There are eyes in the back of your head.

Well, okay, not actual eyes. But at least there are vsion processors for the information that comes from the eyes. The occipital lobe is at the back of the head, just above the neck. This is the primary projection area for vision.

Sensory information comes from the eyes, passes through the LGN, reaches the occipital lobe, is processed and then routed to both the parietal and temporal lobes. The cortex here is striated into 6 layers. Each has its own speciality.

The processed visual information is distributed to both the parietal and temporal lobes. The dorsal path goes from the occipital lobe up to the parietal lobe to help give you a 3D view of the world. The ventral path goes along the side of the head to the temporal lobe where you keep your mental encyclopedia.

 

 

• Eye to Occipital
• 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