Wk 6: Colour perception, colour mixing and types of colour vision

6a: Colour perception

• how colour info is encoded

  • colour mixing

  • metamers

• models of colour processing

• retinal based colour ‘blindness’

  • tests of colour abnormalities

• colour blind visual processing

• ways to define diff colours

colour - perceived colour

• defined by wavelength \lambda lambda

• EMR: electromagnetic radiation 

  • energy that travels in waves and includes a broad spectrum

  • visible 400-700 nm (10^-9m)

wavelength order: ROY G BIV

• red, orange, yellow, green, blue, indigo, violet

definitions

hue:

• the actual colour

• e.g. red, green

saturation: 

• amount of colour relative to white

• great saturation = great vividness

• less sat = washed out colour

brightness

• light intensity

narrow or broadband

cells that mediate (bring about) colour perception can either be broadband or narrow band

• narrowband: 

  • sensitive to a narrow stimulus dimension

  • cell responds strongly to a narrow range of wavelengths

  • typically tuned to a specific colour

• broadband:

  • sensitive to a broad range of stimulus dimension

  • cells respond to a wide range of wavelengths

  • often spanning multiple colour channels

drawback of a narrowband system

• need lots of cells to encode the entire stimulus dimension

• low sampling density

  • with more cell types spread out across the retina, each type becomes less densely packed

  • hence low spatial resolution

  • fine details not detected well

• poor spatial acuity

  • because each cone type is spaced further apart, the system struggles to resolve small or closely spaced visual features

  • this is especially problemati in tasks requiring sharp vision

  • e.g. reading or detecting edges

tuning of individual cones

• individual cones are tuned/selective to colour

  • specific cones need to feed into specific colour-sensitive retinal-ganglion cells to give them their specific colour sensitivity

  • these specific RG cells feed into specific colour sensitive v1 cells

• individual cones are not tuned to orientation or motion

  • v1 simple cells are tuned to orientation so that means a given cone can feed into many simple cells that are tuned to different orienations

  • same for complex cells that are tuned to motion

  • reasons why the brain can have narrow-band tuning for orientation and motion and not impair spatial acuity for those stimulus dimensions

### 🟥 1. What cones are tuned for: Colour

- Cones = colour detectors in your retina (red, green, blue types).

- Each cone connects to a specific retinal ganglion cell that’s sensitive to colour.

- These ganglion cells then pass colour information to V1 cells (in the brain’s visual cortex) that are also tuned to colour.

- 🔑 Key idea: Colour sensitivity is preserved all the way from cone → ganglion cell → V1 cell.

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### 🟦 2. What cones are not tuned for: Orientation or Motion

- Cones themselves don’t care about which direction a line is tilted (orientation) or whether something is moving.

- But V1 cells do care about those things:

- Simple cells in V1 are tuned to orientation.

- Complex cells in V1 are tuned to motion.

- A single cone can send signals to many different V1 cells, each tuned to different orientations or motion directions.

- 🔑 Key idea: This setup allows the brain to be very precise about orientation and motion without losing detail (spatial acuity).

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### 🧠 Summary

- Colour tuning starts early (at the cone level) and stays specific.

- Orientation and motion tuning happen later (in the brain), using flexible input from cones.

- This division helps the brain process different visual features efficiently without overlap or loss of detail.

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Let me know if you’d like a diagram or analogy to go with this—like a “cone-to-brain” signal pathway or a metaphor for how tuning works!

broadband and narrowband: effect on mixing

• mix stimuli that are processed by a broadband system

  • e.g. add red and green together, you’ll perceive a new colour - yellow

  • broadband systems blend input into one unified perception

• mix two stimuli that are processed by a narrowband system

  • e.g. combine vertical and horizontal linesm you’ll perceive both line separately 

  • these systems don’t blend

  • they keep distinct features apart

pooling of broadband and narrow cells

• how the outputs of those cells are pooled

• applies to cells that are looking at the same region of visual space

• broadband tuning

  • e.g. colour

  • total population average

  • relative activation of the three cone types

• narrowband tuning

  • e.g. orientation and motion

  • local populaiton average of cells tuned to similar orientations

  • but not cells tuned to markedly different orientations/direction

  • only pool output from cells that are processing that stimulus and so depends upon the tuning bandwidth of the cells

population response

definition: how a group of neurons responds to a stimulus. The shape of the response (unimodal vs bimodal) depends on how neurons are tuned (broadly or narrowly)

• broadband tuning

  • neurons respond to a wide range of stimulus types

  • e.g. colours

  • when you mix colours, the brain looks at the relative activation of these cones

  • create a unimodal response

  • one clear peak of activity

  • you perceive a single colour as a result 

• narrowband tuning

  • responds to very specific stimuli

  • e.g. line andle or motion direction

  • bimodal response

  • two separate peaks of activity

  • result: perceive both stimuli distinctly

when is a stystem narrow or broadband?

spectrum: energy at each wavelength for a given light

monochromatic light: light of a single wavelength, spectrum is a spike at that wavelength

composition of white light

• can create white light with 2 complimentary colours

• 3 correctly chosen colours can generate all colours

• major colour in the spectrum

  • red, orange, yellow, green, blue, indigo, violet

  • ordered in terms of decreasing wavelength. 

