colour perception Flashcards
3 steps to colour perception
detection
discrimination
appearance
detection
wavelengths must be detected
need photoreceptors to convert light into the nervous system
discrimination
need to be able to tell the difference between one wavelength (or mixture of wavelengths) and another
we need neurons that compare inputs from different kinds of photoreceptors
appearance
clever processing not understood
ie/ perceive colour of rose in sun vs. shade
origin of “colour science”
science is a very new kind of human activity and a new group of people
people have been talking about colour for thousands of years before science existed
when the science of colour developed, it developed from two already existing groups of people - artists and philosophers
not a physical property of the world, but realated to it
humans see wavelength around 400-700nm
what are the primary colours
red, blue, yellow
can’t be described using other colours
like orange being yellowish-red
green is different, don’t really perceive it as yellowish blue
colour space
a three dimensional space that describes all possible colours
depends on how you organize colours
hue
the chromatic (colour) aspect of light
ie/ red
saturation
the chromatic strength of a hue
richness
brightness
the distance from black in colour space
white would be high brightness, black would be low
aspects of colour space
hue
saturation
brightness
range of human sight for wavelength
400-700nm
atmospheric filtering
the sun emits lights across the electromagnetic spectrum
only some types of light make its way through the Earth’s atmosphere
radio waves
used to broadcast radio and tv
microwaves
used in cooking, rada telephone and other signals
visible light waves
object surfaces differentially reflect incoming light
only one that can abosorb and reflect light
ultraviolet waves
absorbed by the skin used in fluorescent tubes
more damaging
if sensitive to it, you won’t have a long life
x-rays
used to view inside of bodies and objects
gamma rays
used in medicine for killing cancer cells
duplicity theory
daytime vision (photopic) cones and night vision (scotopic) rods
daytime vision
photopic
cones
vision in bright light
visible colours
center of visual field is most detailed - move eyes to see high level of detail
peripheral visual field is less detailed and lacks colour
does not require a period of light adaptation for visual functions to stabilize
fovea concentrated in cones
night vision
scotopic
rods
vision in low light
colourless
center of visual field is least detailed - look slightly to side
roughly equivalent detail across periphery. no colour anywhere
requires a period of dark adaptation for visual function to stabilize
photoreceptors
cells in the retina that initially transduce light energy into neural energy
rods
photoreceptors specialized for night vision
more sensitive to lights
low levels of light
respond well in low luminance
are not used to process colour
cones
photoreceptors specialized for daytime vision, fine acuity and colour
respond best in high luminance conditions - high levels of light
are used to process colour
less sensitive to light
purkinje effect
During scotopic vision, only the rods
are active. This shifts your visual
sensitivity into the blue range
▪ At night, red objects appear darker (black),
and blue objects appear brighter (white)
trichromatic theory - thomas young
First person to demonstrate that
light behaves as a wave
“Now, as it is almost impossible to
conceive each sensitive point of the retina
to contain an infinite number of particles,
each capable of vibrating in perfect unison
with every possible undulation, it
becomes necessary to suppose the
number limited, for instance, to the three
principal colours, red, yellow, and blue.”
trichromatic theory - james clerk maxwell
- White light occurs when all wavelengths
are present in an equal amount - Monochromatic light occurs when a ray is
composed dominantly of a single
wavelength (e.g. “red” light) - Maxwell demonstrated that white light
can be created by mixing just three types
of monochromatic light - This suggests that there are only three
colour channels in human vision
Maxwell produced the first colour
photograph (hint: it’s a ribbon)
Steps:
1. Take a B&W photo with a red
filter
2. Repeat with blue and green
filters
3. Separately project each photo
on the same area with the
corresponding coloured light
trichromatic theory - herma von helmholtz
Helmholtz’ book Treatise on Physiological Optics was a landmark publication in the science of visual perception
- In it he refined the Trichromatic Theory in several ways:
1. Receptors had gaussian response curves
2. Colour sensation is encoded in the photoreceptors
3. Each receptor produced a sensation of a “unique hue.”All other colours were mixtures
Demonstrated how this accounted for various types of colour blindness
spectral sensitivity
the sensitivity of a cell or a device to different wavelengths on the electromagnetic spectrum
What are the three types of cone photoreceptor
S cones, M cones, L cones
S cones
maximally responsive to short wavelengths
420 nm max
Blue
M cones
maximally responsive to middle wavelengths
535 nm max
Green
L cones
maximally responsive to long wavelengths
565 nm max
Red
agnosia
person see something, but fails to know what it is
anomia
inability to name, despite the ability to see and recognize them
ie/ colours
synesthesia
where one stimulus evokes the experience of another stimulus
ie/ sounds associate with different colour
achromatopsia
inability to perceive colours that that is caused by damage to the CNS
