Mechanisms of Colour Vision Flashcards
What is the trichromatic theory of color vision?
There are three types of cones in the retina which detect the primary colours of red, green and blue (primary colours), and all other hues are a mixture of these primaries
What are metamers?
matching colours with different spectrums of incoming light (different spectral distributions)
What is the Limitation of Trichromatic Theory?
Even though this theory has some research support (colour matching) and makes sense intuitively, it cannot explain such visual phenomena such as afterimages. Most researchers agree that colour vision is explained by a combination of the trichromatic theory and the opponent-process theory.
What is the opponent-process theory of color vision?
The opponent process theory states that sensory receptors arranged in the retina detect four (not three) primary colours which are organised into two opponent pairs: red/green pairs and yellow/blue pairs (the four unique hues). If one sensor is stimulated, its pair is inhibited from firing.
-opponent colours are mutually exclusive: no hue can be both bluish and yellowish at the same time, or both greenish and reddish
What does the opponent process theory explain?
The theory explains colour afterimages.
E.g if you stare at the colour red for a while, you fatigue the sensors for red and the become adapted (less responsive). Then when you switch your gaze and look at a white image, where you should be stimulating both green and red sensors equally, the opponent part of the pair for red will fire, and you will see a green afterimage. This is because the red sensors have adapted, and so the green sensors will be stimulated more (more responsive).
-Same thing with blue/yellow
How are sensory receptors arranged in the retina according to the opponent process theory?
Sensory receptors arranged in the retina come in red/green, blue/yellow and black/white pairs
Where are colour opponent neurones found?
Found in the retina and LGN -these neurones decrease their signalling from one class of cone but increase it with a different class, demonstrating colour opponency
What is the Structure of neurones that exhibit colour opponency in the retina?
Red/green neurones exhibit colour opponency, and they have a centre-surround receptive field
- centre excited by L cone inputs (L+)
- inhibitory surround excited by M cone inputs (M-)
- also responds achromatically to luminance (white light)
Blue/yellow neurones exhibit colour opponency, but are not centre-surround in their organisation
- excited by S cone input (S+)
- inhibited by L and M cone input (L+M)-
What is the colour opponent circuitry like?
Midget Cells
- projects to the parvocellular layers of the LGN
- circular receptive fields with antagonistic centre-surround organisation (on/off centre), meaning L and M wavelength visible light (red/green) exert opposing effects on centre and surround
- note: not all midget cells are chromatically opponent though
Small Bistratified Cells
- take input from blue cones intermixed with M/L cones
- blue cones and M/L cones connect to separate bipolar cells, with the result that the M/L cone input is inhibitory and the blue cone (S) input is excitatory
Is there neurophysiological evidence for opponent processing?
If colour opponent neurons, expect i puts of opposite sign from cones in different spectral classes
How do centre surrounds receptive fields with colour opponency work?
Parvocellular RGCs have different receptive fields with different cones in them with M cones on the outside and L cones on the inside. Different cones preferentially absorb different wavelengths of light. Excitatory light on both centre and surround tend to cancel out.
What is required to determine the chromaticity of a point in a visual space?
At least 2 receptors are required to determine the chromaticity of a point in visual space. This implies the trade-off between chromatic and spacial acuity. SW cones are sparsely scattered in the retinal cone mosaic and it is not present at all in the central fovea, this means that patterns are purely defined by chromatic contrast (equiluminant)
Can we explain colour sensation by the synthesis of trichromatic theory and opponent processing?
Its a but more complicated than that.
light reflected from an object will differentially affect the three sets of cones according the SSR (surface spectral reflectance). Light does not normall appear coloured, but there is nevertheless variation in its spectral quality. This means that under different illuminants, Light from the object will be distorted from the SSR. Therefore the relative cone responses will change in different lighting conditions.
What does the spectrum of light hitting the eye depend on?
It depends on both SSR and spectrum of the illuminant. For a given surface, wavelength composition of light reaching the ey will change if the wavelength composition of illuminant changes. If the real-world property of colour is SSR, then the job of the visual system may be to estimate the real SSR but this can only be done if the spectrum of the illuminant is known.
What is the wavelength composition of light coded by?
It is coded by relative responses of the three cone classes
