Lecture 3 - Brightness & Colour

Question 1

What is light described as consisting of?

Answer 1

Small packets of energy - photons.

Question 2

What is luminance/light intensity?

Answer 2

• Number of photons per unit space.
• Is associated with perception of brightness.

Question 3

What type of properties are brightness and colour?

Answer 3

Perceptual properties not physical properties.

Question 4

What are different wavelengths of light associated with?

Answer 4

Different colour perceptions.

Question 5

What is absorption?

Answer 5

An interaction of light with an object/surface where photons collide with particles of matter.

Question 7

What is reflection?

Answer 7

An interaction of light with an object/surface where light strikes opaque surfaces.

Question 8

What is transmission?

Answer 8

An interaction of light with an object/surface where light passes through transparent matter.

Question 9

How does the human eye perceive objects?

Answer 9

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

Question 10

What are some key features of rods?

Answer 10

• Located primarily in the peripheral retina.
• Very sensitive to light - capable of operating in low light levels (can detect single photon).

Question 11

What are the two types of photoreceptor?

Answer 11

Rods and cones

Question 12

What are some key features of cones?

Answer 12

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

Question 13

What is the main visual pathway?

Answer 13

Retina -> Optic nerve -> Optic chiasm -> Lateral geniculate nucleus (LGN) -> Primary visual cortex (V1).

Question 14

What processes is brightness perception influenced by?

Answer 14

Both top-down and bottom-up.

Question 15

How does the number of photons (light intensity) affect perception of brightness?

Answer 15

Higher intensity (more photons) is perceived as brighter.

Question 16

What does the bottom-up process for brightness perception consist of?

Answer 16

• Retina does not simply record light intensities.
• Responses shaped by processes occurring within the retina, most notable light/dark adaptation and lateral inhibition.

Question 17

What does the top-down process for brightness perception consist of?

Answer 17

• Brain uses knowledge about how light interacts with objects when determining perceived brightness (e.g shadows).

Question 18

What is brightness constancy?

Answer 18

When we perceive something as the same brightness in both light and dark situations.

Question 19

How does sensitivity in the retina change in different levels of brightness?

Answer 19

• Sensitivity of the retina is constantly adjusted to compensate for changes in mean luminance.
• Sensitivity is increased when it is dark and decreased when it is lighter.
• Sensitivity is reduced when mean intensity of the image is high and increased when it is low.
• Light/dark adaptation dictates that the retina encodes contrast (the ratio of an objects luminance to the mean or background luminance) and plays a critical role in brightness constancy.

Question 20

What are negative afterimages?

Answer 20

• Light/dark adaptation can produce illusions under some circumstances, creating negative afterimages.

Question 21

What is lateral inhibition?

Answer 21

• Early form of information processing in retina.
• Retinal ganglion cells receive both excitatory (+) and inhibitory (-) input from neighbouring photoreceptors.
• Excitatory induces a response, inhibitory reduces a response.
• Arranged in a centre-surround configuration across the retinal image.

Question 22

What influence does lateral inhibition have?

Answer 22

• Makes the visual system sensitive to changes in luminance (important for detecting edges and borders of objects).
• Can have dramatic effects on perceived brightness.
• Example - Herman grid illusion.

Question 23

Why does the world appear devoid of colour in low-light conditions?

Answer 23

• Only rod photoreceptors are sensitive enough to operate.
• Rods contain a single type of photopigment (rhodopsin).
• Light of different wavelength and intensities can elicit identical responses, making it impossible to accurately signal different wavelengths.

Question 24

What is trichromacy?

Answer 24

• Cone photoreceptors contain one of three photopigments.

S-cones: sensitive to short wavelengths (blue cones).

M-cones: sensitive to middle wavelengths (green cones).

L-cones: sensitive to long wavelengths (red cones).

Question 25

What is monochromacy?

Answer 25

• Individuals either have 0 or 1 functioning cone type, resulting in complete colour blindness.
• Extremely rare (approx 1 in 100,000 people).

Question 26

What is dichromacy?

Answer 26

• Only 2 functioning cone types

Protanopia (1% males, 0.02% females) = missing L-cones.

Deuteranopia (1% males, 0.01% females) = missing M-cones.

Tritanopia (0.002% males, 0.001% females) = missing S-cones.

Question 27

What is anomalous trichromacy?

Answer 27

• More common colour perception deficiency.
• Characterised by defect 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.

Trianomaly (0.01% males, 0.01% females) = S-cone defect.

• Commonly assessed using the Ishihara Colour Test.

Question 28

What is colour opponency?

Answer 28

• Retinal ganglion cells receive excitatory and inhibitory input from different cone types.
• Grouped together to create output response.
• Results in distinct red/green and blue-yellow pathways.

Question 29

How do top-down influences affect colour perception?

Answer 29

• Visual system tries to achieve colour constancy by accountign for intensity and composition of light hitting different surfaces.
• Colour constancy- tendency for the perceived colour of objects to remain the same, even if the light changes.
• Can give rise to illusions in which the same wavelength of light is perceived as different colours.