Colour perception

Key Learning Outcomes:

1.    Importance of colour perception.

2.    The role of cone photoreceptors and their evolutionary significance.

3.    Types of colour vision deficiencies.

4.    The concept of cone opponency.

5.    Higher-level aspects of colour perception.

Reading:

• Snowden, R., Thompson, P., & Troscianko, T. (2012). Basic Vision: An Introduction to Visual Perception. Chapter 5.

Why is Colour Important?

• Use of Colour: We use colour for aesthetics, signalling, and recognizing objects, as well as to indicate social and cultural groups.

• Source of Colour: Colour originates from visible light on the electromagnetic spectrum. Varying wavelengths correspond to different colours.

Cone Photoreceptors:

• Human Trichromacy: Humans have three types of cones, each sensitive to different wavelengths:

  • Short (S): Sensitive to blue light.

  • Medium (M): Sensitive to green light.

  • Long (L): Sensitive to red light.

• Retinal Cone Mosaic: A distribution of these cones across the retina enables colour vision.

Evolution of Cone Types:

• Dichromatic vision: Two cone types (S and L) evolved 30-40 million years ago.

• Trichromatic vision: A split of the L cone into M and L cones allows for better colour discrimination, useful for distinguishing ripe fruits from foliage.

• Another theory suggests that trichromacy helps detect changes in blood oxygenation through skin colour.

Colour Vision Deficiency:

• Monochromats: Only one cone type or no cones, resulting in black-and-white vision.

• Dichromats:

  • Protanopia: Absence of L cones.

  • Deuteranopia: Absence of M cones.

  • Tritanopia: Absence of S cones.

• Anomalous Trichromats:

  • Deuteranomaly: M cone shifted toward L.

  • Protanomaly: L cone shifted toward M.

• Prevalence: More common in men due to X-linked inheritance (8% of men, <1% of women).

Cone Opponency:

• Combination of Cones: Signals from the cones are combined and contrasted into three opponent channels:

  • L/(L+M): “Cherry-teal.”

  • S/(L+M): “Violet-lime.”

  • L+M: Achromatic (luminance axis).

• Retinal Ganglion Cells: These channels correspond to cell types in the lateral geniculate nucleus (LGN), such as parvocellular (L/(L+M)) and koniocellular (S/(L+M)) cells.

Higher-level Aspects of Colour Perception:

• Memory Colour: Objects with typical colours, like bananas being yellow, influence our perception. Even when presented in grayscale, we may perceive a slight colour (e.g., blueish tint when attempting to make a banana grey).

• Colour Constancy: The brain adjusts perceived colour based on lighting conditions. For instance, it may perceive surfaces differently under yellow or blue light.

• #THEDRESS Phenomenon: Differences in perception (e.g., Blue & Black vs. White & Gold) can be attributed to assumptions about illumination and the brain’s colour constancy mechanisms.

Aesthetic Preferences:

• Ecological Valence Theory: Colour preferences are influenced by positive or negative associations with objects of that colour.

• Colour Patterns: Natural scenes, often dominated by blue-yellow variations, align with aesthetic preferences.

Diagrams & Visuals:

• Electromagnetic Spectrum: A diagram showing how varying wavelengths correspond to different colours.

• Cone Types and Response Curves: Graphs depicting the response of S, M, and L cones to different wavelengths.

• Retinal Cone Mosaic: Illustration showing the distribution of cones in the human retina.

• Opponent Channels: Diagrams explaining the combination of cone outputs into L/(L+M), S/(L+M), and L+M channels.