colour anomalous
all three cones but their behaviour is atypical
ie/ wavelength shifts
two of them so similar that these people experience world similarly to those with two cones
dichromacy
missing one cone type
types of dichromacy
protanopia
deuteranopia
tritanopia
most common type of dichromacy
protanopia and deuteranopia
protanopia
absence of L cones
deuteranopia
absence of M cones
tritanopia
absence of S cones
cone monochromacy
has only one cone cone
see the world in shades of grey
still have daytime vision
no colour perception, can’t compare cones
rod achromacy
has only rods
lack daytime vision and would hurt (poor acuity in daytime)
missing cones all the time
cerebral achromatopsia
inability to perceive colours, despite functioning photoreceptors and early visual stages; typically due to brain damage
trichromic theory cannot explain
Basic colour terms
Words like blue, not sky blue
Used with high frequency
, and have meanings
Agreed upon by speakers of a language
Common
Higher m cones “green”
Lower m cones “reddish orange”
For a deutemope with no m cones their remainer L and S cones will produce same response
single words that describe colours and have meanings that are agreed upon by speakers of a language
11 terms have broad agreement
(American)
Tetrachromatic
Rare situation where the colour of any light is defined by the relations of 4 numbers—the outputs of those receptor types
Evaluating the Trichromatic Theory: Christine Ladd-Franklin
“[Helmholtz] paid no attention whatever to the fact that, while the necessary stimuli for all the colours in the spectrum (and in the world) can be secured by appropriate mixtures of only three wave-lengths, the distinct, different, sensations that result are not three in number but five – yellow and white are just as good, just as unitary, light-sensations as are red, green, and blue.”
Metamers
We are not typically exposed to single wavelengths
How do we discriminate from injured apples, or ones exposed to sun
Rest of nervous system only knows what the cones tell it
Mixtures of wavelengths that look identical
If two different set of wavelengths stimulate the cones in the same way, even thought the lights are physically different, the bran interprets it as same colour
Green and red identical wavelengths appear yellow
Tells us that colour perception is also determined by how wavelengths interact
Afterimage
a visual image seen after a stimulus has been removed
Positive afterimage
an afterimage whose polarity is the same as would the original stimulus
now & We tell the
Very brief (<500 ms)
Negative afterimage
an afterimage whose polarity is the opposite of the original stimulus
Colour specific adaption
Light stimuli> dark afterimages
Light stimuli produce dark negative afterimages
Colors are complementary(opponent)
Red>Green, Green>Red
Blue>Yellow, Yellow>Blue
Adapting stimulus
A stimulus who’s removal produces a change in visual perception or sensitivity
Neutral point
Point at which an opponent colour mechanism is generating no signal
Ie/ wavelength or combination of wavelengths cancel each other out—visual system will just perceive grey
Opponent Process Theory: The Colours
Ewald Hering (1843 – 1918) proposed an alternative theory of colour vision based on opponent colours
“Orange” is perceived as a reddish-yellow
“Purple” is perceived as a blueish-red
BUT there is no such thing as a reddish-green or yellowish-blue
Hering suggested that colour sensations are produced by the output of three antagonistic processes
Red-green
Blue-yellow
Black-white
Opponent Process Theory: The Mechanism
The Trichromatic theory was correct as a theory of receptors. Three types of photoreceptors do exist.
Opponent process theory predicts that colour is not encoded at the receptors, but later in the system
The input from the cones is carried
to the cortex along two opponent
channels
Retinal Ganglion Cell Receptive Fields
Opponent process required for hue encoding: Colour opponency without spatial opponency
Opponent process found in retina
Colour and spatial opponency
who got it right? helmholtz or hering?
both:
helmhltz was right that the initial stage involves three types of photoreceptors
hering was correct that there was an essentially antagonistic, opponent process in the initial stages of colour vision
Neither:
neither of these processes is sufficient for “hue perception”
the “unique hues” still elude scientists
we still don’t understand the entire process of colour vision
Ladd-Franklin Evolutionary Theory
Neither the opponent process theory, nor the trichromatic theory account for how colour vision evolved.
All animals can discriminate white from black. (oldest system)
Bees (and other animals) have strong yellow-blue discrimination, but weak red- green discrimination. (newer system)
Humans have good, red-green
discrimination. But it gets weaker in the periphery. (newest system)
Contemporary Evolutionary Theory
Contemporary evolutionary theory aligns with the original Ladd-Franklin theory in many respects.
Ladd-Franklin got the physiological mechanism of cone differentiation incorrect
But she did identify the appropriate evolutionary sequence
~stages in evolution of colour vision panel c shows the spectral sensitivity curves of the three photosensitive pigments in the normal human eye. Ancestral mammals are thought to have had dichromatic vision.At an intermediate stage the spectral separation of the long wave/middle wave pigments may have been small, and dependent on a single amino-acid difference between the two proteins~
Does everyone see colours the same way?
