Colour vision Flashcards
Distinguish between the sensory and physical properties of colour
SENSORY - hue, saturation, brightness
PHYSICAL - wavelength, intensity (difference between peaks and troughs, brighter/darker)
What are 2 evolutionary functions of colour discrimination
Detection and signalling
What is the visible spectrum?
400-700nm Short = blue Long = red White = mixture Black = absence of light so no wavelength value
What is meant by “chromatic” light?
All the wavelengths
Selective reflection of light off of an object - some reflected more than others e.g. if blue is reflected most we see a blue object
We can represent this using REFLECTANCE CURVES
What is meant by monochromatic and achromatic light?
MONO = Light source only emits a single wavelength anyway ACHROM = Light composition spread across visible spectrum i.e. object has no hue (white, black or grey)
How do we see colour in transparent objects?
SELECTIVE TRANSMISSION - creation of chromatic colour by selectively transmitting some wavelengths e.g. cranberry juice selectively transmits long wavelengths
What is meant by an additive colour mix?
Happens when mixing lights - remember that white surfaces reflect all wavelengths
So when lights superimposed on white wall we see all of their reflectance so we see white light
What is meant by subtractive colour mixing?
This is what happens when we mix paint
Both paints still absorb their usual wavelengths so we are left with reflectance of wavelengths they both reflect –> different colour
If mix all colours get black because no reflectance
What does the trichromatic theory of colour suggest?
Young and Helmholtz - based on colour matching experiments
Light of a given wavelength stimulates 3 receptor mechanisms, with different spectral sensitivities due to differences in opsin structure, to different degrees and the resulting pattern of activity is what results in perception of a particular colour
S-TYPE - short wavelengths
M-TYPE - medium
L-TYPE - long
What is meant by Metameric colours?
Colours which look alike (e.g. in the colour matching experiments) because trigger same pattern of cone response, but underlying wavelength stimuli actually vary
Physically different stimuli, identical physiological response
Why do we necessarily need multiple receptor mechanisms i.e. different and multiple pigments?
Allows wavelength distinction independent of light intensity (with only one pigment, at a given light intensity the colours will appear the same shade of grey)
Different pigments have different ratios of absorption spectra which remain the same for a given wavelength - the visual system uses these ratios to determine colour (this is the basis of the trichromatic colour theory)
When is colour vision possible?
In dichromats i.e.t those with 2 pigments (not possible with one)
In trichromats a wider range of colours are able to be seen across the visible spectrum
What is the opponent process theory?
Developed by Ewald Hering(1920/1964), the opponent-process theory states that the cone photoreceptors are linked together to form three opposing colour pairs: blue/yellow, red/green, and black/white
We can only detect the presence of one colour from the pair at a time because the two colours oppose one another e.g. we never see “greenish-red”
Opponent neurons will be excited by one and inhibited by the other
Which colour theory is correct?
Both - the trichromatic explains receptor-level processing while opponent process theory explains level of LGN
Each wavelength causes different ratios of response in the 3 kinds of cone receptors - takes a minimum of 3 wavelengths to match any wavelength in the spectrum
Ganglion neurons actively group information from receptors to produce the opponent code
What are afterimages?
Can be considered the product of ganglion cell fatigue - excitation when see red for example, weakens the opponent axis so when we move to a neutral zone e.g. white, the green opponent wins the tug of war
When can we get colour deficiencies?
Brain injury –> inability to perceive colour
Retinal receptor problems –> partial loss of colour perception
How can colour deficiency be determined?
Using colour matching - determine minimum number of wavelengths needed to match any in the spectrum
Monchromats will need 1 (colour blind) - these are hereditary and very rare - no functioning cones at all
Dichromats will need 2
Anomalous trichromat need 3 like normal but mixes them in different proportions and not as good at discriminating between wavelengths close together
How can we establish what specific deficiency a person has?
Using unilateral dichromats - one eye normal trichromatic so compare the colours seen in each eye
What are the 3 forms of dichromatism?
1) PROTANOPIA - Missing long wavelengths, so as wavelength increases just see increasing shade of grey. The wavelength at which see grey is their NEUTRAL POINT, above which just see increasingly saturated yellow
2) DEUTERANOPIA - Missing medium wavelengths (most common), similar pattern as protanopes
3) TRITANOPIA - Missing short wavelengths (very rare and not sex-linked)
What are ISHIHARA TESTS?
Pseudo-isochromatic plates used where number defined by colour contrast
Someone with protanopia i.e. severe red-green would struggle to differentiate the number when green on red background
What is colour constancy?
Our ability to estimate reflectance of an object (perceive its colour) in spite of changes in illumination
Light reflected depends on reflectance curve and light source - sun is all wavelengths but bulbs are generally longer wavelengths so a green sweater under sunlight may reflect more long wavelengths under bulb but we still see it as green
What is believed to be responsible for our colour constancy abilities?
1) Chromatic adaptation - Prolonged exposure to tungsten light bleaches the long wavelength cone pigments (adaptation) so we have decreased sensitivity to long wavelengths and so less effect occurs
2) Effect of surroundings - Constancy is harder when surroundings are masked, works best when other colours visible (mechanism not certain yet)
3) Memory - knowledge about an object’s “usual” colour
What is light constancy and what are the 4 possible causes?
The perception that the apparent brightness of light and dark surfaces remains more or less the same under different luminance conditions
1) Ratio principle - when even illumination across object, lightness determined by reflectance ratio object:surroundings
2) Shadows - Uneven illumination (reliance on penumbra)
3) Orientation of surfaces - Uneven illumination, need full info about arrangement to determine whether illuminance or reflectance edge
4) Perceptual organisation of images (e.g. light and dark mist experiments)
What is meant when we say that colour processing is thought to be an example of distributed processing?
Rather than confined to a specific module, a number of cortical areas may be involved
Cortical mechanisms are complicated - need to process wavelength info and then determine colour perception from that (some individuals can only perform one of these functions)
