15 Notes: Occipital Lobe – biologicalpsych.com

Retina
- Net of layers
- Ganglion cells = to brain
- Amacrine cells = interneurons
- Bipolar cells = receptor output
- Horizontal cells = sharp edges
- Rod & cones
Output channels
- 3 Color receptors (plus B-W)
- 3 Channels of information
- Retina info is sorted into three “channels”
Color
- Molecules absorb light
- Even molecules come in colors
- If hit by light, molecule changes
  - Chromophore
  - Form of Vitamin A
  - Photons changes it shape
- Causes activation of large protein
  - called an opsin
- Opsin
  - Several types, similar process
  - Rods
    - Thermally stable
    - Rhodopsin
  - Cones
    - Less stable
    - Photopsins
    - Long = Red region
    - Medium = Green region
    - Short = Blue region
    - Respond to range of wavelengths
      - Not just one color
      - Varies with light intensity
Photo Receptors
- Different combos of 3 pigments
- Each cone detect all colors
- Level of energy need varies
- Color is pattern of activity
- Yellow = L more than M
- Red = L much more than M
- Green is easy to see
- 3 Color receptors (plus B-W)
  - Long = slow red light
  - Medium = medium green light
  - Short = fast blue light
  - Rods = intensity
Retina output
- Spatially encodes images
- Filters & compresses data
- 100 times more receptors than ganglion cells
- 1.5 million ganglion cells
Fovea Some cones 1:1
- Fovea edge Some cones 5:1
- Periphery Thousands to 1
Spontaneously firing base rate
- Increase rate = excitation
- Decrease rate = inhibition
Types of ganglion cells
- Midget
  - 80% of ganglion cells
  - Small dendritic trees
  - Small center-surround fields
  - Small bodies; slow
  - Mostly from midget bipolar (1:1)
  - Color but weakly to contrast
  - Parvocellular; P pathway
  - B cells
  - Synapse only to LGN
- Parasol
  - Respond well to low-contrast $
  - Center-surround large fields
  - Magnocellular
  - M pathway
  - A cells
  - Respond best to moving stimuli
  - Most synapse to LGN
  - Few to other areas of thalamus
- Bistratified
  - Small as dust cells
  - 10% of ganglions
  - Koniocellular
  - K pathway
  - Moderate # of inputs
  - Moderate resolution
  - Moderate contrast
  - Moderate speed
  - Center but no surrounds
    - Always on to blue
    - Always off to red and green
- Misc
  - Photosensitive Ganglion Cells
  - Giant retinal ganglion cells
    - Melanopsin
    - Light responsive
    - Circadian rhythm
  - Other cells too (more than you need to know now)
Ganglion cells
- Retina output
  - Form the optic nerve (optic tract)
  - Leave eye through blind spot
- Function
  - abstract & enhance cone signals
  - recognize diff in color
  - despite variations in light level = color constancy
Theories of Color
- 1. Trichromatic
  - Young-Helmholtz Theory
  - 3 types of cones
  - Doesn’t explain red-green color blindness
- 2. Opponent-Process Theory
  - Paired opposites:
  - white-black
  - red-green
  - yellow-blue
  - Afterimages from fatiguing
  - Prolonged stimulation
  - Doesn’t explain color constancy
- 3. Retinex Theory
  - Recognize color as light changes
  - Cortex compares inputs
  - Determines appropriate bright
Optic nerve problems
- Multiple Sclerosis
  - One of the places it impacts
  - De-myelinization
  - Blurred vision, etc.
Optic Chiasm
- Transfer neural into
- Left fields to R side
- Inside switches; switch your nose
- Two binocular half images
Geniculo-Striate Pathway
LGN
- Lateral Geniculate Nucleus
  - Part of thalamus collection
  - 90% of fibers go to LGN
  - 10% go to Superior Colliculus
    - controlling eye movements
- LGN input
  - Cortex
  - Input from retina
  - More input from cortex
    - Small signal back to cortex
  - 10 in from retina
  - Sends 4 to cortex
  - Also receives signals from:
    - Other parts of thalamus
    - Other parts of LGN
    - Brain stem
Striate Cortex
- Development of Visual Cortex
- LGN and V1 develop early
- Needs real life to fine-tune them
Primary projection area
- 5 major layers
- Striped look
  - V1 = 1st stage of processing
  - V2 = associations (circle, angles)
  - V3 = lower visual field
  - V4 = color & spatial
  - V5 = motion+
Primary Visual Cortex (V1)
- Striate cortex in occipital lobe
- 1st stage of visual processing
- Most visual input goes into V1
Striate Neurons (Neurons in V1)
- 1. Simple cells
  - Only in V1
  - fixed excitatory & inhibitory zones
  - Most have bar-shaped or edge-shaped receptive fields
- 2. Complex cells
  - In V1 or V2
  - Orientations of light
  - No fixed excitat-inhib zones
  - Input from combos of simple cells
- 3. Hyper-Complex cells
  - End-stopped
  - Bar-shaped recpt. field at 1 end
  - Like complex cells
  - But with strong inhibitory area
Columns of Cortex
- Grouped in columns
- Perpendicular to the surface
- Arrange by specific function
  - Left eye only
  - Both eyes equally
  - One orientation only
Feature Detectors?
