colour perception

Question 1

why is colour important?

Answer 1

aids discrimination and detection

Question 2

what key tasks is colour important in?

Answer 2

natural and human-made signals

scene segmentation

mating rituals

camouflage

visual memory - associating objects with colours

communication - might signal other’s emotions

Question 3

what was the study into how colour helps us recognise objects?

Answer 3

participants asked to label objects

in congruent condition, objects were presented in their usual colour

in the incongruent condition, objects were presented in a very unusual condition

slower reaction time for incongruent

Question 4

what is colour?

Answer 4

colour mainly determined by the wavelengths of light reflected by that object

Question 5

when do achromatic colours (white, black) occur?

Answer 5

when all wavelengths reflected equally

Question 6

what have higher wavelengths than what the human can see?

Answer 6

ultraviolet

x-rays

gamma rays

Question 7

what have lower wavelengths than what the human can see?

Answer 7

infrared

heat

microwaves

FM radio

television

Question 8

what is hue?

Answer 8

the quality that distinguishes red from blue

Question 9

what is brightness?

Answer 9

the perceived intensity of light

sometimes lightness

Question 10

what is saturation?

Answer 10

characterises a colour as pale or vibrant

Question 11

what kind of phenomenon is colour?

Answer 11

purely psychological - entirely subjective

Question 12

what is colour a property of?

Answer 12

our neural apparatus

for an object to appear coloured, we need to have correct photoreceptors and neuronss

Question 13

what is colour perception?

Answer 13

arises from ability of certain light rays to evoke a particular pattern of neural responses in the visual system

Question 14

what are metamers?

Answer 14

sensory stimulus that is perceptually identical to another stimulus but physically different

Newton demonstrated a light that appeared orange was indistinguishable from a light produced by combining a red and yellow light - colour metamers - suggests visual system producing identical neural responses to physically different stimuli

if can discriminate between two lights (appear different) neural representation of these stimuli must differ

if you cannot tell visual stimuli apart, then the physical property that makes them different must not be encoded the visual system

Question 15

what are photoreceptors?

Answer 15

first stage in the processing of visual information

Question 16

what are the three photoreceptor cone types?

Answer 16

S cone

M cone

L cone

Question 17

what are S cones?

Answer 17

short wavelength, blue

peak absorption at 420nm

Question 18

what are M cones?

Answer 18

medium wavelength, green

peak absorption at 530nm

Question 19

what are L cones?

Answer 19

long wavelength, red

peak absorption at 565nm

Question 20

what is principle of univariance?

Answer 20

any single photopigment is colour-blind, since an appropriate combination of wavelength and intensity can result in an identical neural response

Question 21

how do you differentiate between wavelengths and intensities?

Answer 21

need a comparison of signals from two or more cone classes, each with unique spectral sensitivity

wavelength discrimination improves with number of cone classes

Question 22

what is a dichromat?

Answer 22

two pigments

Question 23

what is pentachromats?

Answer 23

five pigments

Question 24

what does it mean that humans are trichromats?

Answer 24

three cone types

the balance of neural activity - how much each cone class is activated - from each of these receptors is sufficient to represent the vast array of natural colours we encounter

Question 25

what is retinal topography - cone mosaic?

Answer 25

far fewer S cones from M or L no S cones in the fovea randomly distributed but clumping is common layout and relative proportions of cones is largely individual topographical patterns of cones vary between people cone distribution does not impact the ability to see colours

Question 26

what is the opponent coding theory?

Answer 26

colours are grouped into opposing pairs - blue and yellow, red and green evident from colour afterimages - adapting to one colour produces its opponent in the afterimage purely a result of our physiology

Question 27

what is the physiology of opponency?

Answer 27

parvocellular RGCs have chromatically opponent receptive fields centre may be excited by red light while inhibitory surround is excited by green light both ON and OFF versions of this arrangement also exists for blue/yellow also cells that respond to red light switched on any anywhere in the receptive field or green light switched off anywhere in the receptive field

Question 28

what is colour tuning in the LGN?

