2. Colour & Lightness Constancy

What is colour?

Visible light forms a narrow band of frequencies in the electromagnetic spectrum

What is the electromagnetic spectrum?

Have different frequencies) or wavelengths) which have different hues, ranging from red (long WL light) to violet (short WL light)

What is 1 millimetre equivalent to?

1,000,000 nanometers

What does the whole spectrum cover?

400nm

What do humans see between?

0.00000390 and 0.00000750 meters

How do objects have different colour?

Different objects absorb and reflect different wavelengths of light

What does the colour of objects depend on?

the light source

What is the wavelength of the light reflected only determined by?

The hue which is seen

What is perceived colour determined by?

1. Intensity of the reflected light (how bright it is) 2. The saturation of the colour (how much white light is mixed in the pure hue)

What is the psychological attribute of the property of light; wavelength?

hue (colour)

What is the psychological attribute of the property of light; intensity?

Brightness (perceived intensity)

What is the psychological attribute of the property of light; spectral purity?

Saturation (i.e. how much colour/ how much white)

What is Helmholtz's Trichromatic theory?

Proposes three types of cones; short, medium and long- Known as red, green, blue but not exactly these colours so refer to them as S,M & L

What is Hering's Opponent Process Theory?

3 opponent processes:- red-green- blue-yellow- black-white, It explains subjective of 4 primary colours and colour after effects

What is the Dual Process Theory?

Trichromatic stage involves the retinal/ photoreceptors and the Opponent- Process stage involves the ganglion cells/ LGN

What is light constancy?

Lightness (greys, whites and blacks) remain constant despite changes in light levels

What is lightness?

The perceived shade of the surface. Concerns the perceptual quality (e.g. white/grey/black)

What is reflectance?

Proportion of light reflected from the surface. Concerns the physical quality (% photons reflected)

What is luminance?

Amount of light reflected from a surface (photons coming off the surface)

What is illumination?

Amount of light emitted from a light source (amount of photons hitting the surface in the first place)

What is the luminance, illumination and reflectance equation?

Luminance = illumination x reflectance

What are the problems with light perception?

Reflectance tells us about the lightness of the surface but we only receive information about luminance, not reflectance

What is an inverse problem with light perception?

A particular luminance could have been produced by infinite combinations of illumination and reflectance

What is lightness constancy?

The perception of constant lightness despite changes in viewing conditions

What are two theories of lightness constancy which have been disproved?

Adaption & unconscious inference

What is the theory of adaption (light constancy)?

Visual system becomes less/more sensitive in bright/ dull conditions. However, adaption is slow so cannot account for fast changes in lighting

What is the theory of unconscious inference (light constancy)?

Prior experience allows us to estimate illumination. However, we are not sensitive to absolute levels of illumination

What are relational theories?

Luminance ratios determine lightness perception (Wallach, 1948)

What did Wallach (1948) find through Relational theories?

Participants matched luminance rations rather than absolute luminance

What was the Craik-Cornsweet- O'Brien illusion (relational theories)?

edges are important. participants perceived different colours near edges. when blue blocks were put in front it showed the same colour

What is the retinex theory (relational theory) (Land & McCann, 1971)

Calculate luminance ratios at edges, Ignore any gradual changes in luminance (assume due to changes in illumination, tells you relative reflectance of surfaces

What are the problems of relational theories?

1. scaling problem 2. illumination edges

What is the scaling problem?

Fails to tell us the absolute level e.g. ratio 5:1 could be an infinite combination of two greys/ black. This can be solved by an anchoring heuristic

What is an anchoring heuristic?

Assumes highest luminance is white then scale all other regions relative to this (white balance)

What is illumination edges?

Retinex theory assumes all changes in illumination are gradual. This cannot account for sudden changes in illumination e.g. edges

What are reflectance edges?

Neighbouring regions have different reflectance e.g. different material or paint

What are illumination edges?

Neighbouring regions receive different amounts of light e.g. shadows, spot lights, change in orientation

What are planarity heuristics?

If depth information indicates that two regions are not coplanar, they are likely to be an illumination edge

What does coplanar mean?

Regions have the same 3D orientation

What are ratio magnitude heuristics?

If luminance ratio is very high, it is likely to be an illumination edge

What is colour constancy?

Perception of constant surface colour despite changes in illumination and viewing conditions

How does light and colour constancy differ?

Colour constancy is concerned about different wavelengths hitting the eye, not the photons hitting the eye

What is illumination spectrum?

Amount of incident illumination at each wavelength

What is reflectance spectrum?

Proportion of light reflected at each wavelength

What is luminance spectrum?

Amount of light reflected at each wavelength

What is the effect of surroundings on colour constancy (Land, 1983)?

Presented Mondrian patterns illuminated by different coloured lights. When whole pattern was visible, single patch not perceived to change colour when the illuminant changed

What is the effect of surroundings on colour constancy continued….(Land, 1983)?

When surroundings were masked single patches were perceived to change colour as the illuminant changed. Shows that surroundings help us to calculate the illumination and correct for it.

What helps our colour constancy?

Prior knowledge of typical colours of familiar objects

What was Hansen et al's study (2006) on colour memory

presented images of grey fruit on grey background, when fruit and background were identical shades of grey, observers still perceived the fruit to be coloured e.g. banana looked yellowish, when ppts had to set the colour of a banana to be grey, they set the colour to a blueish (opposing) colour- colour memory helps us to accurately perceive colour under different illuminations

What does cerebral achromatopsia suggest there is in the brain?

A specialised 'colour centre' that is responsible for colour perception

What do patients with cerebral achromatopsia experience?

1. They perceive the world to be shades of grey 2. Have access to wavelength information but cannot consciously perceive colour

What is the Primary visual cortex (V1)?

'Blobs'/ small patches of cells that respond to colour of stimulus (physical wavelength)

What is Area V4?

Cells respond to perceived colour

When conducting single cell recording, what do V1 and V4 respond to?

V1 cells respond to wavelength and V4 cells respond to perceived colour

What was Hadijkhani et al's study (1998) into brain areas stimulated by colour?

Areas V1, V2 ,V3 and V8 got excited

What was Hadijkhani et al's study (1998) into brain activity of colour after-effects?

Only V8 active, suggesting V8 may be the site of colour consciousness.