Slides Week 5 Flashcards
Basic Principles of Colour Vision
- We see objects when we detect the light reflected from them
- Depending on Atomic Structure objects absorb some wave lengths and reflect others
- It is reflected light that is perceived as colour
- Colour is not a physical property but a psychophysical property
Atomic Structure
- Atoms consist of three basic particles: protons, electrons, and neutrons
- The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge)
- The outermost regions of the atom are called electron shells and contain the electrons (negatively charged)
Visible Spectrum of light
Between 400-700 nm
Three Steps to Colour Perception
- Detection
- Discrimination
- Appearance
Colour Percption - Detection
Wavelengths of light must be detected
Colour Perception - Discrimination
We must be able to tell the difference between one wavelength and another
Colour Perception - Appearance
We want to assign perceived colours to lights and surfaces in the world and have those perceived colours be stable over time regardless of different lighting conditions.
Scotopic Light
Light intensity that is bright enough to stimulate Rod Receptors but too dim to stimulate Cone Receptors
Photopic Light
Light that is bright enough to stimulate Cone Receptors AND saturate Rod Receptors to their maximum responses
Preferential Absorption
- Retina contains three types of cone receptors that differ in sensitivity to light
- They are each most sensitive at
- short wavelenghts
- medium wavelenghts
- long wavelentghs
- There is one type of Rod and it’s peak sensitivity sits between short and medium wavelenghts
Principle of Univariance
- There is an infinite set of wavelength intensity combinations
- They can elicit exactly the same response from a single type of photoreceptor
- A single type of photoreceptor cannot make colour discrimination based on wavelength
- A single photorecptor has different responses to lights of different wavelenths but the same intensity
Trichromatic Theory
- Also known as the Young-Helmholtz theory
- The theory that the colour of any light is defined by:
- Our visual system
- The relationships of numbers
- Outputs of three receptor types now know to be the three cones
Metamers
- Different mixtures of wavelengths that look identical
- Generally any pair of stimuli that are perceived as identical despite physical differences
Additive Colour Mixing
- Physical Definition: A mixture of lights reflected from a surface to the eye
- Psychological Definition: perceiving colour in the effects of those lights added together
- Shining two lights that look one colour each and perceiving a different colour producing and additive colour mixture
Subtractive Colour Mixing
- A mixture of pigments where some of the light from pigment A is subtracted from pigment B
- Only the remaining pigment contributes to the perception of colour
- This is denoted when we use colour filters
Pigment
- A material that changes the colour of reflected or transmitted light.
The limits of a rainbow
- If you combine lights that look red and blue you get purple, But there is no purple on the spectrum
- Purples are non-spectral colours that join the ends of the spectrum into a colour circle
Colour Appearance
What Colours will be perceived?
- Colour Space
- RGB Colour Space
Colour Space
- A three-dimensional space that describes all colours
- There are several possible colour spaces
RGB Colour Space
Defined by the outputs of long, medium and short wavelengths
- Long Wavelength = Red
- Medium Wavelength = Green
- Short Wavelength = Blue
HSB Colour Space
Defined by Hue, Saturation, and brightness
HSB Colour Space - Hue
- The chromatic aspect of light
- The perceived colour of light
HSB Colour Space - Saturation
The Chromatic Strength of a hue
HSB Colour Space - Brightness
The distance from black in colour space
Trichromatic and Opponent Process
- Information about the difference between pairs of cone receptor signals is sent to the brain
- Create afterimages and simultaneous contrast
- Sending separate S, M, L signals to the brain is not particularly useful as M and L have similar sensitivities
- Computing differences between Cone responses is more useful to transmit information
Opponent Cell Physiology
- Lateral Geniculate Nucleus has cells that are most greatly stimulated by spots of light with centre-surround organisation
Cone-opponent cells
- A neuron whose output is based on a difference between sets of cones
- These cone-opponent cells also have a centre surround organisation
Opponent Colour Theory
- The theory that perception of colour depends on the output of three mechanisms
- Each is based on an opponency between two colours:
- Red-Green
- Blue-Yellow
- Black-White
- Some LGN cells excited by L-Cone activation in the centre but inhibited by M-Cone Activation in their surround and vice versa
