Flashcards in Tutorial week 9: colour Deck (46):
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What color do you see when a light passes through both the red filter and the green filter?
yellow
2
what is the problem with colour constancy
a particular wavelength combination reflected from a surface (and the particular combination of excitation in the 3 cone types) can yield experience of different colors in different situations.
3
___________ applies to all sensory systems. it is fundamental to understanding perception and you should make sure you understand it well.
the concept of receptive field
4
______________ is used to describe the part of the sensory surface that an environmental stimulus must impinge upon to make a neuron fire.
receptive field
5
__________ short wavelength
blue
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_________ longer wavelength
red
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_________ middle wavelength
green
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___________ colour is caused by the lightness in the surrounding scene
black
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_____ and ______ is when colour constancy is achieved
V1 and V4
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__________ is where the primary visual cortex is located
VI
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_______ color-coded cells
V4
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______ cells responding to moving objects
V5
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Neurons in the visual cortex work the same way, except they respond when there is a light stimulating the ___________.
retina
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Neurons also have non-spatial ___________________. this describe the type of stimulus necessary to make the neuron work.
tuning characteristics
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for ____________ it's the same, not all touch neurons respond to the same type of touch: if pressure on a patch of skin makes a neuron in this cortex fire, the same neuron might not respond to changes in temperature.
somatosentation
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___________ refers to our ability to perceive different objects as always having the same colour independently of where we see them, or in what lighting conditions.
color constancy
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lighting is __________
artificial
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_____________ is different from artificial light
sunlight
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the light reaching us from the atmosphere is ____________
blue
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colour constancy sometimes discount the ___________ because somehow the variation is discounted.
illuminant
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what lobe is responsible for vision - the visual cortex.
occipital lobe
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the cortical processes that provide colour constancy are not well understand, what is known is that some cells found in the first part of the visual cortex - ___________ - are tuned for a certain combination of wavelength of light, more specifically they sum up the different cones excitation with a different weight for each cone. These cells get their input from the ___________.
area V1
photoreceptors
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In _________cells respond in more complex fashion. in a way that corresponds more to the perceived colour of an object. in other words, cells in ________ have activity that parallels our conscious experience.
area V4
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inside is to _________ as outside is to ________
artificial light
in bluer natural light
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What area is tuned for yellow
V4
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in ___________, the surface appears yellow will reflect lots of long (reddish) and middle (greenish) wavelengths, but not much short (blueish). here, we could find cells that would fire to this combination of reflected light. if the surface is outside, the reflected light will be different, it will contain more short wavelengths because of the blue light and proportionality less long and middle wavelength. in this area, the cells that fired inside will not fire now, different cells will respond, so constancy is NOT apparent. in this area, but it is in V4.
V1
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What have experiments demonstrated that is critical to colour constancy
wavelength distribution
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the perceived colour of an object depends on the relative amount it excited in the 3 kinds of cone and the ____________ in each of the 3 come channels
relative brightness
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humans have 3 types of ________ and that objects in the world reflect light of different ____________
wavelength sensitive photo-receptors
Wavelength
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_______________ is a property of the environment.
reflected light
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the ___________, via the cones, detects reflected light and interprets the light energy as colour.
visual system
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________ is a property of the brain: the perceived colour of an object is a consequence of operations in the brain and does not depend just on the wavelengths of the light from that object.
colour
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the _____________ of reflected light changes under different conditions.
composition
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_____________ is achieved achieved by getting some sense of the illumination color from the reflected light from several objects. exactly how is not clear.
color constancy
35
Colour perception, like all other forms of perception, is ultimately a _________________
subjective experience
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what is the most common-form of coluor-blind?
red-green colour blindness
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how do we see different colors?
a). Electromagnetic Spectrum.
b). Newton's Prism.
c). Colour Wavelengths
d). Absorption and Reflection.
e). The Eye.
f). Rods and Cones.
g). Colour Blindness.
h). Additive Colour System
i). Subtractive System.
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a). Electromagnetic Spectrum.
