lecture 7 Flashcards
what animals have hardware to see colour in a way comparable to us?
catarrhine monkeys
platyrrhine monkeys
how many receptor types do humans have?
three (trichromats)
how many receptor types and some tropical fish have?
4 types (tetrachromatic)
how many receptors to pigeons have?
five, pentachromats
what is colour good for?
scene segmentation
camouflage
perceptual organisation
food identification
scene segmentation
variations in colour often signal object boundaries
camouflage
animals use this fact to disguise themselves by colour markings
perceptual organisation
our visual system uses colour to group elements in a scene
food identification
ripe fruit
correct leaves
harmless vs harmful berries
poisonous or venomous animals
what is colour
wavelength in the visible lights segment of the electromagnetic spectrum
what is the range of the visible light spectrum?
range from red (long wavelength light) to violet (short wavelength light)
how long is the entire electromagnetic spectrum?
1mm (1,000,000nm)
how much of the EM spectrum does visible light cover
just 400nm
4/1000 of a mm
what range do we see between (m)
0.00000750m and 0.00000390m
what is the colour of an object dependent on?
the wavelengths of light the object absorbs and reflects
the light source
what does wavelength of light reflected determine
only the hue
what is perceived colour determined by, other than hue?
intensity of reflected light
saturation of the colour
physiological attribute of wavelength
hue (colour)
physiological attribute of intensity
brightness, perceived intensity
physiological attribute of spectral purity
saturation
saturation
how much white light is mixed in with the pure hue
what is the difference between red and blue
hue
what is the difference between light blue and dark blue (usually)
intensity
what is the difference between red and pink?
saturation
trichromatic theory hypotheses:
1) there are 3 sorts of receptors that respond best to different wavelengths of light
2) colour you see determined by levels of activity in three sorts of receptors
e. g. red objects reflect more long wavelength light than other objects, so red receptors more active
what are the threes receptor types? trichromatic theory
long (red)
medium (green)
short (blue)
s cones respond best to wavelength:
419 nm
m cones respond best to wavelength:
531 nm
l cones respond best to wavelength:
558 nm
support for trichromatic theory:
three primary colours combine to give all possible colours
three forms of dichromatism (colour-blindness)
A mixture of green and red light produces the same perception of yellow colour as monochromatic yellow light (metamerism)
what determines which wavelength a cone will respond best to?
pigments in the cones
how is light encoded from the cones
as an electrical signal
is your red the same as my red?
explanatory gap, we will never know
example of pain
if people have different tastes in colour, does that suggest colours appear different to each other
afterimages
only red and green cones can respond because the blue cones have become fatigued, this produces yellow afterimages
what did hering notice>?
when people are presented with a large number of colour samples and asked to pick those that are pure, they pick a red, a green, a blue and ALSO a yellow
opponent process theory proposal
there are three processes that are opponent in nature
-red/green
-yellow/blue
-black/white
each receptor will signal one colour, but not the other
support of opponent process theory
non existence of certain colours e.g., blueish-yellow
colour confusion in colour blindness (red and green)
complementary colour images
colour context effects
lightness constancy
lightness constancy:
perceive a colour as different depending on surroundings even though the reflected amount of light is the same
what level does the trichromacy theory occur at?
the level of the cones
what level does the opponent, process theory occur at?
LGN in the cortical cells
who was colour blindness first described by?
John Dalton in 1794
what should colour blindness be called and why?
colour deficiency
colour blind people just exhibit colour confusions, their experience of colour is different from normal
cortical colour blindness
view world in black and white
anopias
insensitive to s, m or l wavelengths of light (missing a type of cone)
dichromatism
type of anopia
missing cones
protanopia
L- cone pigment missing
- 3% men
- 02% F
deuteranopia
M cone pigment missing
- 2% M
- 01% F
tritanopia
S-cone pigment missing
- 001% M
- 003% F
anomalies
misalignment of L or M in trichromats
-distribution or deficiency
protoanomoly
L cone pigment deficiency
1.3% M
0.02% F
need more red in red-green mixture to match yellow
deuteranomaly
M cone pigment deficiency
-5% M
-0.35% F
need more green in red green mixture to match yellow
human tetrachromats
very rare in humans
they are predominately female
have 4 pigment cone types
can detect variations in hue that we normally cannot
