PSYU2247 - Colour Vision week 7 PDF

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Colour Vision Wavelength and Colour What is wavelength? –

Light is (kind of) a wave

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Wavelength is the distance between the crests (nm)

What is intensity? –

Intensity is the wave’s “height”

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Approximates to brightness

Different pure, single wavelengths appear to us as different colours –

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Wavelength is an objective property of the stimulus •

Short wavelengths look purpleish/blueish;

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Medium wavelengths look greenish

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Long wavelengths look reddish

Colour is a subjective property of the percept

Colour misconceptions •

“There are 7 colours: Red, Orange, Yellow, Green, Blue, Indigo & Violet” –

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Newton did divide the spectrum into 7 different named categories, but in reality the spectrum is a continuum

“White and black aren't colours: they’re shades” –

Colour is an experience of a combination of wavelengths of light

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Therefore, black and white qualify

Reflection of light  

Objects reflect light, or absorb (subtract) it They appear to be= - …The colour of the light that they reflect - …black if they absorb lots of light at all wavelengths, and reflect little - …white if they absorb little light, and reflect lots at all wavelengths - …yellow if they absorb all else

Additive Colour Mixtures   

We can add various pure wavelengths of light together to form new colour Although the combination of short & long wavelength (blue & red) light makes purple, the wavelengths of light have not changed Light containing all wavelengths together appears white

Subtractive colour mixtures •

Mixing paint is a subtractive process

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Most paint reflects more than just a single, pure wavelength of light

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Blue paint reflects mainly short wavelengths (blue) of light, but also some of the neighbouring wavelengths (purple & green)

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Yellow paint reflects mostly yellow light, but also a bit of green & orange

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Putting both together, the new patch reflects only green

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The more colours of paint you add, the more light is absorbed/subtracted and the darker the patch looks

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Rarely in nature is there one single pure wavelength

Photoreceptors •

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Rods -

Only one type None in central fovea High sensitivity Night vision Scotopic

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Several types, each tuned to a different wavelength Mostly in fovea Lower sensitivity Day vision Photopic

Cones

Scotopic (night time) Colour Vision

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At low luminance (e.g. night vision), cones are not sensitive enough to respond - rods only

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Our colour vision with rods alone is poor: almost non-existent

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“principle of Univariance”

Colour Vision with One Receptor Type •

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Receptors show broad wavelength tuning –

For rods, probability of absorbing (“catching”) photons is highest for wavelength of ~500nm (greenish), smaller probability of absorption for other wavelengths

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Hence, peak response at ~500nm, but smaller response to most wavelengths of visible light

Principle of Univariance –

A receptor’s activity is related to the number of photons it catches, not to the type (wavelength) of photon

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Same response to a single wavelength at 450 (blueish) or to 650nm (orangeish) light at 160cd/m2

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Response to 550 nm (green) light can be the same also if intensity is reduced to 80cd/m2

One lone receptor can only tell light from dark –

Any wavelength can look like any other wavelength, given the right intensity

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Hence bad colour vision at night (rods only)

Photopic (day time) Colour Vision •

Rods are all “bleached out” (over-stimulated): It’s down to the cones

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Unlike rods, there are several different types of cone, each tuned to a different wavelength

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Colour vision in daylight is good: humans can reliably discriminate at least 200 wavelengths

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How many different types of cone are there? 200?

Young’s Colour Theory

The Young-Helmholtz theory states that within your eye are tiny cells that can receive waves of light and translate them into one of three colors: blue, green, and red. These three colors can then be combined to create the entire visible spectrum of light as we see it.

A Two receptor system •

What if we had 2 cone types? –

Colour represented by the relative activation of the 2 cone channels (call them “M” and “L”)

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As wavelength changes, pattern of relative activation changes

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Short wavelengths would produce more activity in M cones than in L cones

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Medium wavelengths would produce equal activity in each cone type (see fig.)

