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Chapter 5

Chapter 5 - study notes

Course Code
Matthias Niemeier

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Describe one experiment that would suggest that faces arent special. Wat were ps asked to
do? Which method was used? Wat was found? Wat does that potentially mean?
Greebles experiment. Where ps were trained to recognize greebles, (new figures). And
results show that (they used fmri) that increase activity in fissform phase area, and this
might meant tat we are lookin at area in brain that not specialized in faces but for any
kinda of visual expertise.
-color is not a physical property of things , it is creation of mind. Visible spectrum run from
400-700nm. Most light tat we see is reflected light. The more light that is absorbed, the
darker surface will appear. Colour of surface depends on mix of wavelengths that reach eye
from the surface. Specific wavelengths of light are NOT specific colours. Color is the result
of interaction of a physical stimulus with a particular nervous system
-a single photoreceptor shows diff responses to lights of diff wavelengths but the same
intensity. With wavelengths as x-axis and receptor response as y axis, we see inverted U
curve. There are 2 pts in the wavelength where the photoreceptors have the same response.
So if we were looking at the output of photoreceptors only, we would have no way of
distinguishing /b/ the 2 lights. But when we look with a normal human colour vision
system, we can see that wavelength A and B are so and so colour
- note that intensity can be reduced. ie the photoreceptors response rate, the highest one,
can be reduced to match the 2 pts in the U curve that had the same response rate. So when
it comes to seeing colours, the output of a single photoreceptor ie the photoreceptors
response is completely ambiguous. An infinite set of different wavelength and intensity
combinations can elicit exactly the same response. So output of a single photoreceptor cant
by itself tell us anything about the wavelengths stimulating it. =problem of univariance
= one photoreceptor type cant make colour discriminations based on wavelength
- 2 kinds of photoreceptors: rods and cones. Rods sensitive to low, scotopic, light levels. All
rods contain the same type of photopigment molecule: rhodopsin thus they all have same
sensitivity to wavelength. Consequence, thought rods able to tell light from dark under
scotopic conditions, problem of univariance make it not possible to discriminate colors. This
is hint that color is psychophysical and not physical . at night, dim light stimulate only rods
and they only have 1 kind so cant tell colours apart

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-cone photoreceptors are sensitive to higher, daylight light levels, photopic. They come in 3
forms that contains a slightly diff photopigment. These diff pigments give each type of cone
a distinctive wavelength sensitivity: cones that have peak at about 440nm = short
wavelength s-cones. Middle wave length M-cones peak at about 535nm. Long wavelength
L-cones peak at 565nm. It may be tempting to rename these cones, blue, green, red. But
dont. We do perceive long-wavelength light to be red but if u only have L-cones u would not
see red, and due to univariance u will see a colorless world or gray, work like ur rods. The L-
cones are more sensitive to long wavelengths than the other cones but they work together
With the 3 cones we can tell the diff /b/ light of diff wavelengths. Univariance not happen in
the 3-cone world. even if we change the intensity, the response size may change but the
relationship /b/ the cones remain and it is those relationship that defines the colour
Trichromatic theory of color vision, trichromacy = theory that the color of any light is
defined in our visual system by the relationship of 3 #s, the outputs of 3 receptor types, the
3 cones. This also known as the Young-Helmholtz theory
-we do not see pure wavelengths most of the time
-metamers different mixtures of wavelengths that look identical. Any pair of stimuli that
are perceived as identical in spite of physical differences
-mixing wavelengths does not change the physical wavelengths. If we mix 500 and 600 nm,
it not contain the average of the 2, nor the sum. It contains 500 and 600. Color mixture is a
mental event, not a change in physics of light
additive color mixture = mixture of lights. If light A and B are both reflected from a
surface to the eye, in the perception of the color the effects of those 2 lights together
subtractive color mixture = a mixture of pigments. If pigment A and B mix, some of the
light shining on the surface will be subtracted by A and some by B. Only the remainder
contribute to the perception of color. So a patch looks yellow cuz the colour is being reflected
off surface to eye and the other colours are either absorbed or being subtracted and thus
cant see it
3 diff types of cone photoreceptors, light reaching any part of the retina will be translated
into 3 responses, one for each local population of cones

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If 2 light rays reflecting off 2 surfaces produce same set of cones responses, the 2 surfaces
must and will appear to be exactly same color. They will be metamers meaning their
physical characteristics are diff but look identical. Ie red blood is not same as a red
tomatoes physical characteristic but do set off same cone response
We can discriminate 10 million diff colors. The range of our experience of color can be
described by reference to a 3D color space = 3D space established cuz color perception is
based on the outputs of 3 cone types, that describes the set of all colors
Ex. Of color space: red, green and blue. Hue = chromatic (colourful) aspect of colour ie red,
blue, green etc, saturation (chromatic strength of a hue, white has zero saturation, pink is
more and red has the most), brightness = distance from black (zero brightness) to color
Nonspectral hues = hues that dont exist as a pure form of light but only as a mixture of
diff wavelength
-we can send separate L, M, S signals to brain but that be less useful that one thinks. We
should look at diff /b/ cone response. (L-S), (M-S) signals. But since L and M are so similar,
a single comparison /b/ S and (L+M) can capture almost the same info as above. Thus,
convert the 3 cone signals into 3 new signals (L-M), ([L+M]-S) and (L+M) = blue, green, red
-lateral geniculate nuleus LGN = A structure in the thalamus, part of the midbrain, that
receives input from the retinal ganglion cells and has input and output connections to the
visual cortex
-just like ganglion cells in retina and LGN of the thalamus prefer certain receptive field
ways, some of the retinal and LGN ganglion cells are excited by the L cone onset in their
enter and inhibited by M cone onsets in their surround. (L-M) cells are one type of colour
opponent cell = a neuron whose output is based on a diff between sets of cones. There are
also (M-L), ([M+L]-S) AND (S-[M+L]) cells, the cells we like to have to support a 3 process
opponent color system
-opponent colour theory perception of color is based on the output of 3 mechanisms, each
of them based on an opponency between 2 colours; red-green, blue-yellow, black-white (but
it can be a colour ie grey)
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