PY359: Chapters 6 - 8

Module Overview: Dimensions of Visual Perception

Module 5 covers Chapters 6 to 8.

• Section 1 (Chapter 6): Color Perception

• Section 2 (Chapter 7): Depth and Size Perception

• Section 3 (Chapter 8): Movement and Action

Continuing Themes:

• Properties of light that influence visual perception

• Photoreceptors and their connections in the retina, which play a critical role in how we perceive light and color

• Receptive fields that respond to specific stimuli within the visual field

• Specialized regions of the visual cortex responsible for processing different aspects of visual information

• The organizational properties of the visual cortex that allow for complex processing

• The distinction between dorsal and ventral visual streams that govern how we perceive motion and object recognition

Section 1: Color Vision

This section focuses on the intricate processes related to color perception within the visual system:

Stages of Processing:

• Processing occurs in the eyes and is initiated by light entering through the cornea and lens, focused onto the retina.

• Key stages of processing include:

  • Retina: Where phototransduction occurs, converting light into neural signals.

  • LGN (Lateral Geniculate Nucleus): A relay center in the thalamus for visual information received from the retina.

  • V1 (Primary Visual Cortex): The initial area of the cortex involved in processing visual information.

Dorsal and Ventral Visual Pathways:

Higher-stage visual processing occurs through two main pathways:

• Dorsal Pathway: Associated with the processing of motion and spatial awareness, leading to visually-guided actions.

• Ventral Pathway: Primarily concerned with object identification, including color, form, and texture recognition.

Higher-stage visual processing includes:

• Processing of complex edges, integration of color, and identification of shapes in area V4.

• Recognition of object identities occurs in the Inferotemporal (IT) cortex, crucial for interpreting what we see.

• Motion processing is concentrated in area MT, important for perceiving the direction and speed of moving objects.

• Visually-guided movement is managed in the Intraparietal Sulcus, linking perception to action directly.

Basic Definitions in Perception:

• Visible Spectrum: The range of electromagnetic wavelengths between approximately 400 nm (violet) to 700 nm (red) that human eyes can perceive.

• Color Vision: The ability to differentiate between wavelengths of light, resulting in the perception of distinct colors.

• Heterochromatic Light: Composed of multiple wavelengths, this is typical of most light sources like sunlight and incandescent bulbs.

• Monochromatic Light: Contains a single wavelength, found in rare instances such as lasers and specific LED lights.

Object Colors and Spectral Reflectance:

• Spectral Reflectance: Refers to the proportion of light reflected by a surface at each wavelength, which is dictated by the surface's molecular structure, determining its color.

• Depth in Applications: For instance, plants absorb blue and red wavelengths for photosynthesis while reflecting green light, contributing to their green appearance.

• Atmospheric scattering causes the blue hue of the sky, as short wavelengths scatter more than long wavelengths.

• Chemists and scientists apply spectral analysis to characterize various materials based on their reflective properties.

Dimensions of Color:

• Hue: The actual color perceived, with monochromatic colors existing purely within the visible spectrum while nonspectral colors require mixtures of wavelengths.

• Saturation: Refers to the purity of a hue; higher saturation means more vibrant colors.

• Brightness: Represents the amount of light perceived, impacting overall visual experience and color interpretation.

Color Mixing Techniques:

• Additive Color Mixing (Light):

  • Involves mixing colored lights, commonly seen in televisions and digital displays.

  • Primary Colors in this model: Red, Green, Blue (RGB), from which any color can be created by combination.

• Subtractive Color Mixing (Physical Substances):

  • This occurs when merging colored substances, such as paints.

  • Primary Colors are Magenta, Cyan, Yellow (CMY), which absorb certain wavelengths and reflect others, creating color combinations.

Color Model Confusion in Art History:

• The science of color mixing is often at odds with historical practices in art, where traditional primary colors such as red, blue, and yellow were inefficient for full-color reproduction as per modern color science.

Spectral Reflectance Recap:

• Spectral Reflectance: Understanding this is crucial as it defines how color is perceived based on reflected light across different wavelengths.

Perception of Color:

• Color perception hinges not just on the wavelengths absorbed by individual receptors, but rather on combined responses across multiple receptor types present in the retina.

Color Vision Theories:

1. Trichromatic Theory (Thomas Young and Hermann von Helmholtz):

   • This theory posits that three types of cones in the retina detect wavelengths corresponding to red, green, and blue light, enabling the perception of all colors through their combination.

2. Opponent Process Theory (Ewald Hering):

   • This theory explains that colors are perceived in opposing pairs (red vs. green, blue vs. yellow) and accounts for color afterimages and the perception of color opposition.

Studying Color Vision:

• Metameric Color Matching Experiments: Metamers are physically different stimuli perceived as identical colors due to the way light interacts with our visual system. These studies involve comparing test and standard patches to evaluate how many wavelengths are needed to match various colors.

