W3: Colour Perception

THE DRESS: Why do people see it in different colours?

• Over-exposure of the image.

• Different viewing angles/screens affecting color perception.

• Varying color terminology used among individuals.

• Color blindness.

• Individual differences in individual color processing

• Individual differences in individual experiences.

• Failures in color constancy leading to discrepancies in perception.

(1) The image is overexposed

• Over-exposure makes colors in the photograph differ from the actual dress.

• However, people varied in how they saw the colours in the PHOTOGRAPGH not between the pic + irl

• SO = over-exposure = not the only cause

(2) Different viewing conditions

• Evident variations in perceived colors due to:

  • Changes in image size

  • Changes in viewing angle

• Laboratory findings (Sousa, Hermann and Conway, 2015):

  • 53% of individuals perceive the dress as blue-black

  • 40% as white-gold

• SO = not just viewing conditions

(3) Problems naming colours

• Different names used for perceived colors.

• It was a forced-choice question

  • ‘is this white and gold or blue and black?’

• Forced-choice questions can lead to variances in responses.

Investigating colour naming

1. Naïve observers tend to provide similar answers in open-ended questions = forced-answer q is not the issue

2. Distinctions appear when asked to match colors; groups favor different matches.

3. SO = Naming discrepancies do NOT fully explain color perception differences.

Visible Spectrum

• Overview of light wavelengths and how they correspond to different types of light, including:

  • Cosmic rays, X-rays, ultraviolet, infrared, and visible spectrum.

Where is colour processed?

• Color processed across various areas:

  • Retina ➔ LGN ➔ V1 ➔ V4/V8

• Dorsal stream = ‘Where / Action’

  • involved in motion detection

  • 3D shape recognition

  • actions (events, sequences)

  • emotions

• Ventral stream = ‘What / Perception’

  • involved in form perception

  • object identification

  • facial recognition

  • colour

Photoreceptor types: Rods and Cones

• Rods: Responsible for night vision, monochromatic (one colour only)

• Cones: Three types (S, M, L) for color vision + daylight

  • Blue = S-type

  • Green = M-type

  • Red = L-type (daylight vision only)

• Sensitivity analysis of cones across different wavelengths (blue, green, red).

Colour Vision and the Visible Spectrum

• Visible spectrum = the part of the spectrum that human eyes are sensitive to

• Most mammals = only 2 cone types

• Humans, apes, monkeys = 3 cone types

• Many other animals = multiple (>3)

• Snakes = can see infra-red (we don’t bc we’re warm blooded)

• Birds + Insects = can see ultra-violet (we don’t bc it damages retina over long periods so we screen most of it out - birds don’t live as long as us)

Summary of Photoreceptors

• 3 types of cone photoreceptor for daylight vision

• Each type responds to a range of colours = but most strongly to one wavelength (colour)

• With 1 cone type = can see a range of colours BUT cannot discriminate between them

• With 2 tuned to blue + yellow = can discriminate blues from yellows BUT not green-yellow-orange-red

• With 3 = can discriminate all colours in the visible spectrum

• We only have one type of rod photoreceptor for night vision = we are colour blind at night

With 1 cone type we can…

see a wide range of colours but cannot discriminate between them

With 2 cone types tuned to blue + yellow we can…

discriminate blues from yellows but not green-yellow-orange-red

With 3 cone types we can…

discriminate between all the colours in the visible spectrum

Colour blindness

• most ‘colour blind’ people can see colour - just not properly

• some have mis-tuned cone type = they can’t discriminate certain colours well

• some have a missing cone type = dichromats

• common colour-blindness = mis-tuned green cone

  • red-green cone colour genes carried on X chromosome = red-green colour blindness is more common in men than women

• some women have 4 cone types = tetrachromats

• monochromats = 1 cone type (rare)

• v rare = rod monochromats = no cones = truly colour blind + cannot see in daylight

Common colour deficiencies

• Protanopia = no red

• Protanomaly = miss-tuned red

• Deuteranopia = no green

• Deuteranomaly = miss-tuned green

• Tritanopia = no blue

True colour blindness

• Cone monochromates = only 1 cone type

• Rod monochromats = no cones at all (cannot see in daylight)

• = both very rare

Cerebral achromatopsia

= cortical colour blindness

• following a stroke / injury = patient cannot see colour at all

• photoreceptors, retina, LGN, V1 = all intact

• stroke affects V8 region of visual cortex

(4) Colour blindness and the dress

• the number of people who are colour blind is much LOWER than the people who saw the dress as an ‘odd’ colour

• the colours in the photograph = blue + yellow/brown

• the number of people with blue cone colour blindness = v low (<0.01% of population)

• SO, colour blindness CANNOT explain the dress illusion

How colours are processed in human vision = colour opponency

2 colour systems:

1. Ancient luminance + yellow/blue system (common in most mammals)

2. Newer red/green systems = genetic adaptation of the yellow part of the older system

Colour opponency

• One member of the color pair suppresses the other color.

