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cognition > Brightness and colour > Flashcards

Brightness and colour Flashcards

1
Q

What is visible light?

A

A band of energy within the electromagnetic spectrum (400-700)

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2
Q

What is wavelength of light associated with?

A

Different colour perceptions

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3
Q

What is photons?

A

Light can also be described as consisting of small packets of energy

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4
Q

What is luminance?

A

This is the number of photons per unit of space which is associated with perception of brightness

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5
Q

What is absorption?

A

This is where photons collide with a particle of matter

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6
Q

What is reflection?

A

This is where light strikes opaque surfaces

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7
Q

What is transmission?

A

Light passes through transparent matter

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8
Q

What is the process of projecting an image onto the retina?

A
  1. Single chambered eye uses convex cornea and lens to project an image onto the retina
  2. Enables directional sensitivity - can represent the spatial structure rather than sum total of light
  3. Photoreceptors transduce light into an electrical potential
  4. These signals then flow through a network of neurone to retinal ganglion cells and then out the back of the eye via the optic nerve
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9
Q

What are the two types of photoreceptors?

A

Rods
Cones

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10
Q

What is a rod?

A
  • Located primarily in peripheral retina
  • Capable of operating in low light levels (detect a single photon)
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11
Q

What is a cone?

A
  • Concentrated in centre of retina known as fovea
  • Require higher light levels (daylight, 100s of photons) to respond
  • 3 different photopigments, sensitive to short, medium, and long wavelengths of light
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12
Q

What is the visual pathways?

A

Visual information is transmitted from the retina to the brain

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13
Q

What is the stages of the visual pathways?

A
  1. Retina
  2. Optic nerve
  3. Optic chiasm
  4. LGN
  5. Primary visual cortex (V1)
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14
Q

What is brightness perception?

A

This is where light intensity is related to perceived brightness

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15
Q

How does bottom up link to brightness perception?

A
  • The retina does not simply record light intensities
  • Responses are shaped by processes occurring within the retina, most notably light/dark adaptation and lateral inhibition
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16
Q

How does top down link to brightness perception?

A

The brain uses knowledge about how light interacts with objects when determining perceived brightness eg shadows

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17
Q

Brightness perception - light/dark adaptation

A

Consider a piece of paper with black text viewed under indoor lighting or outside on a bright sunny day
We perceive the text as black and white in both situations

18
Q

What is brightness constancy?

A

This is where you perceive the test as both black and white because the white will reflect less light indoors than the black will reflect outdoors.

19
Q

What is adjusted to compensate in changes in mean luminance?

A

The sensitivity of the retina

20
Q

When is sensitivity reduced?

A

When the mean intensity of the image is high and increased when it is low

21
Q

How does the retina encode contrast?

A

The ratio of an objects luminance relative to the mean or background luminance, playing a critical role in achieving brightness constancy

22
Q

What can brightness perception produce after?

A

Negative afterimages and illusions

23
Q

What is lateral inhibition?

A

This is an early form of information processing in retina

24
Q

What do retinal ganglion cells recieve?

A

Excitatory (+) and inhibitory (-) input from neighbouring photoreceptors

25
Q

How is lateral inhibition arranged?

A

Arranged in a centre surround configuration across retinal image

26
Q

How does the herman grid illusion link to brightness perception/lateral inhibition?

A

The intersections are surrounded by more high intensity (white) resulting in more inhibition from the surround in on centre and off surround receptive fields

27
Q

How does top down influence brightness perception?

A

Our visual systems uses knowledge of how light interacts with 3d objects when determining brightness

28
Q

Examples of top down influence brightness perception?

A

E.g it tries to maintain brightness constancy when the amount of light falling on a surface is affected by shadows resulting in 2d 2d images portraying 3d scenes like the checker shadow illusion

29
Q

Why does the world lack colour under low light images?

A

Only rod photoreceptors are sensitive enough to operate as they contain a single photopigment (rhodopsin)

30
Q

Why can we not accurately signal different wavelengths?

A

Light of different wavelengths and intensities can elicit identical responses

31
Q

What is trichromacy?

A

This is where cone photoreceptors contain one of three different photopigments each with different wavelength sensitivities

32
Q

What are the 3 different cones and their wavelengths?

A

S cones - cones that are sensitive to short wavelengths (blue cones)
M cones - cones that are preferentially sensitive to middle wavelengths (green)
L cones - Cones that are preferentially sensitive to long wavelengths (red )

33
Q

What is monochromacy?

A

Individuals have either 0 or 1 functioning cone type resulting in complete colour blindness which is rare (1 person in 100000)

34
Q

What is dichromacy?

A

Only 2 functioning cone types

35
Q

What is protanopia?

A

1% males - 0.02% females = missing L cones

36
Q

What is deuteranopia?

A

1% males, 0.01% females = missing M cones

37
Q

What is tritanopia?

A

0.002% males, 0.001% females = missing S cones

38
Q

What is anomalous trichomacy and how is it tested?

A

More common form of colour perception deficiency
Characterised by defects in one of the cone types
Protanomaly (1.3% males, 0.02% females) = L-cone defect
Deuteranomaly (5% males, 0.35% females)= M-cone defect
Tritanomaly (0.01% males, 0.01% females) = S-cone defect

Assessed using Ishihara colour test

39
Q

What is colour perception - Opponency

A

Like brightness, perception of colour is shaped by bottom up processing of visual information in retina and beyond
This is where retinal ganglion cells recieve excitatory (+) and inhibitory (-) input from different cone types

40
Q

What does colour opponency result in?

A

Red/green pathways
Blue/yellow pathways

41
Q

Why does negative afterimages occur?

A

E.g staring at a red object will result in green afterimage
- Adaptation to red causes a reduction in the sensitivity of long wavelength cones, creating an imbalance in the inputs to red/green opponent retinal ganglion cells

42
Q

What is the definition of colour constancy?

A

The tendency for the percieved colour of objects to remain the same, even if the lighting changes

cognition (5 decks)

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