Ventral and Dorsal Pathways

• Ventral and Dorsal Streams:

  • Ventral = the What pathway (DOWN)

  *V1 - Inferotemporal Cortex

  *Prosopagnosia = facial blindness

  *V2 and V4

  • Dorsal = the Where pathway (UP)

  *V1 - Parietal Cortex - Frontal Cortex

  *Optical Ataxia = difficulty locating an object

  *V2 , V3 , V5

  ! Pathways are connected via visual streams/channels
• Outputs from V1 - V2 - HIGHER AREAS:

  • 4B (direction of motion) - V2 (thick stripes - movement direction and binocular disparity) - V3 (depth processing) and V5/MT (motion processing)
  • Interblobs (layers 2&3 = Orientation) - V2 (pale stripes - real/illusory orientation/contrast) - V4 (colour and form processing)
  • Blobs (layers 2&3 = Colour) - V2 (thin stripes - colour and contrast) - V4 (colour and form processing)
• Inferior Parietal Lobule: (IPL)

  • Inside parietal lobule (where MST projects to) - translates visual space to motor space
  • Split into 2 - Lateral Intra Parietal Cortex (LIP) and Area 7a

  1)Fixation spot appears - eye move to this spot

  2)In periphery, another spot appears briefly

  3)7a responds briefly as to where the spot is whereas LIP maintains a response to peripheral target even after it has disappeared

  4)Eye moves to take up fixation on where peripheral spot was via 7a

  5)Both 7a and LIP respond to eye movement

  *LIP has memory - maintains a response

  *7a has no memory
• Dorsal Pathway:

  • V1 - V2 - V3 (depth) - V5 (motion) - Parietal Cortex (MST - Optic Flow) (IPL - LIP and 7a Eye Movements) - Frontal Cortex (Frontal Eye Fields/FEF - Eye Movements

  *Both IPL and FEF have strong links to Superior Colliculus (EYE MOVEMENT)

  *FEF also has memory
• The Role of the Inferotemporal Cortex: (WHAT - VENTRAL)

  • Responds to components of face and hands
  • Helps read facial expressions and body language
  • Very large receptive fields, all include fovea
  • Cells may be more/less responsive according to angle of view or familiarity
  • Cells generally do not care about size or position of object within receptive field
• Receptive Field Sizes:

  • As you move further away from V1, receptive field size increases
  • They also increase in complexity (responding to shapes and not just edges)

  V2 - Illusory Contours:

  • Orientation selective cells respond the same to illusory and real contours - strength of response increases with number of inducers
  • Responds to more complex shapes - curves

  V4 - Even More Complex Shapes:

  • respond to spikes - toilet brushes
  • responsive to combinations of colour and form