General agreement on colours
Basic colour terms: single words that describe colours and have meanings that are agreed upon by speakers of a language
11 terms have broad agreement
(American)
But, various cultures describe colour differently
Colour boundaries are pretty sharp
More likely to be confused if the colour has a different label
cultural relativism
sapir whorf hypothesis
Cultural relativism
in sensation and perception, the idea that basic perceptual experiences (e.g., colour perception) may be determined in part by the cultural environment
Sapir whorf hypothesis
Your perception is limited by your
range of concepts. If you don’t have a concept for something, you cannot see it.
If you didn’t have a term for blue, you wouldn’t be able to see it
Qualia
private conscious experiences
of sensations or perception
Only your experience
Colour Contrast
When many colours are present, as is typical in natural scenes, they can influence each other
Colour contrast: a colour perception effect in which the colors in surrounding colour of one region induces the opponent colour in a
In the environment neighbouring region
colors in in surrounding In the environment push colour in a certain direction
Unrelated colour
a colour that can be experienced in Isolation.
Related colour
a colour seen only in relation to other colors
Ie/gray, brown
Colour assimilation
When many colours are present, as is typical in natural scenes, they can influence each other
a colour perception effect in
which two colours bleed into each other, each taking on some of the chromatic quality of the other
Scenes can contain colours that can’t be experienced in isolation
Apparent colour comes from the stripes passing the the object
Lightness Constancy
Lightness (whiteness) constancy: the tendency of a surface to appear the same lightness despite variations in the intensity of illumination
Achieved by discounting the illuminant and determining the true lightness of a surface regardless of how it appears
Achromatic (Black > White)
Lightness/whiteness – the apparent reflectance (albedo) of a surface
Black reflects little
Grey reflects an intermediate amount
White reflects a lot
Not dependent on the actual amount of light reaching your eye
Edges;
Reflectance edge: edge where reflectance properties of two
surfaces change
Illumination edge: edge where illumination changes
Visual system has to determine if a change in the amount of light
reaching the retina is due to a reflectance edge (lightness change)
or illumination edge (shadow)
Identifying Illumination Edges;
Meaningful shape,
Penumbra,
Change in surface direction
Explanation;
Adaption,
Photoreceptors will adjust to average light level—within an environment
Anchoring Principle-
The brightest part of a scene will be seen as “white,” the darkest will be seen as “black”
Ratio principle-
Comparison of reflectance ratios of nearby surfaces
reflectance edge
edge where reflectance properties of two surfaces change
illumination edge
edge where illumination changes
illuminant
the light that illuminates a surface - it is not constant
adaptation
photoreceptors will adjust to average light level within an environment
anchoring principle
the brightest part of a scene will be seen as “white,” the darkest will be seen as “black”
ratio principle
comparison of reflectance ratios of nearby surfaces
Colour consistency
the tendency of a surface to appear the same colour despite variations in the colour of illumination - changes in lighting conditions
Achieved by discounting the illuminant and determining the
true colour of a surface regardless of how it appears
Illuminant: the light that illuminates a surface— is not constant
Explanations;
Memory—common objects have a known colour regardless of illumination
Chromatic adaptation—exposure to a particular wavelength of light decreases sensitivity to wavelengths similar to it
Comparison to surroundings—factor out illumination by using
a ratio principle
Other Assumptions;
Qualities and position of the illuminant and
Directions, orientations, and shapes of surfaces
The “Retinex” Theory of Colour Constancy
Retinex = Retina + Cortex
The rods alone are capable of lightness constancy
The Retinex Theory suggests that each cone engaged in an independent lightness constancy process independent of the other cones
Once combined, these three independent values encoded the determinate colour of an object surface
Retinex Physiology
The physiology of the retina does not support the Retinex theory
Information does leave the eye in three pathways.
But the three pathways do not correspond to the three cones
trichromat
regular vision
have all three types of cones and can distinguish a wide range of colours
light is two dimensional
wavelength and intensity
cone output is one dimensional
it fires more or less
can a single cone detect colour on its own
no
concept of univarience
explains lack of colour in dim lighting - cause there is only a single rod
we can detect differences because we have more than one kind of photoreceptor
colour constancy explanations
Memory – common objects have a known colour regardless of
illumination
▪ Chromatic adaptation – exposure to a particular wavelength of
light decreases sensitivity to wavelengths similar to it
▪ Comparison to surroundings – factor out illumination by using
a ratio principle
▪ Other Assumptions
▪ Qualities and position of the illuminant and
▪ Directions, orientations, and shapes of surfaces
