Visual Perception Flashcards

Color Vision

Functions of Color

• Classification of objects:

  • Colors help us identify and categorize objects.
    • Example: Oranges are classified as orange because of their color.
    • Example: The sky is classified as blue because of its color.
    • Example: Grass is classified as green because of its color.
  • Salient color features enable us to identify objects.

• Perceptual organization:

  • Color helps in grouping elements into different objects.
  • Objects of the same color are perceived to belong together.
  • Example: In an image, red fruits are grouped together in the foreground, while a green bush is grouped in the background.

• Searching for food:

  • Color vision may have evolved to help us find food.
  • Plants may have developed bright and colorful fruits to attract animals for seed dispersal.
  • Example: Red apples distinct from a green bush capture our attention, indicating they are good to eat.

Color as a Property of the Mind

• Color is not an inherent property of the world but a construct of our minds.
• Objects do not possess color until we perceive it.
• Example: A grayscale image can be perceived as colorful.
• Objects absorb various wavelengths of light and reflect the remaining wavelengths.
• We perceive the reflected electromagnetic radiation wavelength as color.
• Color perception is related to the wavelength of light.
  • Violet/blue objects reflect wavelengths between 400-500 nanometers.
  • Red objects reflect wavelengths between 590-700 nanometers.
  • These wavelengths themselves do not have color; their sizes are what we perceive as colors.

Perceptual Qualities of Color

• Color is composed of three main perceptual qualities:
  • Hue
  • Saturation
  • Brightness

Hue

• Hue is determined by the dominant wavelength of light reflected by an object.

• This can be understood through reflectance curves.

• Reflectance Curves:

  • X-axis: Wavelength of light (e.g., 400-450 nm for blue, 650-700 nm for red).
  • Y-axis: Percentage of light reflected.
  • Blue pigment: Reflects mostly in the 400-500 nm range, so it's perceived as blue.
  • Green pigment: Reflects mostly around 500 nm, so it's perceived as green.
  • Yellow pigment: Reflects across green, yellow, and red wavelengths, so it's perceived as yellow.
  • Example: A tomato reflects mostly light between 600-700 nm, so it's seen as red.

• White:

  • White occurs when an object reflects all wavelengths of light.
  • The reflectance curve shows a high amount of all wavelengths being reflected.

• Gray:

  • Gray occurs when an object absorbs some wavelengths of light.
  • The white reflectance curve is shifted down.
  • Black occurs when an object absorbs all wavelengths of light.

• Primary Colors and Wavelengths:

  • Short wavelength (around 400 nm): Perceived as blue.
  • Medium wavelength: Perceived as green.
  • Long wavelength: Perceived as red.
  • Long and medium wavelengths combined: Perceived as yellow.
  • All wavelengths presented together at high intensity: Perceived as white light.

Saturation

• Saturation refers to the intensity of a color in a stimulus.
• In reflectance curves, saturation is indicated by the height of the peak for a certain color.
  • Example: Green pigment with a peak at 60% reflectance has a standard saturation.
    • If the peak drops to 40%, saturation is lower.
    • If the peak climbs to 80%, saturation is higher.
• High saturation = vivid, distinct colors.
• Low saturation = more grayscale, colors are less distinguishable.
• Zero saturation = grayscale (flat reflectance curve).
• Saturation can also be increased beyond 100%, making colors more distinct.

Brightness

• Brightness refers to the amount of pure white light in a stimulus.
• It corresponds to the overall amount of light reflected across all wavelengths.
• In reflectance curves, brightness is indicated by the base level of the curve.
  • Example: If the green pigment starts at 20% reflectance, shifting the entire curve up increases brightness, while shifting it down decreases brightness.
• Baseline brightness: Medium amount of white light across the entire spectrum.
• Darkness: Curve is shifted lower, approaching zero reflectance.
• 100% darkness: Whole curve flattens at the zero line, absorbing all light.
• Increased brightness: Curve shifts up the reflectance curve.
• 100% brightness: Curve is a flat line at the top, showing white light.

Trichromatic Theory of Color Vision

• Proposed by Young and Helmholtz in the 1800s.
• States that there are three different receptor mechanisms responsible for color vision.
• Derived from behavioral experiments.

Color Matching Experiment

• Two visual fields: a test field and a comparison field.

• Test field:

  • Presented a pure color of light (e.g., 500 nm of green light).

• Comparison field:

  • Made up of three different nanometer wavelengths (420 nm blue, 560 nm yellowy-green, and 640 nm red).

• Task:

  • Participants adjusted the three wavelengths in the comparison field to match the color in the test field.

• Findings:

  • Participants could match the color in the test field by adjusting the three wavelengths.
  • Observers with regular color vision needed at least three primaries to make their matches.
  • Having only two wavelengths was insufficient for color matching.

Conclusions

• This led to the conclusion that there are three photoreceptors.
• Colors are only matched in perception.
• The test field consists of 500 nm pure green light, while the comparison field uses a combination of three different wavelengths.
• Physiologically, the colors are not the same, but perceptually they are.

Metamers

• The phenomena where colors are physiologically different but perceptually equivalent.

Biological Validation

• In the 1960s, researchers found three cones that matched the findings of Young and Helmholtz.

• These cones respond to three different kinds of wavelengths:

  • Short wavelength: Peaks around 419 nm (blue).
  • Medium wavelength: Peaks around 530 nm (green).
  • Long wavelength: Peaks around 564 nm (red).

• Later research found genetic differences for coding proteins in the three pigments, providing genetic evidence for these different cones.

Side Notes

• The medium and long cones are closer to each other than the short and medium cones.

• Early in evolution, humans were dichromatic, with only short and medium wavelength cones.

• A genetic mutation 30-40 million years ago shifted the medium cone to the long cone, resulting in trichromatic vision.

• Tetrachromatic Vision:

  • Occurs in a small percentage of females.
  • The long wavelength cone has mutated again, shifting slightly to the left.
  • Results in a more rich and complex perception of color with four different kinds of cones.

Cone Response Profiles

• Blue Signal:

  • Predominantly short cone firing, with a little medium cone firing and not much long wavelength cone firing.

• Green Signal:

  • Middle cone fires the most, with long and short cones firing less.

• Red Signal:

  • Longer wavelength cone fires, with other two firing less.

• White Light:

  • All cones fire at the same time.

• Color perception depends on the combinations and rates at which these three cones fire.

Metamers Revisited

• Colors may be physiologically different but perceptually similar.

• Example:

  • A yellowy color made up of 580 nm light results in specific firing rates for each cone:
    • Short cones fire at 1.
    • Medium cones fire at 5.
    • Long cones fire at 8.

• A combination of 530 nm and 620 nm light results in the exact same firing rates.

• The lights are different but produce the same perceptual experience because the cones are firing at the same rate.

Summary of Visual Perception Lectures

• Described the physical nature of light and identified major components of the human visual system.
• Discussed challenges of object perception and resolved them using Gestalt laws.
• Analyzed the properties of color experience (hue, saturation, and brightness) in relation to reflective curves to explain the neural physiology of vision.

Next Week

• Vision in relation to size and depth and how these are interrelated in our visual system.