Psychophysics Quiz: Methods and JND

Psychophysical Methods

• Constant Stimuli uses predetermined stimulus strengths in random order to determine a participant's perception.
  • The intensity that corresponds to a 50% chance of accuracy is found by fitting responses to a curve.
• Limits involve starting with a strong or weak stimulus and gradually increasing or decreasing it until the participant can no longer detect it.
  • The average intensity where the participant's response changes is used.
• Adjustment lets the participant control the stimulus strength until it is visible or not visible.
  • The average point of change in the participant's responses is recorded.

Just Noticeable Difference (JND)

• JND: The smallest difference in stimulus strength needed to distinguish between two stimuli.
• Weber's Law: JND changes with stimulus intensity, but the proportion remains constant.
  • JND gets larger as intensity increases, meaning we become less sensitive to changes at higher intensities.
• Fechner's Law: A revision of Weber's Law that measures sensory magnitude.
  • Shows that sensitivity to change decreases as stimulus strength increases.
• Steven's Law: More flexible relationship than Weber's and Fechner's laws, capturing cases where perception is directly proportional or greater than physical change.

Psychological Dimensions Of Light

• Hue: Related to wavelength, but not always directly.
• Brightness: Determined by the amount of light reflected.
  • More reflected light results in a brighter color.
• Saturation: Represents the purity of the wavelengths of light.
  • Colors with more white have multiple wavelengths, resulting in less saturation.

Functions Of Color Vision

• Fruit Theory: Proposed that color vision evolved to help us detect ripe fruit.
• Object Recognition and Identification: Colors aid in recognizing and identifying objects in space.

Phenomena In Color & Color Vision

• Color Mixing:
  • Additive color mixing (light): Combining different wavelengths results in white light.
  • Subtractive color mixing (pigments): Pigments absorb light, reflecting the color they represent. Combining pigments leads to increased absorption and a darkening effect.
• Color Contrast:
  • Simultaneous contrast: Same color perceived differently based on background color.
  • Successive contrast (color afterimages): Viewing a color for an extended period results in an inverse afterimage.
• Color Intuition: Specific color combinations feel natural and intuitive, while others do not.
• Color Blindness: Vision without three different cone types (trichromacy).
  • Red-green color deficiencies: Most common, caused by a lack of the L-cone (red) or M-cone (green).
  • Yellow-blue color blindness: Tritanopia, caused by a lack of the S-cone (blue).

Theories Of Color Vision

• Trichromatic Theory (Young-Helmholtz Theory): Color vision relies on three primary color sensations (red, green, blue).
  • Supported by the discovery of cones and color-matching experiments.
• Opponent-Process Theory (Hering Theory): Color vision is the product of three opponent systems (red-green, blue-yellow, black-white).
  • Four primary color perceptions (red, green, yellow, blue) organized into opponent pairs.
  • Activating one color in a pair inhibits the perception of the other.

Edge Detection

• Edges: Regions of contrast (color, brightness, spatial frequency, etc.) marking object boundaries.
• Challenges to edge detection:
  • Integrating local edges into global contours.
  • Differentiating between true object boundaries and other edge features.
• Junctions: Intersections of edges, providing information about edge relationships in space.
  • T Junctions: One object occluding another.
  • X Junction: Transparent object occlusion.
  • L Junction: Corners.
  • Y Junction: 3D object corners.
• Boundary Detection: Uses edges to separate objects from their background.

Form & Object Perception

• The process of combining parts to create an object, even in cases where sections are separate.

Segmenting & Grouping

• Processes that identify separate and connected objects in a visual scene.
• Gestalt Principles: Classical approach to segmenting and grouping objects.

Gestalt Theory

• "The whole is greater than the sum of its parts."
• Humans naturally organize visual information into meaningful wholes.
• Properties:
  • Reification: Perceived object has more spatial information than the sensory stimulus.
  • Multistability: Ambiguous stimuli can have multiple interpretations.
  • Invariance: Recognition of objects regardless of orientation or position.