Psyc 367 (UAlberta- Farley) Ch. 9 Colour

Lightness Constancy

The constancy of our perception of achromatic colours as remaining relatively constant under different intensities of illumination.

determined by reflectance, not intensity

Hue Cancellation

Procedure in which a subject is shown a monochromatic reference light and is asked to remove, or "cancel", the one of the colours in the reference light by adding the opposing pair

Achromatic Colours

Colour without hue. White, black, and all the grays are examples.

perceived when light is reflected equally across the spectrum

Additive Colour Mixture

The creating of colours that occurs when lights of different colours are superimposed.

occurs when mixing lights of different wavelengths

Anomalous Trichromatism

A person who needs to mix a minimum of 3 wavelenths to match any other wavelength in the spectrum but mixes these wavelengths in different proportions than a trichromat.

Cerebral Achromatopsia

A loss of colour vision caused by damage to the cortex.

colour blindness

can co-occur with prosopagnosia

Chromatic Colours

Colour with hue, such as blue, yellow, red, or green.

perceived when certain wavelengths are reflected by objects more than others

Colour Circle

Perceptually similar colours located next to each other and arranged in a circle.

Colour Constancy

Perception of colours as relatively constant in spite of changing colour sources

works best when an object is surrounded by many colours

Colour Matching

A procedure in which observers are asked to match the colour in one field by mixing 2 or more lights in another field.

Colour Solid

A solid in which colours are arranged in an orderly way based on their hue, saturation, and value.

Complementary Afterimages

An afterimage that is on the opposite side of the colour circle from the inducing colour.

Desaturated

Low saturation in chromatic colours as would occur when white is added to a colour.

Dichromatism

Colour-blindness in which only 2 of 3 primary colours can be discerned

Dichromats

A person who has a form of colour deficiency.

Can match any wavelength in the spectrum by mixing 2 other wavelengths.

Double-Opponent Neurons

Neurons that have receptive fields in which stimulation of one part of the receptive field causes an excitatory response to wavelengths in one area of the spectrum and an inhibitory response to wavelengths in another area of the spectrum, and stimulation of an adjacent part of the receptive field causes the opposite response.

Hering's Primary Colours

The colours red, yellow, green and blue in the colour circle.

HSV Colour Solid

A solid in which colours are arranged in an orderly way based on their hue, saturation, and value.

Hue

The experience of a chromatic colour, such as red, green, yellow, or blue, or combinations of these colours.

another term for a chromatic colour/pure colour

Illumination Edge

The border between two areas created by different light intensities in the two areas.

changes in lighting

Ishihara Plates

A display of coloured dots used to test for the presence of colour deficiency.

The dots coloured so that people with normal (trichromatic) colour vision can perceive number in the plate, but people with colour deficiency cannot perceive these number of perceive different numbers than someone with trichromatic vision.

Lightness

The perception of shades ranging from white to gray to black.

not related to the total amount of light reflected by an object, rather the %

Memory Colour

The idea than an object's characteristic colour infliences our perception of that object's colour.

Metamerism

The situation in which 2 physically different stimuli are perceptually identical. In vision, this refers to 2 lights with different wavelength distributions that are perceives as having the same colour.

Metamers

Two lights that have different wavelength distributions but are perceptially identical.

colors that appear identical to the eye under some lighting conditions, but different under others

Monochromatism

Rare form of colour blindness in which the absence of cone receptors results in perception only of shades of lightness (white, gray, and black), with no chromatic colour present.

Monochromats

A person who is completely colour-blind and therefore sees everything as black, white, or shades of gray.

Can match any wavelength in the spectrum by adjusting the intensity of any other wavelength

poor visual acuity

very sensitive to bright light

Nonspectral Colours

Colours that do not appear in the spectrum because they are mixtures of other colours.

Opponent Neurons

A neuron that has an excitatory response to wavelengths in one part of the spectrum and an inhibitory response to wavelengths in the other part of the spectrum.

located in the retina + LGN

Opponent-Process Theory of Colour Vision

A theory originally proposed by Hering, claimed that our perception of colour is determined by the activity of 2 opponent mechanisms:

• blue-yellow, red-green, white/black.