Answer 28

LGN layers 1 and 2 get their input from M RGCs - input for achromatic luminance channel layers 3-6 get theirs from P RGCs - input for the two chromatic channels called cardinals at LGN, nearly all cells prefer stimuli that are modulated along the cardinal directions of colour space (red-green, blue-yellow)

Question 29

what is colour tuning in the visual cortex?

Answer 29

cortical cells show a preference for a wide range of hues, not just the cardinals tuning width remains fairly consistent across cortical areas (V1, V2, V3) some cortical cells have double opponent receptive fields - centre is excited by red and inhibited by green, surround is excited by green and inhibited by red

Question 30

what is colour constancy?

Answer 30

ability to assign a fixed colour to an object even though the actual spectral information entering the eye changes in different illumination conditions

Question 31

what is the change in a perceived colour called?

Answer 31

chromatic induction

Question 32

are we good judges of colour in different illumination conditions?

Answer 32

highly debated participants asked to name the colour of each piece of paper at 24 different locations some had artificial illumination only, others only natural daylight, or both kinds of illumination participants correctly identified the paper on 55% of the trials (above chance level)

Question 33

how is colour constancy achieved through memory colour?

Answer 33

subjective colour experience mediated by our knowledge of object-colour association might only be true for objects with a specific colour tend to judge the colour of familiar objects as richer compared to unknown objects reflecting same wavelength

Question 34

how is colour constancy achieved through chromatic adaption?

Answer 34

observer's visual sensitivity to a given light illuminating the visual scene (illuminant) decreases over time

Question 35

how is colour constancy achieved through the effect of the surroundings?

Answer 35

visual system uses information about how light affects objects in a scene allows brain to estimate lighting conditions and make adjustments so that colours appear the same, even under different lighting

Question 36

how do humans get colour vision disorders?

Answer 36

congenital or acquired acquired CVD (cerebral chromatopsia) typically due to damage to V4

Question 37

what is the congenital CVD X-linked recessive gene?

Answer 37

XY chromosomes - 8% chance of colour blindness XX chromosomes - 0.5% of colour blindness people affected have normal cone numbers but fewer photopigments usually affect M or L cones, rather than S cones

Question 38

what happens if M or L cones missing?

Answer 38

green and red will be confused

Question 39

what happens if S cones are missing?

Answer 39

blue becomes hard to distinguish

Question 40

what is monochromat?

Answer 40

rare form of CVD characterised by no functioning cones see everything in black and white shades because their vision relies on rods

Question 41

what are dichromats?

Answer 41

miss one pigment

Question 42

what does it mean if someone is missing M cone pigment?

Answer 42

individuals have difficulties distinguishing between green and red colours perceive greens as more neutral or brownish and often confuse reds with greens and some shades of orange

Question 43

what does it mean if someone is missing L cone pigment?

Answer 43

individuals have difficulty distinguishing between red and green shades often perceive red as darker or even black can also affect ability to distinguish between colours with red components, like purple or orange might not see the full spectrum of red hues, but instead see them as shades of beige, grey or brown

Question 44

what does it mean if someone is missing S cone pigment?

Answer 44

individuals have difficulty distinguishing blue and yellow (blue is missing) as a result, have a difficulty distinguishing between blue and yellow and may confuse shades of green with blue or red may see blue as green and yellow as light pink or grey

Question 45

what is the most common form of CVD?

Answer 45

anomalous trichromat

Question 46

what is an anomalous trichromat?

Answer 46

all three types of cone pigments are present but one of them has altered sensitivity to light results in impaired colour discrimination, but not as severe as dichromacy subtypes depend on which cone type has reduced sensitivity can still perceive colours but with diminished clarity or differentiation between certain colour shades

Question 47

how can colour vision deficiency be detected?

Answer 47

Ishihara colour plates each dot in these images is different only in hue, not in luminance combination of numbers that are visible to patient is indicative of the type of deficiency