Task 3: Perceiving colour Flashcards
Colour of light
wavelengths in the visible spectrum
Colour of objects
which wavelengths are reflected (for opaque objects) or transmitted (for transparent objects)
Differences in pigment underlie
different absorption spectra => colour perception
Subtractive colour mixture
mixing paints
- fewer wavelengths are reflected
=> only the WL that is in both colours appears
Additive colour mixture
mixing lights
- more wavelengths are reflected
=> white
The perceptual dimensions of colour
- Hue = colour on spectrum (dominant wavelength)
- Saturation or chroma = more white leads to a desaturated colour (spread of WL)
- Value or brightness = value decreases as colour become darker (amplitude of WL)
Trichromatic theory of colour vision (Young-Helmholtz)
- occurs at the receptor level (retina)
- offers ratio information (receptors respond to different WL)
- does not fully explain colour blindness (dichromacy)
- three colours : green cone, blue cone, red cone
Colour matching experiments
- just amount of 3 different wavelengths mixed together in a comparison field until one single WL is perceived
- possible with 3 WL but not 2
Metamerism
two different stimuli (different WL) are perceived as being the same
Principle of unvariance
- a photon of light is absorbed => identity of WL is lost
- absorption of a single photon has the same effect no matter the WL or receptor type
Different receptors can give different responses to the same wavelength because
different receptors have different pigments => different absorption spectra
Population or distributed coding
analysis of stimulus by multiple types of receptor
- not ambiguous
- colour changes if ratio of activity between the 3 WL changes
Opponent-process theory (Hering)
- neural level process (lateral geniculate nucleus)
- difference information
- colour experience is arranged into two opponent pairs
Yellow -Blue
Red-Green
The two opponent pairs are linked through
lateral inhibition
Complementary afterimage
seeing the opposite colour of the colour circle
1 - looking at red => L cones get used up
2- L cones less active => green afterimage seen on a white surface
Hue cancellation procedure
how much of a opponent colour needs to be added to cancel all perception of the opponent colour (opponent pairs)
Difference information
neurons integrate inhibitory (-) and excitatory (+) signals of receptors
Opponent LGN-Neurons
- in the retina and lateral geniculate nucleus
- excitatory response to light from one part of the spectrum => inhibitory response to the opponent light
- cones send excitatory or inhibitory input to bipolar cell
Types of opponent neutrons in the cortex
Single-opponent neurons
Double-Opponent neurons
Single-opponent neurons
- colours within broad regions
- centre-surround antagonism
- ganglion cells, LGN, visual cortex
- best stimulus = higher firing rate
Double-opponent neurons
- boundaries between colours
- colour contrast
- colour constancy
- visual cortex
- side by side receptive fields
Chromatic adaptation
prolonged exposure to chromatic colour
- adaptation to red light => reduced sensitivity of L-cones => a less saturated red
Chromatic adaptation leads to
colour constancy
Colour constancy
perceiving the colours of objects as relatively constant even under changing illumination
Actual reflected light depends on
object reflectance curve and illumination
Other factors influencing the way we perceive colour
a) Surroundings (colour constancy works best when object is surrounded by many colours)
b) Memory and prior knowledge (familiar objects have a more saturated colour than other objects that reflect the same WL)
Dual-process theory
- Trichromatic stage (photoreceptors)
- Opponent-process stage (ganglion cells, LGN, VC)
- Cortical processing
“The dress” experiment explains
individual variability => perceived colour depends on the context
Warm illumination = blue/black dress
Cool illumination = white/gold dress
