Visual Perception and Colour Constancy

Colour Constancy

• The ability of the human visual system to maintain a consistent perception of colour despite changes in illumination or surrounding colours
• Allows us to recognize objects as having a constant colour, even when viewed under different lighting conditions
• Achieved through a combination of neural processing and adaptation to the visual environment

Visual Processing

• Trichromatic Theory: The human retina contains three types of cones sensitive to different parts of the visual spectrum (long, medium, and short wavelengths)
• Opponent Process Theory: The visual system processes colour information through opposing neural responses to different colours (e.g., red-green, blue-yellow)
• Colour Opponency: The way the visual system encodes colour information through the opposing responses of different types of cones

Colour Illusions

• Contextual Effects: The surrounding colours and context can influence our perception of a specific colour
• Colour Contrast: The perceived colour of an object can be influenced by the colours surrounding it
• Adaptation: Prolonged exposure to a colour can lead to adaptation, causing the colour to appear less intense or different

Colour Blindness

• Types:
  • Red-green colour blindness: Most common, affecting approximately 8% of males and 0.5% of females
  • Total colour blindness: Rare, affecting approximately 1 in 30,000 people
• Causes: Genetic mutations affecting the cones in the retina, or acquired through age-related or disease-related degeneration
• Effects: Difficulty distinguishing certain colours, especially in the red-green spectrum

Mixing Paint and Light

• Additive Colour Mixing: When combining light of different colours, the resulting colour is brighter and more saturated
  • Red + Green = Yellow
  • Red + Blue = Magenta
  • Green + Blue = Cyan
• Subtractive Colour Mixing: When combining paints or inks of different colours, the resulting colour is darker and less saturated
  • Red + Green = Brown
  • Red + Blue = Purple
  • Green + Blue = Olive

Colour of Opaque and Transparent Objects

• Opaque Objects: Colour is determined by the pigments or dyes present in the object
• Transparent Objects: Colour is determined by the wavelengths of light transmitted through the object
• Transparency and Reflection: Transparent objects can exhibit both transmission and reflection of light, affecting our perception of their colour

Colour Constancy

• Human visual system maintains consistent perception of colour despite changes in illumination or surrounding colours
• Recognize objects as having a constant colour under different lighting conditions
• Achieved through combination of neural processing and adaptation to visual environment

Visual Processing

• Human retina contains three types of cones sensitive to different parts of visual spectrum (long, medium, and short wavelengths)
• Visual system processes colour information through opposing neural responses to different colours (e.g., red-green, blue-yellow)
• Colour information encoded through opposing responses of different types of cones

Colour Illusions

• Surrounding colours and context influence our perception of a specific colour
• Perceived colour of an object influenced by colours surrounding it
• Prolonged exposure to a colour leads to adaptation, causing it to appear less intense or different

Colour Blindness

• Types: red-green colour blindness (most common), total colour blindness (rare)
• Causes: genetic mutations affecting cones in retina or acquired through age-related or disease-related degeneration
• Effects: difficulty distinguishing certain colours, especially in red-green spectrum

Mixing Paint and Light

• Additive colour mixing: combining light of different colours results in brighter, more saturated colour
• Subtractive colour mixing: combining paints or inks of different colours results in darker, less saturated colour
• Examples of additive and subtractive colour mixing results

Colour of Opaque and Transparent Objects

• Opaque objects: colour determined by pigments or dyes present
• Transparent objects: colour determined by wavelengths of light transmitted
• Transparency and reflection: transparent objects exhibit both transmission and reflection of light, affecting our perception of their colour

Trichromatic Theory of Vision

• States that the human retina has three types of colour receptors (cones) that respond to different parts of the visible spectrum
• The three types of cones respond to:
  • Red light (600-700 nm)
  • Green light (500-600 nm)
  • Blue light (400-500 nm)
• The combination of signals from these three types of cones allows the brain to perceive a wide range of colours

Colour Perception

• The brain can perceive a wide range of colours using only three primary colours (red, green, and blue)
• Demonstrated through colour matching experiments, where participants adjust the intensity of primary colours to match a given colour

Colour Deficiency (Colour Blindness)

• A condition where an individual has difficulty perceiving certain colours due to a deficiency in one or more types of cones
• Types of colour deficiency:
  • Red-green colour blindness (most common)
  • Blue-yellow colour blindness (rare)
  • Total colour blindness (rare)
• Colour deficiency can be inherited or acquired through disease or injury

Afterimages

• A visual phenomenon where an image continues to appear after the original stimulus has been removed
• Caused by prolonged exposure to a colour, leading to fatigue of the corresponding cones
• Types of afterimages:
  • Positive afterimage (image appears in the same colour as the original stimulus)
  • Negative afterimage (image appears in the complementary colour of the original stimulus)
• Demonstrates the trichromatic theory of vision and the way the brain processes colour information

Colour Perception: Monochromatism and Dichromatism

Monochromatism

• Monochromatism is a rare condition where an individual has only one type of functioning cone cell in their retina.
• People with monochromatism see the world in shades of grey, as they are unable to perceive colours.
• Also known as complete colour blindness or achromatopsia.
• Caused by a defect in one or more of the genes that code for the light-sensitive pigments in cone cells.
• Characteristics include:
  • Increased sensitivity to light (photophobia)
  • Blurry vision
  • Nystagmus (involuntary eye movements)
  • Squinting or avoiding bright lights

Dichromatism

• Dichromatism is a condition where an individual has only two types of functioning cone cells in their retina.
• People with dichromatism see the world in a limited range of colours, often with difficulty distinguishing between certain colours.
• Also known as partial colour blindness.
• Caused by a defect in one of the genes that code for the light-sensitive pigments in cone cells.
• Types of dichromatism include:
  • Protanopia (red-green colour blindness): difficulty distinguishing between red and green colours
  • Deuteranopia (red-green colour blindness): difficulty distinguishing between red and green colours
  • Tritanopia (blue-yellow colour blindness): difficulty distinguishing between blue and yellow colours
• Characteristics include:
  • Difficulty with colour-based tasks, such as selecting ripe fruit or matching colours
  • May have difficulty with certain occupations, such as graphic design or fashion
  • May have adaptations to compensate for colour perception difficulties, such as using technology or relying on brightness and saturation cues

Colour Constancy

• Colour constancy is the ability of the human visual system to maintain a consistent perception of colour despite changes in the colour of the illumination.

Key Factors

• Contextual influence affects colour perception, influenced by surrounding objects and overall lighting conditions.
• Adaptation allows the visual system to adjust to prevailing illumination, making colours appear relatively constant.
• Chromatic adaptation helps the visual system adjust to the colour of the illumination, allowing colours to appear more constant.

Mechanisms

• Colour opponent cells in the visual cortex respond to colour differences, helping maintain colour constancy.
• Gain control adjusts the sensitivity of colour receptors to compensate for changes in illumination.

Examples

• A white shirt appears relatively constant in colour despite changes in lighting conditions due to colour constancy.
• Fruits and vegetables appear to have consistent colours despite changes in lighting due to colour constancy.

Theories

• Land's Retinex Theory proposes separating the colour signal from the illumination signal to achieve colour constancy.
• Helson's Adaptation-Level Theory suggests colour constancy results from the visual system adapting to the average colour of the surroundings.

Importance

• Colour constancy is essential for object recognition, colour-based decision making, and overall visual perception.
• Colour constancy has implications for colour reproduction, display technology, and art/design.