38 Questions
What is the pathway that extends from the retina to the primary visual cortex?
Slow-conducting parvocellular pathway
What type of ganglion cells are responsible for our perception of colour?
Parvocellular ganglion cells
Which type of ganglion cells project to the two magnocellular layers of the LGN?
Magnocellular ganglion cells
What is the primary function of the magnocellular pathway?
Mediating our perception of stimulus change (including motion)
What is the primary function of the parvocellular pathway?
Mediating our perception of colour and acuity
What is the primary function of rods in the retina?
Mediating perception in low illuminance conditions
What is the characteristic of magnocellular ganglion cells?
Achromatic
What is the role of the luminance channel in colour processing?
Coding for brightness and darkness
What is the function of cones in the retina?
Supporting colour vision and acuity in daylight conditions
What is the function of the geniculostriate pathway?
Processing form and colour information
What type of cells are C-units and L-units?
Horizontal cells
What is the characteristic of the C-units?
Respond to colours in an opponent fashion
What is the characteristic of the L-units?
Code only for brightness
How do rods converge on bipolar cells?
At high rates
What is the primary benefit of bipolar cells in the retina?
Providing the brain with high spatial resolution
What type of bipolar cells are attached to rods?
Only On type
What is the primary purpose of the experiments conducted by Young and Helmholtz?
To determine the number of wavelengths needed to match any other wavelength in the visible spectrum
What is the primary function of dark/light ganglion cells?
Detecting the intensity of light without regard to wavelength
What is the main idea behind the opponent-process theory?
That the cone photoreceptors are linked together to form opposing color pairs
What is the difference between dark/light ganglion cells and white/not white cells?
Dark/light cells detect intensity, while white/not white cells detect colour
What would trigger a dark/light ganglion cell?
A lot of just yellow waves
How many different wavelengths are needed to match any other wavelength in the visible spectrum?
3
What is the benefit of having a single cone in the retina?
Improved spatial resolution
What is the name of the scientist who developed the opponent-process theory?
Ewald Hering
Why do rods distinguish slightly lighter shapes on a darker background better than they distinguish slightly darker shapes on lighter backgrounds?
Because they are specialized for detecting contrast
What is the purpose of the test field in the experiments conducted by Young and Helmholtz?
To compare the mixture field with a single wavelength
What is the role of white/not white cells in the retina?
Detecting the presence of all three primary colours
What is the relationship between the trichromatic theory and the opponent-process theory?
They are complementary theories
What is the result of activating one member of an opposing color pair in the opponent-process theory?
It inhibits the activity of the other member
What are the three opposing color pairs in the opponent-process theory?
Blue/yellow, red/green, black/white
What does Newton's colour circle provide in the context of colour matches?
A qualitative description of colour matches
What is the purpose of adding the third primary colour in a colour match?
To desaturate the colour and make the match
What is the name of the theory that explains colour vision and is supported by colour matching and colour mixing studies?
Trichromatic Theory
What is the result of adding the third primary colour to a colour mixture?
Negative tristimulus values
What is the purpose of using 'negative' colours in colour matches?
To make the colour match possible
What is the alternative name of the trichromatic theory?
Young-Helmholtz Theory
What is the minimum number of colours required to make a colour match?
Three
What is the result of using blue and green primaries alone to match a spectral colour of 500 nm?
A desaturated colour
Study Notes
Colour Vision Theories
- The trichromatic theory and the opponent-process theory are two complementary theories that explain processes operating at different levels of the visual system.
- The trichromatic theory states that three different wavelengths (i.e., primaries) are needed to match any other wavelength in the visible spectrum.
- Individuals with normal colour vision need three different wavelengths to match any other wavelength in the visible spectrum.
Opponent-Process Theory
- Developed by Ewald Hering in 1920/1964
- The theory states that cone photoreceptors are linked together to form three opposing colour pairs:
- Blue/yellow
- Red/green
- Black/white
- Activation of one member of the pair inhibits activity in the other.
The Retina's Role in Colour Vision
- The retina contains two types of photoreceptors: rods and cones.
- Rods are more numerous than cones, converge at high rates, and have poor acuity but are suitable for mediating perception in low illuminance (i.e., scotopic) conditions.
- Rods are colour-blind.
- Cones are sensitive to colour information and are responsible for acuity and vision in daylight (i.e., photopic) conditions.
- Cones feed into three neural channels for colour processing, each composed of opponent pairs:
- Red-green
- Yellow-blue
- Black-white
- The first two mediate the perception of hue and form, while the latter is responsible for brightness and darkness (i.e., luminance).
The Geniculostriate Visual Pathways
- The geniculostriate is the primary visual pathway for the processing of form and colour.
- It extends from the retina through the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex (i.e., V1).
- The slow-conducting parvocellular pathway mediates our perception of colour and acuity.
- The fast-conducting magnocellular pathway is responsible for our perception of stimulus change (including motion) and is largely colour-blind.
Chromaticity System
- Newton's colour circle provides a qualitative description of colour matches.
- It can be used to explain why two colours may not be sufficient to make colour matches and also the use of 'negative' colours.
- The addition of a third primary is necessary to desaturate the colour mixture and make the match.
- Negative tristimulus values result when the third primary is added to desaturate the colour mixture.
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