
450nm looks violet.
Problem of universe: The fact that an infinite set of different wavelength-
intensity combinations can elicit exactly the same response from a single
type of photoreceptor. One photoreceptor type cannot make colour
discriminations based on wavelength.
The Trichromatic Solution
We can detect differences between wavelengths or mixtures of wavelengths
precisely because we have more than one kind of cone photoreceptor.
With our three cone types, we can tell the difference between lights of
different wavelengths.
Any wavelength from about 420 to 660 nm produces a unique set of three
responses from the three cone types. This combined signal, a triplet of
numbers for each pixel in the visual field, can be used as the basis for
colour vision. (we can see from about 400- 700nm, but the very long and
very short wavelengths stimulate only one types of cone.
Trichromatic theory of colour vision (or trichromacy): The theory that the
colour of any light is defined in our visual system by the relationship of three
numbers the outputs of three receptor types now known to be the three
cones. Also known as the Young-Helmholtz theory.
Metamers:
Almost every light and every surface that we see is emitting or reflecting a
wide range of wavelengths.
The key point is that the rest of the nervous system knows only what the
cones tell it. )t the mixture of red plus green lights produces the same
cone output as the single wavelength must look identical. Mixtures of
different wavelengths that look identical are called Metamers.
1. Mixing wavelengths does not change the physical wavelengths. If we mix
500- and 600-nm lights, the physical stimulus contains 500 and 600 nm.
It does not contain the average (550 nm). It does not contain the sum
(1100 nm (which we would not be able to see anyway). Colour mixture is
a mental event, not a change in the physics of light.
2. For the mixture of a red light and a green light to look perfectly
yellow, we would have to have just the right red and just the right green.
Other mixes might look a bit reddish or a bit greenish.
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The History of Trichromatic Theory
Such research has cemented our understanding of the physical basis of
trichromacy, but the basic theory was established by psychophysical
experimentation, and theorizing started with )saac Newton’s great discovery
that a prism would break up sunlight into the spectrum of hues, and a second
prism would put the spectrum back together into white. In 1666, Newton
understood that the rays to speak properly are not coloured. )n them is
nothing else than a certain Power and Disposition to stir up a Sensation of
this or that Colour. Newtown knew that colour is a mental event.
The three-dimensional nature of the experience of colour was worked out in
the nineteenth century by Thomas Young, and subsequently by Hermann von
(elmholtz. )n their honour, trichromatic theory is often called the young-
Helmholtz theory.
The central observation from these experiments was that only three mixing
lights are needed to match any reference light.
A Brief Digression into Lights, Filters, and Finger Paints
Additive colour mixture: A mixture of lights. If light A and light B are both
reflected from a surface to the eye, in the perception of colour the effects of
those two light add together.
Substrate colour mixture: A mixture of pigments. If pigments A and B mix,
some of the light shining on the surface will be subtracted by A, and some by
B. Only the remainder contributes to the perception of colour.
From Retina to Brain: Repackaging the Information
To tell the difference between different lights, the nervous system will look at
differences in the activities of the three cones types. This work begins in the
retina.
L and M cones have very similar sensitivities, so most of the time they are
in close agreement.
Computing differences between cones responses turns out to be a much
more useful way to transmit information to the brain. The nervous system
computers two differences: (L M) and ([L + M] S). The difference between
L and M responses contains considerable information about colour. One
speculative idea is that (L M) differences are particularly well suited to
appreciating the differences between different amounts of blood in skin.
In addition to (L M), we could create (L S) and (M S) signals. However,
because L and M are so similar, a single comparison between S and (L + M)
can capture almost the same information that would be found in (L S) and
(M S) signals. Finally, combining L and M signals is a pretty good measure of
the intensity of the light (S- cones make a rather small contribution to our
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