Light can be thought of as waves of energy. Light waves are part of the ___________ and waves of the spectrum differ in _________. The __________ (i.e. the distance
between identical points, such as between two peaks or between two troughs, on successive
waves) of electromagnetic radiation varies over an enormous range from trillionths of a
centimetre to many kilometres.
Energy with very _____ wavelengths (e.g. gamma rays) is not visible, nor is energy with very long wavelengths (e.g. radar, TV broadcasting signals).
The section of the electromagnetic spectrum that is visible to humans is quite small (about 1/70th of the total spectrum), ranging from wavelengths of about __________. It is important to understand that _________ is composed of a range of different wavelengths
because humans perceive different wavelengths of visible light as different ______________.
1. electromagnetic spectrum
2. differ in wavelength
3. wavelength
4. short
5. 10-6 to 10-7m (m = metre)
6. visible light
7. colours
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b). Newton's Prism.
_______ light is composed of different colours or wavelengths of light. Light bends or refracts when passing through a prism, and the amount of bending depends on the wavelength of the light: Longer wavelengths bend _____ than shorter wavelengths (to make an analogy, a semi-trailer cannot turn around a corner as tightly as a small car). When white light is beamed through a prism, the prism separates the white light into the different wavelengths of which it is composed.
1. white
2. less
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c). Colour Wavelengths.
As noted above, the wavelengths of visible light range from about ________. Because these
are small measures it is usual to express wavelengths in terms of ________. A nanometre is a ________ of a metre and is usually abbreviated nm (1 m = 1,000,000,000 nm). The module specifies wavelengths in terms of millimicrons that is an old term now replaced by the equivalent __________. As indicated, different wavelengths in the visible range of the electromagnetic spectrum corresponds to different colours (hues). Under normal viewing conditions, the relationship between the wavelength of a light and its perceived colour is as
follows:
(fill in the blanks – see Wickens, 2009)
750 infrared (nonvisible)
1. 10-6 to 10-7m
2. nanometres
3. billionth (10-9)
4. nanometres (i.e. 1 millimicron = I nm)
5. 450 = blue
6. 570 = yellow
7. 750 = red
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d). Absorption and Reflection (SUBTRACTIVE COLOUR MIXING)
The colours of surfaces are determined by the wavelength composition of the light reflected off
the surface. A black surface looks black because it absorbs all the wavelengths of light falling (or
incident) on the surface, so there is no light reflected. A white surface reflects all the
wavelengths in the incident light and therefore looks white. Similarly, a red surface looks red
because it reflects the long wavelengths and absorbs the other wavelengths in the incident light.
Yellow reflects _____________________and absorbs ___________________.
Magenta reflects _____________________and absorbs ___________________.
Cyan reflects _____________________and absorbs ___________________.
Yellow reflects red and green and absorbs blue
Magenta reflects red and blue and absorbs green
cyan reflects green and blue and absorbs red
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e). The Eye.
Fovea: ____________
Retina: ____________
Optic disk: _________
FOVEA: responsible for central sharpest vision and ability to see in detail
RETINA: receive light that the lens has focused on and convert it to neural signals
OPTIC DISK: a point of exit for ganglion cell axons leaving the eye; the blind spot - no rods and no cones
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f). Rods and Cones.
In the human retina, there are two types of light-sensitive cells, or photoreceptors, called the rods and the cones. The photoreceptors contain pigments that absorb light. All rods have the same pigment and so all work in the same way. Each cone, in contrast, contains one of three types of pigment, each of which works in different light mixes. Thus, there are three categories of cones and only one type of rod. Rods are distributed across the retina, except on the fovea and the blind spot, and are more sensitive to light than the cones. As a consequence of this sensitivity they work well in low light levels and thus are active during night vision. The rods are not involved in colour vision. That is, rods only provide information in shades of gray. This is why, at night, it is not possible to see colour.
