-perceptions not in agreement (“pure” blue?) Perception of Colour Defining colour: ”a quality such as red, blue, green, yellow, etc., that you see when you look at something” (Mirriam Webster) -”perceptual attributes that correspond to differences in wavelengths of electromagnetic energy”? Nanometre = billionth of a metre Problems: -perceptions not in agreement (“pure” blue?) -no guarantee that the internal experience is similar between us -colour perception is not in a 1:1 relationship with wavelength

Physical/psychological properties: wavelength colour intensity brightness How many colours can we experience/see? ~200 jnds

The problem with that method: Not all ‘colours’ are represented in the “visible spectrum”

Physical/psychological properties: wavelength colour intensity brightness Amount of white light mixed in Saturation

there are over 7 million distinguishable colours. When you factor in variations in the intensity and purity of the light, there are over 7 million distinguishable colours.

Colour-perception phenomena requiring an explanation:

Colour-perception phenomena requiring an explanation: 1) Complementary colours A version of Newton’s Colour wheel Generally speaking, Red-green and blue-yellow are the two basic complements

Subtractive mixing

Subtractive mixing

Subtractive mixing

Subtractive mixing

Subtractive mixing

Subtractive mixing

Subtractive mixing

Colour-perception phenomena requiring an explanation: 1) Complementary colours 2) Negative Afterimages x

Colour-perception phenomena requiring an explanation: 1) Complementary colours 2) Negative Afterimages 3) Colourblindness

Theories of Colour Vision (Young/Helmholtz, circa 1800)

Colour-perception phenomena requiring an explanation: 1) Complementary colours 2) Negative Afterimages 3) Colourblindness

Theories of Colour Vision (Young/Helmholtz, circa 1800) (Hering/Hurvich, circa 1860)

Colour-perception phenomena requiring an explanation: 1) Complementary colours 2) Negative Afterimages 3) Colourblindness

Hurvich demarkates the opponent-process ‘mechanisms’ via psychophysics

560+630: r/g cancels out, left with yellow

Early evidence: three cone types!

Colour-perception phenomena requiring an explanation: 1) Complementary colours 2) Negative Afterimages 3) Colourblindness (red receptor deficit) (green receptor deficit) (blue receptor deficit)

Early evidence: three cone types! Subsequent work: opponent-process cells in visual pathway (e.g. LGN and beyond)

Proposed neural wiring to go from trichromatic to opponent-processing codes:

But these two models can’t account for all of colour perception either. Problem: both rely on the decoding of wavelength information. In impoverished situations, they work just fine

Anomalous Colour Illusions

Anomalous Colour Illusions

Anomalous Colour Illusions

Edwin Land: “Mondrians” But these two models can’t account for all of colour perception either. Problem: both rely on the decoding of wavelength information. In impoverished situations, they work just fine Edwin Land: “Mondrians”

100 100 100

Choice: 100 100 100

100 100 100

100 90 100 9 100 10

100 90 100 9 100 10 90% 9% 10%

100 90 100 9 100 10 90% 9% 10%

100 90 100 9 100 10 90% 9% 10 10% 90 10

100 90 100 9 100 10 90% 9% 10 10% 10% 90% 90 10% 10

100 90 What if we make the “green” diamond reflect the same proportions as the red triangle? 100 9 100 10 90% 9% 10 10% 10% 90% 90 10% 10

100 90 100 9 100 10 90% 9% 90 10% 10% 90% 9 10% 10

900 90 100 9 100 10 90% 9% 90 10% 10% 90% 9 10% 10

900 90 10 9 100 10 90% 9% 90 10% 10% 90% 9 10% 10

Choice: Why? The answer appears when we look at what the triangle is reflecting back under these illumination conditions 900 90 10 9 100 10 90% 9% 90 10% 10% 90% 9 10% 10

Choice: 900 810!!! 10 .9!! 100 10 90% 9% 90 10% 10% 90% 9 10% 10

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another.

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another. Whichever retinex is ‘most predominant’ for an object, will be assigned that colour by the brain

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another. Whichever retinex is ‘most predominant’ for an object, will be assigned that colour by the brain

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another. Whichever retinex is ‘most predominant’ for an object, will be assigned that colour by the brain 20% 90% 15% 90% 10% 10% 20% 80%

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another. Whichever retinex is ‘most predominant’ for an object, will be assigned that colour by the brain 20% 90% 15% 2 2 1 90% 10% 3 3 1 10% 20% 80% 3 1 2

A third alternative: Land’s Retinex model There are three ‘retinexes”, which examine how much red, green, and blue light is reflected off of all the surfaces relative to one another. Whichever retinex is ‘most predominant’ for an object, will be assigned that colour by the brain 20% 90% 15% “BLUE”! 2 2 1 90% 10% 3 3 “RED”! 1 10% 20% 80% “GREEN”! 3 1 2

A third alternative: Land’s Retinex model Choice: 900 810!!! 10 .9!! 100 10 90% 9% 90 10% 10% 90% 9 10% 10

McCullough Effect