- Prolonged exposure decreases sensitivity
- Stare at waterfall illusion
- Looks like flowing upwards
Damage to V1
- No conscious vision or visual imagery, even in dreams
- Blind sight
2 pathways from occipital lobe
- 1. Dorsal Path (where)
  - To parietal lobe
    - 3D view of the world
  - Damage
    - Have most normal vision
      - can read
      - recognize faces
      - describe objects in detail
    - Know what things are but not where
    - Can’t reach out and grab
- 2. Ventral Path (what)
  - To temporal lobe
  - Encyclopedia
  - Damage
    - See where but not what
    - Can see and grab
    - Can’t watch TV
    - Can’t tell what is what
Inferior Temporal Cortex
- Underside of temporal lobe
- Input from occipital lobe
- Identifying objects
- Cells R to physical $
- Also to what viewer perceives
- Figure & background
- R same way even if change position, size and angle
- Important for shape constancy
- Face recognition
  - fusiform gyrus of inferior temporal cortex
Fusiform gyrus
- Face recognition
- Car model identification
- Bird species
Object Recognition
- Able to identify objects
- Changes in orientation
- Moderately occluded
- Changes in size
- Novel examples of objects
- Degraded images
Retina image varies
- Size of retinal image impacted by
- Distance from image
- Which retina part $ impacted by
- Vantage point viewed
- Relative loc. of object-viewer
Rotational Invariance
- Different angles & vantage points
- Even if never seen before
- More local features
Size Invariance
- Actual or apparent size variations
- But not at extremes
Translational Invariance
- Moved to a new position
- $ different part of retina
- Still recognize it
- Not absolute position in environment
- Not relative position to objects
Objects with missing parts
- Correctly ID if have 2 or 3 parts
- Missing 1 sail is easy
- Not when 1 part only
Geons Theory
- The major idea
  - visual system extracts geons
    - (basic shapes)
    - cubes, spheres & wedges…
  - Stored in brain as structural descriptions?
  - Which geons
  - How interrelate (cube on top of triangle)
  - Parse object into geons
  - Determine interrelations
  - Maybe as few as 36 geons
  - Local features = not enough
Dual Recognition Theory
- Primal recognition
  - fast-acting
  - not higher-level cognitive processes
- Higher-level processing
  - shading, texture, or color
  - top-down processing of environmental cues
- Use context to ID difficult ones
Agnosia
- Lose ability to recognize
  - Objects and shapes
  - Faces
  - Sounds
  - Smells
- Visual agnosia
  - Can’t recognize objects
  - Lesion in
    - Left occipital lobe
    - Left temporal lobe
- Form agnosia
  - Can’t perceive whole
  - Only recognize parts
Lateral fusiform gyrus
- Top of temporal lobe where meets occipital lobe
- Vital for object & face recognition
  - processing color info
  - word recognition
  - number recognition?
  - within-category identification
- Left & Right work together
  - Interconnected
  - Left = recognizes “face-like” features in objects
  - Right = determines if actual face
Face Recognition
- Infant Vision
  - Infants strongly prefer: Faces
  - 2 days old, mimic expressions
    - Not aware of emotional content
  - At 2 months
    - want parts in right places
  - Five-month olds,
    - Pay same attention to happy and fearful faces
  - Seven-months
    - Focus more on fearful faces
Face Recognition is a very difficult task
- Lots of info to process
- Gender, expression, age, pose…
- Estimating age from face images is hard
Faces are so similar
- Greebles
- Complex 3D objects
- Organized into two categories: gender & family
- Expert greeble identifiers
- Activity in right middle fusiform gyrus is similar to when recognizing faces
- Novice greeble idenfiers
- Not similar
Right hemisphere
- holistic strategy
Left hemisphere
- analytic strategy
Right lateral fusiform gyrus
- hallucinations of faces
- Charles Bonnet syndrome
- Hypnagogic hallucinations
- Peduncular hallucinations
- drug-induced hallucinations
- perception of emotions in facial $
- may be related to face blindness (prosopagnosia)
Prosopagnosia = Impairment in recognizing faces
- usually caused by brain injury
- differ in abilities to understand face
- Inability to recognize faces
- No loss of vision or memory
- Can identify young-old
- Can indentify male-female
- Not know who they are
Lateralization in face identification
- Male use right hemisphere
  - men are right lateralized for object and facial perception
- Women use left hemisphere
  - left lateralized for facial tasks
  - right or neutral for object perception
Sex differences
- Men tend to recognize fewer faces of women than women do
- No sex differences with male faces
- Several independent sub-processes working in unison?
Best at familiar faces
- People we know
- People related to
- People who look like us
- Same ethnicity

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