Sensation and Perception Final Flashcards
(112 cards)
Why Do We Perceive Colour?
Aids in perceptual organization: Helps segregate elements.
Classification and identification: Helps us recognize and distinguish objects.
Evolutionary advantage: Helps in food foraging (e.g., spotting ripe fruit).
What is Colour?
Colour = Light wavelength reflected, emitted, or transmitted by an object.
Chromatic colours (hues): Objects reflect some wavelengths more than others (selective reflectance).
Achromatic colours: White, black, and grey (reflect all wavelengths equally).
Key Colour Properties
Wavelength: Determines hue (e.g., blue, red).
Intensity: Changes perceived brightness.
Saturation: Adding white reduces saturation (e.g., red → pink).
Color and Light - Newton’s Discovery
White light = mixture of all colours.
Different wavelengths are bent differently by a prism.
Reflectance curve
% of light reflected at each wavelength.
Transmission curve
% of light transmitted through a transparent object.
Additive colour mixing
Mixing lights of different wavelengths.
Example: Blue + yellow light = white.
Subtractive colour mixing
Mixing paints/pigments (fewer wavelengths reflected).
Example: Blue + yellow paint = green.
Perceptual Dimensions of Colour
Hue: Colour type (e.g., red, green).
Saturation: Intensity/purity of colour.
Value/Lightness: Brightness of the colour.
Trichromatic Theory (Young-Helmholtz)
Three cone types in the retina:
Short-wavelength cones (S-cones): Peak at 419 nm.
Medium-wavelength cones (M-cones): Peak at 531 nm.
Long-wavelength cones (L-cones): Peak at 558 nm.
Colour perception = combination of cone responses.
Evidence: Colour-matching experiments (Maxwell) show people need at least 3 wavelengths to match any colour.
Opponent-Process Theory (Hering)
Opponent pairs:
Red ↔ Green
Blue ↔ Yellow
White ↔ Black
Opponent neurons: Found in the retina & LGN, they respond to one colour while inhibiting the opposite.
Evidence:
Colour afterimages: Seeing green after staring at red.
Simultaneous contrast: A grey square looks greenish on a red background.
How the Theories Work Together
Trichromatic theory: Explains cone responses in the retina.
Opponent-process theory: Explains neural signals processed “higher up” in the visual system (e.g., LGN, visual cortex).
Both of these are correct and expain different stages of visual processing
Metamers
Different wavelengths can produce the same colour perception if they stimulate cones in the same way.
Colour Deficiency & Vision Disorders -Monochromatism
One receptor type (rods only).
See only shades of grey.
Poor visual acuity, very sensitive to light.
Colour Deficiency & Vision Disorders - Dichromatism
Two cone types: See chromatic colours like blue and green but with limitations.
Protanopia
missing L-cones
No red-green distinction.
Deuteranopia
missing M-cones
No green-yellow distinction.
Tritanopia
missing S-cones
No blue-yellow distinction.
Colour Deficiency & Vision Disorders - Anomalous Trichromatism
Three cones, but one is abnormal.
Needs different wavelength proportions to match colours.
Colour Deficiency & Vision Disorders - Unilateral Dichromatism
One eye trichromatic, the other dichromatic.
Why Are Three Cone Types Necessary
One receptor type: No colour vision (same response to any wavelength).
Two receptor types: Limited colour vision (like dichromats).
Three receptor types: Full colour spectrum perception.
Color Constancy
Perception of colors remains relatively constant even under varying lighting conditions.
Examples of Light Sources
Sunlight: Balanced energy across visible wavelengths.
Tungsten Lighting: More energy in long wavelengths (red/yellow tones).
Fluorescent Lighting: More energy in short wavelengths (blue tones).
Chromatic Adaptation
Prolonged exposure to a particular color reduces sensitivity to that color and helps maintain color constancy
