L3: Colour

Course Title: PSYC 236 - Perception & Cognition

Receptors to Color

Slide Overview

  • Original Slides by Mark Schira

Introduction to Color Perception

  • Color is essential in our daily lives and influences decisions like choosing food and fruits from trees.

  • Difficulties arise when individuals cannot perceive specific colors, for instance, red.

Trichromacy vs. Dichromacy

  • Trichromacy: Three color receptors (cones) in human vision.

  • Dichromacy: Two color receptors, leading to certain visual limitations, especially in distinguishing colors against similar backgrounds (e.g., green leaves vs. red fruit).

Importance of Color Perception

  • Color perception is significant for navigating and interpreting the environment, illustrated through the design and understanding of the Tokyo subway map, which relies heavily on color to convey information.

Color Perception Mechanisms

Color Discrimination

  • Color discrimination becomes challenging when individuals lack the ability to differentiate certain wavelengths.

  • Awareness of color deviations informs our understanding of visual differences in our surroundings.

Light, Wavelength, and Energy

  • Electromagnetic waves: Ranges from radio to gamma rays.   - Long wavelengths correlate with low energy, while shorter wavelengths have higher energy per photon.   - There is no fundamental difference between wavelengths except for visual frequencies, which are abundant from sunlight.   - The number of photons affects light intensity.

Nature of White Light

  • White light is not a distinct entity but a combination of all visible wavelengths.

Color and Light Reflection

  • Color perception occurs based on light absorption and reflection mechanisms.   - Example: Red light does not absorb but reflects, which allows us to perceive it.   - Seeing blue light: Similar processes apply to different wavelengths, where cones respond based on the intensity of light.

Cone Functionality

  • One cone cannot independently discriminate colors based on wavelength; it can only respond to the intensity of light.   - Requires multiple cones to accurately perceive colors.

  • Cones functions:   - S-cone (short), M-cone (medium), and L-cone (long) respond differently to wavelengths, thus contributing to color analysis.

Color Analysis by Cones

  • The three different cones process light within specified wavelengths (e.g., 420 nm for S, 530 nm for M, and 560 nm for L).

  • Each cone perceives the environment in a monochromatic manner, activating optimally at specific wavelengths.

Metamers

  • Metamers: Different spectral compositions that activate cones equally, resulting in identical color perception despite different light spectra.   - For example, various combinations of wavelengths can yield the same visual perception through specific cone activation ratios.

Achieving Monochromatic Light

  • Adjusting light source intensities allows for creation of monochromatic light conditions, guiding how we understand and see color based on additive mixtures of primary colors (red, green, blue).

RGB Color Space and Reflectance Patterns

  • In RGB color space, color perception does not fully encapsulate what the human eye can discern, indicating limitations of projector technology in reproducing colors.

  • The reflectance of natural elements must be understood through complex multidimensional color spectra correlating wavelengths to perceived colors.

Color in Nature and Animal Perception

  • Different species perceive colors uniquely; for instance, zebrafish and bees can see beyond the human visible spectrum (e.g., UV light) and have tetrachromatic vision.

Purple and Color Mixing

  • Purple: A mixture of red and blue, lacks monochromatic manifestation, as no single frequency defines it.

  • Color Wheel: Historical tool illustrating color composition but lacked brightness information, essential for capturing the full color spectrum.

Color Mixing Techniques

  • Subtractive Color Mixing: Involves surfaces absorbing light.

  • Additive Color Mixing: Light sources combined to create colors. Examples include the interaction of magenta, yellow to generate red, and cyan to produce green.

Human Color Vision Diversity

  • Majority of mammals are dichromatic compared to humans who possess trichromatic vision, enabling better color differentiation. About 10% of males and 0.5% of females have color vision deficiencies, specifically red/green color blindness.

Impact of Color Blindness

  • Exploring how dichromatic vision alters perception, making aspects of survival challenging, as in distinguishing food from foliage.

Tests for Color Vision

  • Ishihara Color Test Plates: Essential in diagnosing color blindness by leveraging contrast in yellow/blue and red/green visual tasks.

Visual Disorders: Achromatopsia

  • Condition involving the absence of cone types leading to monochromatic vision or shades of luminance.   - Cortical achromatopsia caused by brain injuries can lead to similar perceptual deficits.

Perception and Brain Functionality

  • Color perception extends beyond cone function; brain processing plays a pivotal role in structuring how color is perceived and how metamers are understood.

Opponent-Process Theory

  • Colors are analyzed through outputs of two opposing channels (red-green and blue-yellow), added by a luminance channel, facilitating color differentiation while explaining phenomena like afterimages.

Color Perception Challenges

  • Varied light sources (daylight, fluorescent, incandescent) greatly affect the perception of color due to their distinct spectral distributions.

  • Color Constancy: The ability of the visual system to maintain the perception of consistent color despite changes in lighting conditions, aided by information from surrounding color context.

Summary

  • Understanding color perception encompasses both physiological (cone responses) and psychological (brain processing) aspects, leading us to appreciate the complexity involved in how we perceive the colorful world around us.