Results of Color Matching:

• Trichromats: Individuals with normal color vision who can match the full spectrum using three monochromatic lights.

• Monochromats: Individuals who perceive color by intensity from a single monochromatic light, lacking the ability to see color properly.

Photoreceptors in the Retina:

• Types:

  • Cones: Approximately 7 million per retina, subdivided into L (red), M (green), and S (blue) types, responsible for color vision in bright light conditions.

  • Rods: Approximately 120 million per retina, provide monochromatic vision in low-light conditions and are more sensitive to light than cones.

• Spectral Sensitivities of Photopigments: Different cones have pigments that absorb light optimally at different wavelengths, contributing to color differentiation.

Evidence for Trichromacy:

• As demonstrated by George Wald (1964), spectral sensitivity functions of cones show an uneven distribution, confirming the presence of three types of color receptors within the retina.

Principle of Univariance:

• This principle explains how photoreceptors respond based solely on the amount of light absorbed rather than specific wavelengths.

Evidence for Opponent Process Theory:

• The limitations of trichromatic theory to explain perceptual phenomena such as color afterimages led to the establishment of opposing color pairs: red-green and blue-yellow.

Hue Cancellation Experiments:

• Hue cancellation demonstrates color opposition, where adding light from one color can neutralize another, providing insight into how colors are perceived in relation to one another.

Individual Differences in Color Perception:

• Genetic factors impact color perception, particularly affecting opsins situated on the X chromosome (linked to red and green cones) and on chromosome 7 (linked to blue cones), leading to varying color sensitivity and perceptions among individuals.

Color Vision Deficiencies:

• These deficiencies can occur when individuals possess fewer than three functional receptor types, often assessed using the Ishihara color test.

• Dichromacy: The condition where only two types of cones function properly.

• Monochromacy: The condition in which no functional cones exist.

• Subtypes Include:

  • Protanopia: L-cones missing (red deficiency).

  • Deuteranopia: M-cones missing (green deficiency).

  • Tritanopia: S-cones missing (blue deficiency, a rare condition).

Strategies to Help with Color Deficiencies:

• To assist those with color deficiencies:

  • Avoid using red and green for important distinctions;

  • Enhance brightness differences for clearer interpretation;

  • Utilize specialized glasses designed to improve color discrimination as needed.

Depth and Size Perception:

• Visual perception relies on depth cues that assist in determining size and spatial orientation, essential for object localization and navigation.

Definitions in Depth Perception:

• Various cues guide our depth perception informed by properties of retinal images, oculomotor feedback, and comparisons derived from each eye.

• Cue Types Include:

  • Oculomotor cues: Based on muscle feedback from eye movements such as accommodation and vergence.

  • Monocular cues: Cues that require one eye, including pictorial cues (like occlusion and relative height/size) and motion cues.

  • Binocular cues: Rely on the differences in images from two eyes due to their spatial separation, known as disparity.

Oculomotor and Monocular Cues:

• Depth perception utilizes differences in retinal images between the left and right eye, linked with muscular adjustments when focusing on objects.

Motion-Based Cues:

• Motion Parallax: This effect occurs when closer objects appear to move faster than distant ones as an individual navigates through an environment.

• Optic Flow: Refers to the patterns of movement perceived when one is moving toward or away from objects in their environment, important for grasping spatial relationships.

Motion Detection in the Retina:

• Motion detection involves specialized cells in the retina sensitive to continuous movement across various layers of the visual system, categorized under magnocellular and parvocellular processing pathways.

Key Elements for Motion Perception:

• Identifying the direction and speed of motion is crucial for object recognition and interaction.

• Distinguishing between actual object movement and movements of the observer's eyes is necessary for accurate perception.

Understanding Eye Movements:

• Types of eye movements include:

  • Saccadic movements: Quick shifts in focus from one point to another.

  • Smooth pursuit movements: Following the motion of a moving object smoothly, allowing the visual system to track movement effectively.

Corollary Discharge Theory:

• This theory postulates that signals about eye movements are sent to the visual cortex, which allows for accurate attribution of motion in the visual field, preventing misinterpretation due to eye movements.

Area MT and Motion Detection:

• Known as the "motion center", area MT houses large receptive fields optimally designed for detecting movement and plays a crucial role in interpreting motion cues, essential for perception navigation.

Motion Aftereffects:

• This perceptual phenomenon occurs when stationary images appear to move following exposure to motion, exemplified by the famous waterfall illusion, where viewers perceive the motion of objects after looking away from the moving stimulus.

Visually-Guided Action and Optic Flow:

• The interaction between perception and action is essential for guiding movements and spatial orientation during tasks like navigation, where visual input directly influences behavioral responses related to movement.