Lecture 03 Visual Perception Flashcards
Perception
- Picking up sensory information from external environment
- Process into perceptual experience
Human Eye
- Photoreceptors
- Ganglion cells
Photoreceptors
- Rods
- Cones
Rod Cells
- Monochromatic
- Sensitive
Cone Cells
- Colors
- Sharpness
What are the differences between rod and cone cells
- Distribution
- Connection
Ganglion Cells
Rod convergence: light sensitivity, less acuity
Neural Processing
Found early stage in the eyes
- Lateral inhibition
- Edge enhancement
Lateral Inhibition
Stimulation of a cell inhibit the activity of neighboring cells
Edge Enhancement
Chevreul illusion or staircase illusion
Visual Pathway
- Retina
- Pre-optic tracts: separated left and right eyes
- Optic chiasma
- Post-optic tracts: combined left and right eyes
- Lateral geniculate nuclei (LGN)
- Visual cortex
Retinotopic Mapping
RETINAL cells correspond to points of
surface of the visual cortex
Receptive field
Visual space that correspond to retinal region
- at Fovea
3 Specialized Neurons
- Dot detectors
- Edge detectors
- Movement detectors
Parallel Processing
Different steps are processed simultaneously
- MT: speed and movement
- V4: share and color
V1 & V2
Early stage processing
- Color
- Form
V3
Form and shapes but not color
V4
Color and line orientation
V5
Visual motion
Medial temporal cortex (MT)
Speed and movement
Two Visual Pathways
- Ventral
- Dorsal
Ventral Stream
Occipital-Parietal: What
- Object identification
- Visual agnosia
Dorsal Stream
Occipital-Temporal: Where
- Object location
- Visually guided action
- Optic ataxia
Visual Agnosia
Inability to recognize object
- No difficulty in object orientation or reaching
Optic Ataxia
Difficulty in reaching
- No object identification
Visual Illusions
Vision-for-action (how pathway) should not be affected by visual illusions
Ebbinghaus illusion
- Verbal judgement → stronger illusion effect
- Grasping → weaker illusion effect
3D object sensitivity
Ventral pathways damage doesn’t affect
The Holistic Processing of Faces Brain Part
TMS on parietal cortex (dorsal) disrupts
Color Vision
- Hue – colors
- Saturation – vividness (amount of white)
- Brightness (value) – intensity of light-to-drak
Color Vision Functions
- Recognize
- Categorize objects
E.g., under- or overripe fruits
Trichromatic Theory
- Three types of cone receptors
1) Short-wavelength (blue)
2) Medium-wavelength (yellow- green)
3) Long-wave length (orange-red) - Perceived color is determined by relative stimulation levels
Opponent-process Theory
- Red-green channel
- Blue-yellow channel
- Achromatic channel (white-black)
- Negative afterimage [link] cannot be explained by the trichromatic theory
- Adaptation of presented color -> higher sensitivity of opponent color in white light
- Opponent color cannot be seen together (e.g., bluish yellow or reddish green).
- Color blindness for both color in a channel
Color Constancy
- Surface or object is perceived as having the same color when the illuminant (light source) is changed
- Color perception does not depend solely on the wavelengths of the light
Familiarity effect
- Chromatic adjustment method/Chromatic adaptation
- Over-adjustment
Scene Illumination
- Reflected wavelength is influenced by light source
- Light source estimation is accurate -> High color constancy
- The Dress: White/gold or blue/black
1) Ambiguous light source
2) Depend on how observers assume light source
Form Perception: Gestalt Approach
- Perceptual organization principle
- Figure-ground organization
- Law of prägnanz (good figure)
- Innate and intrinsic
Organization and Features
- Bottom-up process
- Marr’s Computational Approach
- Biederman’s Recognition-by Components Theory - Top-down process
Marr’s Computational Approach
- Primal sketch – 2-D description
- 2.5-D sketch – depth and orientation
- 3-D model representation – shape and relative position
Biederman’s Recognition-by Components Theory
GEON: Basic shapes (geometric ions)
Top-down Process
- Incoming signals from cortex to primary visual cortex.
- Strong effects for ambiguous figures
- Interactive-iterative framework
Depth Perception
- Transformation of 2-D retinal images to 3-D representations.
- Ambiguous information
- Cues
Cues
1) Monocular cues
2) Binocular cues
3) Oculomotor cues
Monocular Cues
“Pictorial cues”
- Interposition
- Linear perspective
- Familiar size
- Blur
- Texture gradient
- Motion parallax
Binocular Cues
Binocular disparity
- Stereopsis: ability to perceive in 3-D
- Decrease with distance
Oculomotor Cues
- Vergence: Eye Rotation
- Accommodation: Eye focus
Vergence
Eye Rotation
Accommodation
Eye focus
Size Constancy
Distance -> different retinal images
- Haber & Levin (2001)
- Invariant size - e.g., bicycle
- Variable size - e.g., TV
- Unfamiliar - e.g., ovals
Helmholtz’s theory of unconscious inference
Humans unconsciously made inference based on retinal image size and distance
Accuracy of Size judgment
- Invariant size > Variable size > Unfamiliar
