3. Colour Vision

Question 1

can individual photoreceptors see colour

Answer 1

no

Question 2

What type of response do photoreceptors have

Answer 2

Univariant

Question 3

What is “colour” in terms of physics and perception?
A:

Answer 3

Electromagnetic radiation of varying wavelengths (400–700 nm)
Perceptual experience generated by neural processing of wavelength-dependent signals

Question 4

Why can a single photoreceptor not distinguish wavelength from intensity?

Answer 4

A cone’s output varies only in magnitude, not in spectral shape
Different wavelength–intensity combinations can elicit the same response

Question 5

What evidence supports Young–Helmholtz trichromacy?

Answer 5

Colour matches achieved with three primaries
Existence of three cone types (S, M, L) with distinct spectral sensitivities

Question 6

What are the three human cone types and where are they concentrated?

Answer 6

S-cones (~420 nm), M-cones (~530 nm), L-cones (~560 nm)
~6–7 million cones, densely packed in the fovea

Question 7

How does chromatic vision vary across species?

Answer 7

Monochromacy: one photoreceptor type (e.g., dolphins)

Dichromacy: two types (e.g., dogs)

Trichromacy: three types (humans)

Tetrachromacy: four types (some birds, potentially 2–12% of women)

Question 8

What are the main forms of dichromatic colour blindness in humans?

Answer 8

Protanopia (no L-cones, ~1% males)
Deuteranopia (no M-cones, ~1% males)
Tritanopia (no S-cones, very rare)
Tags: Colour Blindness, Dichroma

Question 9

What distinguishes anomalous trichromats from dichromats?

Answer 9

All three cone types present but one has shifted sensitivity
e.g., deuteranomalous M-cones shifted toward L spectrum (~8% males)

Tags: Colour Vision, Anomalous Trichromacy

Question 10

How do additive and subtractive colour mixing differ?

Answer 10

Additive: Combining lights → sums spectral power (RGB monitors)

Subtractive: Combining pigments → absorbs wavelengths (paints, inks)
Tags: Colour Mixing, Physic

Question 11

What is defined by the CIE 1931 colour space?

Answer 11

Mapping between spectral stimuli and human colour matches

Chromaticity coordinates derived from three-primary matching tasks
Tags: Colourimetry, CIE

Question 12

What are the primary opponent channels in Hering’s theory?

Answer 12

Red vs. green
Blue vs. yellow
Plus luminance (light–dark)
Tags: Opponency, Hering

Question 13

How do retinal ganglion cells encode red–green opponency?

Answer 13

Centre receives excitatory L-cone input, surround inhibitory M-cone input (or vice versa)
Tags: Retina, Opponency

Question 14

Where are red–green and blue–yellow opponent cells found in the LGN?

Answer 14

Red–green: parvocellular layers 3–6
Blue–yellow: koniocellular layers between magno/parvo
Tags: LGN, Opponency

Question 15

What distinguishes single-opponent from double-opponent cells?

Answer 15

Single-opponent: colour opponency without spatial opponency

Double-opponent: spatially and spectrally selective—edges and colour contrasts
Tags: Opponency, Receptive Fields

Question 16

What underlies complementary-colour afterimages?

Answer 16

Prolonged stimulation fatigues one opponent channel

Post-stimulus balance shifts → perception of opposite hue
Tags: Adaptation, Afterimages

Question 17

What is colour constancy and where is it proposed to occur?

Answer 17

Perceived surface colours remain stable under different illuminants

Achieved by contextual processing, likely in area V4
Tags: Colour Constancy, V4

Question 18

What did Shipp & Zeki (1985) demonstrate about V4?

Answer 18

Method: Varied illumination wavelength during fMRI in monkeys

Finding: V1 codes raw wavelength; V4 shows invariance (constancy) to illumination changes
Tags: V4, Colour Constancy

Study Reference:
Authors & Year: Shipp & Zeki (1985)

Method: fMRI contrast of varying illumination
Key Finding: Neural correlate of colour constancy in V4

Question 19

What did Lafer-Sousa & Conway (2017) find about #TheDress?

Answer 19

Method: Categorical perception task on ambiguous image

Finding: Observer differences arise from inferred illuminant assumptions

Tags: Colour Perception, Individual Differences

Study Reference:
Authors & Year: Lafer-Sousa & Conway (2017)

Method: Psychophysical categorization of #TheDress
Key Finding: Perceptual categories driven by top-down illumination estimates

Question 20

What did Hadjikhani et al. (1998) reveal about colour regions?

Answer 20

Method: fMRI contrast of colour vs. luminance gratings in humans

Finding: Distributed colour-selective regions beyond V1, notably in V4/V8
Tags: fMRI, Colour Mapping

Study Reference:
Authors & Year: Hadjikhani et al. (1998)

Key Finding: Multiple cortical areas show colour selectivity

Question 21

What characterizes cerebral achromatopsia?

Answer 21

Loss of colour vision despite intact retinal mechanisms
Caused by lesions in ventral occipital cortex (e.g., V4)
Tags: Neuropsychology, Achromatopsia

Question 22

How does colour agnosia differ from achromatopsia?

Answer 22

Preserved colour perception but impaired ability to name or associate colours

Evidenced by cases (Miceli et al., 2001) with intact cones/LGN but semantic deficits
Tags: Neuropsychology, Agnosia

Study Reference:
Authors & Year: Miceli et al. (2001)
Method: Case studies of patients with colour naming deficits
Key Finding: Dissociation between perception and semantic knowledge of colour