Color Vision and Perception

Color Vision: An Overview

• Color plays a vital role in visual perception, aiding in object classification, perceptual organization, and potentially, food search.
• Color is created by our minds; objects themselves are not inherently colored until perceived.

The Functions of Color

• Object Classification: Color helps us identify objects (e.g., oranges are orange, the sky is blue, grass is green).
• Perceptual Organization: Color groups elements into objects. Objects of the same color are perceived as belonging together.
  • Example: In an image, red fruits stand out against a green bush because of color differences.
• Food Search: Color vision may have evolved to help us find food. Plants make fruits bright and colorful to attract us.

Color as a Property of the Mind

• Color isn't an inherent property of the world but a construct of our minds.
• Objects absorb certain wavelengths of light and reflect others. We perceive color based on the reflected wavelengths.
• Example: A grayscale image can be perceived as colorful because our brain interprets the wavelengths reflected.
• Violet or blue objects reflect wavelengths between 400 and 500 nanometers.
• Red objects reflect wavelengths between 590 and 700 nanometers.

Perceptual Qualities of Color

• Color vision is composed of hue, saturation, and brightness.

Hue

• Hue is determined by the dominant wavelength of light reflected by an object.
• Reflectance curves illustrate the percentage of light reflected at different wavelengths.
  • Blue pigment reflects light mostly in the 400-500 nm range.
  • Green pigment reflects light mostly around 500 nm.
  • Yellow pigment reflects a wider range, including green, yellow, and red wavelengths.
• Example: A tomato reflects light mostly between 600 and 700 nm, appearing red.
• White occurs when an object reflects all wavelengths of light.
• Gray is when an object absorbs some wavelengths, but reflects all wavelenghts equally.
• Black occurs when an object absorbs all wavelengths of light.
• Key wavelengths and their perceived colors:
  • Short (400 nm): Blue
  • Medium: Green
  • Long: Red
  • Long and Medium: Yellow
  • All wavelengths at high intensity: White

Saturation

• Saturation refers to the intensity or purity of a color.
• In reflectance curves, saturation is indicated by the height of the peak for a certain color.
• Higher peaks indicate higher saturation; lower peaks indicate lower saturation (more grayscale).
• At 0% saturation, the image appears grayscale.
• Above 100% saturation, colors become more vivid.

Brightness

• Brightness refers to the amount of pure white light in a stimulus.
• In reflectance curves, brightness is indicated by how high the entire curve shifts on the graph.
• Shifting the curve upwards increases brightness; shifting it downwards decreases brightness.
• At 0% brightness, the image is completely dark.
• At 100% brightness, the image is pure white.

Trichromatic Theory of Color Vision

• Proposed by Young and Helmholtz in the 1800s.
• States that three receptor mechanisms are responsible for color vision.
• Based on behavioral experiments, not neuroscience.

Color Matching Experiment:

• Participants adjusted three different nanometer wavelengths (blue, yellow-green, and red) in a comparison field to match a pure color in a test field.
• Observers with normal color vision needed at least three primaries to make their matches.
• Led to the conclusion that there are three photoreceptors.
• Colors are matched perceptually, not necessarily physiologically (metamers).

Metamers

• Colors that are physiologically different but perceptually identical.

Biological Validation

• In the 1960s, researchers found three cones in the human eye that responded to different wavelengths, validating the trichromatic theory.
  • Short wavelength cone: peaks around 419 nm
  • Medium wavelength cone: peaks around 530 nm
  • Long wavelength cone: peaks around 564 nm
• Genetic differences were found for coding proteins in the three pigments.

Evolution of Color Vision

• Early humans were dichromatic, with only short and medium wavelength cones.
• A genetic mutation about 30-40 million years ago shifted the medium cone to the long cone, resulting in trichromatic vision.

Tetrachromatic Vision

• Rare condition (mostly in females) where a long wavelength cone mutates slightly, resulting in four different cones and a richer color perception.

Cone Response Profiles

• Different cones fire depending on the incoming signal (blue, green, red).
• Blue signal: short cone fires the most.
• Green signal: medium cone fires the most.
• Red signal: long cone fires the most.
• White light: all cones fire at the same time.

Metamer Example

• A yellow color made up of 580 nm light results in a specific firing rate for the short, medium, and long cones.
• A combination of 530 nm and 620 nm light can produce the exact same firing rate, resulting in the same perceptual experience (yellow), even though the wavelengths are different.

Summary of Visual Perception Lectures

• Described the physical nature of light and the components of the human visual system.
• Discussed the challenges of object perception and applied Gestalt laws to resolve them.
• Analyzed the properties of color experience (hue, saturation, brightness) in relation to reflectance curves and neural physiology.

Next Steps

• Discuss vision in relation to size and depth and their interrelation in our visual system.