- Other cells excited by S-Cone activation in centre but inhibited by (L+M)-Cone activation in there surround
Double Opponent Cells
- First found in the Primary Visual Cortex
- Are more complicated, combining the properties of two colour opponent cells from LGN
- Opponent cell information is processed further to allow for opponent colours
Hering’s Idea of Opponent Colours
- All the colours on the Colour Circle” could be represented by two pairs of opposing colours
- We can have bluish green (cyan)
- We can have reddish yellso (orange)
- We can have bluish read (Purple)
- We cannot have reddish green or bluish yellow
Forbidden Colours
- Crane & Piantanida (1983) used eye tracking to force blue and yellow to maintain a stable position on the retina
- This forced two colours to merge causing an impossible colour to appear between blue and yellow
- Subsequent efforts to replicate the findings failed
Equiluminance
- A more recent replication of Forbidden Colours
- Billock & Tsu (2010)
- suggests Forbidden Colours may be created by using Equiluminance
Hue Cancellation Experiments
- Developed by Hurvich & Jameson (1957) as a way to quantify opponency
- Measured how much of the opposing colour (red) is needed to be added to cancel out the green
Unique Hue
- Any of the four colours that can be described with only a single colour term:
- Red
- Yellow
- Green
- Blue
- We can use the hue cancellation paradigm to determine the wavelengths of unique hues
Qualia
- Private conscious experiences of sensation and perception
- The question ‘is my perceptin of blue the same as your perception of blue?’ is a question of qualia
Does everyone see colours the same way?
- Yes - Lindsey & Brown asked Americans to name colour patches and everyone used the 11 Basic Colours
- Maybe - Various cultures describe colour differently
Cultural Relativisim
- In sensation and perception it is the idea that basic perceptual experiences may be determined in part by the cultural environment
Colour Anomalous
- About 8% of the male and 0.5% of the female population has some form of colour deficiency (colour blindness)
- Most colour blind individuals can still make discriminations based on wavelenght
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Types of Colour Anomalous
- Deutaranope
- Protanope
- Tritanope
- Cone Monochromat
- Tetrachromat
- Rod Monochromat
- Achroatopsia
- Anomia
Deuteranope
An absence of L-Cones
Protanope
Due to an absence of L-Cones
Tritanope
Due to an absence of S-Cones
Cone Monochromat
Has only one cone type; truly colour blind
Tetrachromat
Due to having an extra cone
Concetta Antica - tetrachrome painter
Rod Monochromat
- Has no cones of any kind or type
- Truly colour blind and very visually impaired in bright light
Achromatopsia
Inability to see due to cortical damage
Anomia
- Inability to name objects or colours
- In spite of the ability to see and recognise them
- Typically due to brain damage
Colour Contrast
- A colour perception effect in which the colour of one region induces the opponent colour in a neighboring region
- When many colours are present, they can influence each other
Colour Assimilation
- A colour perception effect in which two colours bleed into each other, each taking on some of the chromatic quality of the other.
- Colours very rarely appear in isolation. Usually, many colours are present in a scene.
Afterimages
A visual image seen after a stimulus has been removed
Negative Afterimage
- An afterimage whoes polarity is the opposite of the original stimulus
- Light stimuli produce dark negative afterimages
- Colours are complementary
- This is a way to see opponent colours in action
Colour Constancy
- The tendency of a surface to appear the same colour under a fairly wide rangeof illuminants
- To achieve colour constancy we discount the illuminant and determine what the true colour of a surface is regardless of how it appears
Why is colour constancy possible
- We assume light sources and surfaces are constant in colour
- Bananas always look yellow in the dark and bright light
- One study showed ppl perceived a yellow banana even when the picture was grey.
- Produced a blue banana to remove the visual illusion
- Example of Opponenet colours
Colour Vision and Human Behaviour -
Ellio & Niesta, 2008
Males perceived women as more socially desirable and attractive when they were wearing Red clothes (Ellio & Niesta, 2008) or lipstick (Gueguen, 2012).
Colour Vision and Human Behaviour -
Hill & Barton 2005
- Analysed outcomes in four sports in the 2004 Olympics and found athletes clad in red were more likely to win.
Colour Vision and Human Behaviour -
Sanches-Vives et al., 2013
Found that shining a red light on a virtual arm lowered pain tolerance for heat stimuli.
Colour Vision and Human Behaviour -
Elliot et al., 2007
Participants who were exposed to the colour red performed significantly worse on an IQ test compared to green or grey
