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.