• We do see yellowish-greens and reddish-yellows, but we never see reddish-green or yellowish-blue color hues.

• Opponent Colors: Our vision system doesn’t just detect individual colors—it compares them.

• There are two main opponent color pairs:

  • Red vs. Green

  • Blue vs. Yellow

    This means that when one color in a pair is active, the other is suppressed. For example, if you see red, green is reduced.

Colour opponency image

Colour opponency in retina and LGN

• the Red-Green, Blue-Yellow appearance is misleading

• Red-Green is closer to what we would call reddish and greenish orange

• Yellow-Blue is closer to lime-violet

• the centre-surround properties of retinal ganglion cells = provides the mechanism for colour opponency

• these cells calculate the ratio of colours and provide edge detection mechanism

(5) Individual differences in colour processing and the dress

• blue + brown dots are measured colours from the dress image

• most of the variation is on the blue-yellow system

• the red dots show peoples setting for neutral grey/white

• SO - what people see as neutral varies between the colours in the dress image

• SO - individual variations in the blue-yellow system could explain people’s perception of the dress

Inhibition over space + centre-surround + simultaneous colour contrast

• perhaps the spatial context in which you view the dress can change its colours

Inhibition over time: colour after-effects

• perhaps the lighting conditions over time in which you view the dress can change its colours

Excitation in cortex - colour filling in

• colour centre-surround cells in the retina only transmit the colour edges = the cortex must reconstruct the body colour of the object

• water colour illusion: these images show only the edges outlined in colour

  • they pass through the retina to the cortex

  • cortex automatically fills in the gaps with a sense of colour

• SO - maybe the stripes in the dress could facilitate filling in + alter the perception of the dress

(6) individual differences - experience and the dress

• things that alter colour perception

  • simultaneous colour contrast

  • colour after-effects

  • water colour illusion (filling in)

• simultaneous colour contrast + filling in might explain any effects = due to surrounding colours

• colour after-effect might explain = short term changes over-time

• surrounding + ambient colours do change over time (e.g. daylight vs indoor)

• SO - these effects suggest variability over time (but within observers)

• SO - not enough to explain the data collected under lab conditions

Colour constancy

• the colour of light reflected from objects depends on:

  • their colour

  • and the colour of the light that shines on them

• we don’t notice objects changing colour when ambient light changes (dawn-dusk)

• to do this = we ‘ignore’ the light source

• we do not know how this is done

• where = occurs in V8 (sub-section of area V4)

• colour constancy = the ability to perceive colors of objects, invariant to the color of the light source

  • called white balance correction (on a camera)

Cortical areas V1, V2, V4 + V8

• colour sensitive cells in V1 + V2 cluster into regions of cortex that are separate from the rest of the cells

• Colour signals are then passed to a sub-section of V4 = called V8

• V8 = responsible for the conscious perception of colour

Area V4/V8 + Colour Constancy

V1 vs. V4/V8 Areas in the Brain:

• V1 (Primary Visual Cortex) = processes basic color information but does not adjust for changes in ambient light.

• V4/V8 (Higher Visual Areas) = help correct for lighting changes, making colors appear more stable.

Colour perception:

• V1 response to white patch when ambient light its green = response changes bc V1 does NOT discount ambient light

• V4 response to white patch when light is green = stays the same bc V4 DOES discount ambient light

(7) Colour constancy and the dress

• In dress image = no context to tell actual colour of ambient light

• SO - subconsciously pick either yellowish or blueish light

• light source colour you pick = determines perception of dress

  • people who saw white-gold = assumed it was daylight = brain ignored bluer wavelengths

  • people who saw blue-black = assumes warm, artificial light = brain ignored redder wavelengths

• this likely depends on the yellow-blue system = which is tuned to highlights and shadows

• SO - individual differences in the yellow-blue system may promote one precept over the other

• contrast effects and after-effects may produce additional biases over time = causing some people to change their precept

Key points about colour vision

• We get colour vision by having 3 cone photoreceptor types

• rods = only support night vision (we are all colour blind at night)

• most colour-blind people just have one missing or mis-tuned cone type

• cerebral achromatopsia / cortical colour blindness = results from a loss of area V8

  • suggests this area is responsible for conscious colour perception

• the colour system is arranged like spatial vision with:

  • centre-surround cells in the retina - centre-surround prefer diff colours

  • filling in (probs in area V1)

  • SO - we get simultaneous colour contrast + colour after effects + water colour illusion

• colour constancy = our ability to ignore the colour of the light source (in area V4/V8)

Protanopia

no red cone

Protanomaly

mis-tuned red cone

Deuteranopia

no green cone

deuteranomaly

mis-tuned green cone

tritanopia

no blue cone