LGN and V1

• Primary Visual Cortex Outputs:

  • Feedback from LGN = 6
  • To Extrastriate Areas (outside V1) = 2 , 3 , 6
  • To Superior Colliculus (eye movement control) = 5

  
• Primary Visual Cortex Inputs: FROM LGN

  • Contralateral - opposite side

  *Layer 1 (M) = 4ca

  *Layers 4+6 (P) = 4cb + 4a

  *Interlayers (K) = 3

  • Ipsilateral - same side

  *Layer 2 (M) = 4ca

  *Layers 3+5 (P) = 4cb + 4a

  *Interlayers (K) = 2/3

  • M = Magnocellular (2 layers)
  • P = Parvocellular (4 layers)
  • K = Koniocellular (in b/w them)

  
• Lateral Geniculate Nucleus:

  • 1 on each side of brain - inside thalamus
  • Optic Nerve - Optic Chiasm - LGN - V1
  • Left Visual Field = LE nasal retina & RE temporal retina = projects to Right LGN
  • Right Visual Field = RE nasal retina & LE temporal retina = projects to Left LGN

  *ONLY NASAL FIBRES SWAP OVER AT OPTIC CHIASM

  • 3 types of ganglion cells = Magnocellular , Parvocellular, Koniocellular

  
• 1)MAGNOCELLULAR GANGLION CELLS:

  • 2 layers - 10% of total
  • Large receptive fields = more photoreceptor inputs
  • Not sensitive to colour - only black and white
  • Transient Response = high quick response which does not remain constant
  • Some are sensitive to directions of visual motion
  • Sensitive to low contrasts and saturate when contrast is high
• 2)PARVOCELLULAR GANGLION CELLS:

  • 4 layers - make up 70-80% of total
  • Smaller receptive fields = less photoreceptor inputs

  *One-to-one in fovea (photoreceptor and ganglion cell)

  • Sensitive to colour and fine details (can pinpoint where light hits retina as less photoreceptors)
  • Sustained response = short response but this remains constant for longer
  • Not sensitive to low contrasts and do not saturate at high contrasts
• 3)KONIOCELLULAR GANGLION CELLS:

  • make up 8% total
  • sensitive to colour - very first sites for yellow-blue comparison
  • they are the only cell class known to carry the s-cone signal (responds to blue light)
• V1 - PRIMARY VISUAL CORTEX:

  • Axons leaving the LGN make up the optic radiations that project into V1/Striate Cortex/Area 17
  • Retinotopy = nearby cells in V1 represent nearby locations in the visual field (ascending from lower visual areas to higher)

  *RIGHT = 1+2 (fovea) , 5+6 , 9+10

  *LEFT = 3+4 (fovea) , 7+8 , 11+12

  • Cortical Magnification = mm of cortical surface per degree of visual angle

  *Fovea has the highest mag factor = highest density of photoreceptors (therefore resolution)

  • V1 has 6 layers, 4 divided into 3 (4a,4b,4c) , 4c divided into alpha and beta
• Structure of V1:

  • Ocular Dominance Columns - each column in layer 4 receives input from LGN per eye (arranged LE,RE,LE,RE..)

  *Temporal columns correspond to the same area of retina for that eye

  *Underpins stereopsis - depth perception

  • Simple Cells - orientation selective and contains columns of blobs (code for colour)
  • Complex Cells - detect movement (orientation slabs)
  • Hypercomplex Cells - determine length (formed from complex cells)

  COML

  
• Receptive Fields for Orientation:

  • V1 has receptive fields that have a centre and surround but they are not circular (circular ones only found in retina and LGN)

  *Maximum response occurs at 45 deg

  • Bar Detectors = 3 columns (has centre)
  • Edge Detectors = 2 columns (doesn't really have centre)

  ! Wide Receptive Fields like Low Spatial Frequencies

  ! Narrow Receptive Fields like High Spatial Frequencies

  
• Simple Cells Receptive Fields: LAYERS 4+6

  • Circular receptive fields added together to make orientation selective receptive fields
  • Helpful for edge enhancement
  • Centre dominates colour of edges

  Complex Cells: LAYERS 2,3,5

  • Sum up multiple simple cells of same orientation to produce receptive field with overlapping ON/OFF responses
  • Cell only responds to input from simple cells
  • Detects movement

  Hypercomplex/End-Stop Cells: LAYERS 2-4

  • Max response = defined length
  • Min response = stimulus is longer than them
  • Formed from 3 complex cells with a centre