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Long wavelengths: L activity > M activity

As intensity changes, activity changes equally in both channels: hence the relative activation remains the same •

Bright medium wavelengths produce equal high activity in each cone type

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Dim medium wavelengths produce equal low activity in each cone type

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Allows us to tell a wavelength change from an intensity change

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Two different physical stimulus configurations that appear identical

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Physics is different but perception is identical

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For a 1- receptor system (e.g. rods at night) all wavelengths can be metamers for each other (so long as you get the intensities right)

Metamers

With a 2-cone system… – …we could match any given wavelength by adjusting the intensities of almost any two other wavelengths (metamers) – …there would be a single wavelength that appears grey. This is known as a “neutral point”, where the firing of both channels is equal Colour matching

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Colour Matching Experiments (Young, 1802) –

To match any single, pure wavelength of light (i.e. create a metamer), humans need 3 (or fewer) “primaries” of adjustable intensity

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Any 3 wavelengths of light can be used (so long as they are suitably different)

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Like your TV screen, or the projector in this room

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Most use single wavelengths that appear Red, Green & Blue

From this we infer that most observers have 3 cone types

Young-Helmholtz Trichromatic Theory of Colour Vision •

Developed many decades before neurophysiologists discovered & measured the 3 separate cone types

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There are only 3 receptors, each broadly tuned to wavelength

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Perceived color depends on the relative strength of their activation

Three Cone Types •

Long (peak 560 - reddish)

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Med. (peak 530 - greenish)

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Short (peak 420 - blueish) – Blue light is often blurred •

Few in the fovea

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More sparse

Colour Misconceptions #3 •

“Red, Yellow and Blue are the primary colours”

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“Red, Green and Blue are the primary colours” •

If we’re talking about the 3 different photoreceptor types, we should really refer to Short (~420nm), Medium (~530nm) & Long (~560nm) wavelengthtuned photoreceptor types.

Trichromatic Metamers

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Colour encoded by the pattern of relative activation of each channel

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Like population coding in spatial vision, olfactory perception, etc.

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Observe the levels of cone activation for 600nm (red), 505nm (green) and 545nm (yellow)

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Imagine what the cone activation levels would be like if red (600) and green (505) wavelengths were presented together –

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Add together the Short wavelength cone response to red & to green, then do the same for the other two cone types

Activation pattern for red + green presented together is the same as for yellow presented alone

Colour Deficiency •

Different types associated with different cone types

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Anomalous Trichromats –

Have 3 types of cone, but responses are different from most (normal) people, due to deficiency in one cone type •

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Dichromats –

Lack one cone type entirely •

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Protanomaly (red/long cone), Deuteranomaly (green/medium cone), Tritanomaly (blue/short cone)

Protanopia/Protanopes (red/long cone), Deuteranopia/Deuteranopes (green/medium cone), Tritanopia/Tritanopes (blue/short cone)

But some people are truly colour blind (i.e. can only tell light from dark) –

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Monochromats •

Cone monochromats have only one type of cone & lack colour vision day & night

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Rod monochromats have no cones at all Highly sensitive - need dark glasses in daylight

Cerebral Achromatopsia •

Colour Misconceptions

Damage to area V4 can cause complete “colour blindness”

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Colour blind people see only black and white”

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Depends what you mean by “colour blind” –

The majority of colour deficient people (dichromats and anomalous trichromats) can discriminate many colours

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Monochromats (only one cone, or only rods) and those with complete Cerebral Achromatopsia can only discriminate light from dark (truly “colour blind”)

Colour Opponency (Hering, 1872) •

Observations –

When asked to select “pure” colours, people pick 4, not 3

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Humans describe the world using 4 colours •

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Reddish-blue, or yellow-green, but never reddish-green or blueish-yellow

Inference –

Two opponent colour axes: red-green and blue-yellow

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For full colour space, we need to include the black-white axis

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Grey is neutral: neither red nor green; neither blue nor yellow; neither black nor white

Dual Process Theory •

Hurvich & Jameson (1957): dual process theory wherein trichromatic receptors (cones) feed an opponent process – Luminance (B&W) Green

=

Long + Mid

or

Red +

– Red&Green Green

=

Long ÷ Mid

or

Red ÷

– Yellow&Blue Blue

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(Long + Mid) ÷ Short or

(Red + Green) ÷

Physiology of Opponent Processing

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Cells have been found in LGN and V1 which show antagonistic cone inputs

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Some also show a centre-surround arrangement - “double opponent” cells

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R/G cells are Parvocellular, B/Y are Koniocellular, B/W (luminance) are Magnocellular

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V1 hypercolumns contain “blobs”: cylinders where opponent colour cells are located

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Blobs - class of neurons with very simple circular-surround fields; projections from parvocellular LGN that retain color information

Colour constancy •

Objects generally look the same colour in a wide variety of lighting conditions

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Land (1977) – Isolated coloured patch appears to change colour when lit by different wavelengths – When this patch is part of a Mondrian Pattern, it appears to be the same colour regardless of the illumination...