  The responses to the 2 colours in each mechanism oppose each other, one being an excitatory response and the other an inhibitory response.

explains neural response for cells connected to the cones further in the brain

Partial Colour Constancy

A type of colour constancy that occurs when changing an object's illumination causes a change in perception of the object's hue, but less change than would be expected based on the change in the wavelenths of light reaching the eye.

Principle of Univariance

Once a photon of light is absorbed by a visual pigment molecule, the identity of the light's wavelenth is lost.

absorption of a photon causes the same effect the all receptors

This means that the receptor does not know the wavelength of the light that is absorbed, only the total amount of light it has absorbed.

Ratio Principle

A principle stating that 2 areas that reflect different amounts of light will have the same perceived lightness if the ratios of their intensities to the intensities of their surroundings are the same.

Reflectance Curves

A plot showing the % of light reflected from an object versus wavelength.

Reflectance Edge

An edge between 2 areas where the reflectance of 2 surfaces changes.

changes in material

Reflectance

The % of light reflected from a surface.

Saturation

The relative amount of whiteness in a chromatic colour. The less whiteness a colour contains, the more saturated it is.

more white = desaturation

more dark = value decreasing

Selective Reflection

When an object reflects some wavelengths of the spectrum more than others.

Selective Transmission

When some/certain wavelengths pass through visually transparent objects or substances and others do not.

Associated with the perception of chromatic colour.

Single-Opponent Neurons

Neurons that increase firing to long wavelengths presented to the centre of the receptive field and decrease firing to short wavelengths presented to the surround (or vice versa).

Spectral Colours

Colours that appear in the visible spectrum.

Subtractive Colour Mixture

The creation of colours that occurs

paints of different colours are mixed together.

adding more pigments to a mixture = fewer wavelengths being reflected + more being absorbed

Trichromatic Theory of Vision

A theory proposing that our perception of colour is determined by the ratio of activity in 3 receptor mechanisms with different spectral sensitivities.

all human colour vision is based on 3 principle colours

explains the responses of the cones in the retina

Trichromats

A person with normal colour vision. Can match any wavelength in the spectrum by mixing 3 other wavelengths in various proportions.

Unilateral Dichromat

A person who has dichromatic vision in one eye and trichromatic vision in the other eye.

People with this rare condition have been tested to determine what colours a dichromat perceive by asking them to compare the perceptions they experience with their dichromatic eye and their trichromatic eye.

Value

The light-to-dark dimension of colour.

Functions of colour perception

can be adaptive, help with object identification

facilitates perceptual organization of elements into objects

evolutionary advantage

Trichromatic theory: behavioural evidence

Colour-matching

observers with normal colour vision could match any reference wavelength by mixing the proportion of 3 wavelengths

Trichromatic theory: physiological evidence

cone pigments

came from measuring the absorption spectra of visual pigments in receptors: pigments were found to respond maximally to short, medium and long wavelengths

V4

originally proposed as a sort of “colour centre”

there’s areas that show considerable overlap between neural mechanisms for processing colour + other visual properties

type of dichromate: protanopia

affects 1% of males + 0.02% of females

missing long wavelength pigments

form of red-green colourblindess: red looks more green + less bright

type of dichromate: deuteranopia

affects 1% males + 0.01% females

missing medium wavelength pigment

form of red-green colour blindness: green looks more red

type of dichromate: tritanopia

affects 0.002% males + 0.001% females

missing short wavelength pigment

form of blue-yellow colour blindness: difficulty separating blue + green and red + yellow

chromatic adaptation

occurs with prolonged exposure to chromatic colours

ability of the eyes to adjust to change in light conditions + maintain the appearance of object colours

Intensity of light from an object

1. intensity of illumination; total amount of light hitting the object

2. object’s reflectance; proportion of light reflected back by the object