The cones are concentrated in the fovea and are sparse in the periphery (edge) of the retina. Cones function mainly in daylight, as there is not enough light at night to make them work. Only the cones provide colour vision, explaining why things look colourless at night. Each of the three cone types responds to a band of wavelengths in the visible spectrum. The main difference between the cone types is in their sensitivity or responses to the different wavelengths, a consequence of their different photopigments. The “blue” cones contain a pigment that is maximally sensitive to wavelengths in the short wave region of the spectrum, the “green” cones contains a pigment that is maximally sensitive to wavelengths in the middle of the spectrum, and
the “red” cones are maximally sensitive to the longer wavelengths. “Green” ,“red”, and “blue” are in quotation marks because in fact these cones respond to a large range of colors, just to different degrees
If the light falling on the retina is from the blue
end of the spectrum, there will be maximal
activity in the “blue” cones. The other cone
types might also be active, but much less so.
If the light is from the orange-red end of the
spectrum it will elicit maximum activity from
the “red” cone type whose sensitivity is
greatest in the long-wave region, with the
other cones also active but less so. In this
way, each wavelength of the visible spectrum
will produce a different pattern of neural
activity among the three cone types. It is this pattern of activity among the three cone types that determines the colours we perceive.
If the light falling on the retina is from the blue
end of the spectrum, there will be maximal
activity in the “blue” cones. The other cone
types might also be active, but much less so.
If the light is from the orange-red end of the
spectrum it will elicit maximum activity from
the “red” cone type whose sensitivity is
greatest in the long-wave region, with the
other cones also active but less so. In this
way, each wavelength of the visible spectrum
will produce a different pattern of neural
activity among the three cone types. It is this pattern of activity among the three cone types that
determines the colours we perceive.
You should now understand that the perception or the experience of colour is created by our
nervous system. There is nothing 'red' about long wavelengths or 'blue' about short wavelengths
of light: Light rays are electromagnetic forms of energy and have no colour at all. Our visual
cortex transforms that information into a colour experience.
What is the difference between rods and cones?
______________________________________________
In your own words: Our experience of colour is the result of:
________________________________________________
1. RODS: has only one type of rod; are active during night vision; found in the retina (except the fovea and the blind spot); are not involved with colour vision; only provide information in shades of gray
CONES: function mainly in daylight; concentrated in the fovea and in the periphery; involved with colour vision;
The main difference between the cone types is in their sensitivity or responses to the different wavelengths
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g). Colour Blindness.
Colour deficiencies come in various forms and these deficiencies are most often associated with _________________ (i.e. one or more of the three cone types are missing or not functioning). The most common, ________ colour blindness, is caused by the absence of ___________. It is much more common in ______ , because the genes for the red and green color receptors are located on the__________, of which males have only____ and females have _____.
1. missing or malfunctioning cone types
2. red-green
3. “red” or “green” cones
4. males
5. X chromosome
6. one
7. two
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h). Additive Colour System.
The additive colour system, or additive colour mixing, describes the colour of light resulting
from the mixture or addition of two or more wavelengths of light. For example, if we project and superimpose on a white screen a red light and a green light of equal intensity, the resulting mixture appears ______. This is an instance of additive colour mixture. The two wavelengths do not affect each other. Rather, the colour of the superimposed lights reflects the activity of the colour coding mechanisms in our nervous system, its a combination of the activity generated in the cones by the red light and at the same time the activity generated by the green light. The combined activity is perceived as the colour yellow.
One of the most important, and oldest, laws of colour perception is that the additive mixture of
just three properly selected lights of different wavelengths can create every color we can
experience. One possible set (sometimes called primaries) consists of lights of ____ (reddish), ____ (greenish) and _____ (bluish) nm. The choice of primaries is arbitrary, however. Primary colours need only be reasonably far apart in terms of wavelength, and meet the additional requirement that the additive mixture of any two of the primaries cannot generate the third. That is, for a set of primaries, any two cannot be used to make the third.
Y = ____ + ____
M = ____ + _____
C = ____ + _____
1. yellow
2. 650
3. 540
4. 460
5. YELLOW = RED AND GREEN
6. MAGENTA = RED AND BLUE
7. CYAN = GREEN AND BLUE
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