6b: Colour Mixing

what do we perceive when we add diff colour together 

how does the brain process colour iinfo

metamers: physically diff stimuli but perceived the same

• colour mixing studies tell us: people determined how the colour system worked

• behavioural studies tell us: a function map of the system

metamers

e.g. yellow

• spectral yellow

  • a single wavelength of light (a)

• mixed yellow

  • combine multiple long-wavelength light (red+green) (b)

why this happens

• our visual system uses three types of cones (red, green, blue)

• diff light combinations activate these cones in the same way

• hence we perceive the same colour

types of colour mixing

additive mixing

primary colours: RGB

• combining light sources of different colours

• resultant spectrum is the sum of the component spectrums

• more light in the mixture than in the components

• brain interprets the combined cone activation as a new colour

methods

• projection

  • project different colour onto the same region

• spatial summation

  • project to points in close proximity

  • e.g. red, green, blue points of light that are so close together the brain can’t spatially resolve them, so the brain averages them 

  • like a TV/phone scren

• temporal summation

  • temporally interleave different colours faster than the brain can resolve them

  • faster than our temporal acuity limit

  • the brain adds up signals that arrive close together in time

  • if brief flashes of light occur rapidly enough, they’re perceived as continuous or brighter

  • if diff colours are flashed quickly in sequence, the brain can average them over time

subtractive mixing

primary colours: CMY cyan magenta yellow

• light is removed, so there is less light in the mixture than in the components

• only the light that is common to both components remain

• combining materials that absorb certain wavelengths

• perceives wavelengths that are left over after absorption

methods

• add colours to remove light

• less light in the mixture than in either component

• e.g. adding paints together, stacking lenses together

mixing the spectral colours

what if we mix all ROYGBIV colours

colour matching

process of recreating a target colour by combining other colours

• minimum no. of primary colours

  • 3 to maky any colour

  • 2 to make white

implications for colour mechanisms

• for people with standard colour vision, any colour can be matched by three appropriately chosen primaries

• related to having three cones types

  • long, medium, and short LMS lambda sensitive

  • also called RGB

6c: Types of Colour Vision

consider the type of vision a person would have if they had one, two or three diff cone types

monochromatic vision

• only one type of cone

• this system cannot discriminate colours, only light intensity

• the response of the receptor to diff wavelengths can be matched by varying the relative intensities of the lights

  • e.g. it might abosorb more light at 500nm than 550nm

• a dim light at one wavelength can produce the same response as a brighter light at another wavelength

  • e.g. 500nm has same response as 550nm with twice the intensity

• can only encode intensity: black and white vision

  • shades of grey

  • similar to how the rod system works

• monochromatic light at neutral point will drive them both equally

  • ratio of response 1:1

• monochromatic light at neutral point is perceived as white

  • white light also drives both types of detectors equally and so is signalled b relative activity of 1:1

dichromatic vision

• two types of cones

• can encode colour

• limited range of distinct colours that can be perceived

• only two distinct colours

• need only two primaries to match any colour

• presence of a neutral point

  • wavelength at which the two curves cross over

  • excites both receptors equally

trichromatic

• colour perception based the relative activity of three receptor types

• need three primaries to match all colours

• no neutral point

• no wavelength excites all receptors equally

tetrachromatic

• four types of cones

• affects on human colour perception

  • subdivide the spectrum into more bands (10 vs 7)

trichromatic theory

• three types of ‘fibres’ in the eye

• perceived colour is based on the ratio of LMS activity

• physical ratio of LMS cones are about 32:16:1

results that can’t be accounted for by trichromatic theory

• colour afterimages

  • e.g. view red, green afterimage

• colour naming

  • asked to assign a name to monochromatic lights

  • can’t describe all colours using RGB

  • need to add Y

  • can’t get certain combinations

    • e.g.can’t get RG but can get RB

opponency model

colour vision is mediated by three opponent mechanisms

opponent channels: 

• red, green

• blue, yellow

• white, black

trichromacy vs opponency

pairing

our colour perception is based up

• having three cone types and their relative activation

• opponent mechanisms is how the brain encodes their relative activity

• note RGB the same as LMS

• LMS often used to make it clear that they are not only sensitive to RGB

colour bliind processing

• Magnocellular and parvocellular distinction

  • M: only luminance sensitive (colour blind), no colour info

  • P: colour and luminance sensitive