Visual pathways
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Visual Pathways. visual hemifields project contralaterally exception: bilateral representation of fovea! Optic nerve splits at optic chiasm about 90 % of fibers project to cortex via LGN about 10 % project through superior colliculus and pulvinar but that’s still a lot of fibers! .

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Visual Pathways

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Visual pathways

Visual Pathways

  • visual hemifields project contralaterally

    • exception: bilateral representation of fovea!

  • Optic nerve splits at optic chiasm

  • about 90 % of fibers project to cortex via LGN

  • about 10 % project through superior colliculus and pulvinar

    • but that’s still a lot of fibers!

Note: this will be important when we talk about visuospatial attention


Visual pathways1

Visual Pathways

  • Lateral Geniculate Nucleus maintains segregation:

    • of M and P cells (mango and parvo)

    • of left and right eyes

P cells project to layers 3 - 6

M cells project to layers 1 and 2


Visual pathways2

Visual Pathways

  • Primary visual cortex receives input from LGN

    • also known as “striate” because it appears striped when labeled with some dyes

    • also known as V1

    • also known as Brodmann Area 17


Visual pathways3

Visual Pathways

  • Primary cortex maintains distinct pathways – functional segregation

  • M and P pathways synapse in different layers

W. W. Norton


The role of extrastriate areas

The Role of “Extrastriate” Areas

  • Different visual cortex regions contain cells with different tuning properties


The role of extrastriate areas1

The Role of “Extrastriate” Areas

  • Consider two plausible models:

    • System is hierarchical:

      • each area performs some elaboration on the input it is given and then passes on that elaboration as input to the next “higher” area

    • System is analytic and parallel:

      • different areas elaborate on different features of the input


The role of extrastriate areas2

The Role of “Extrastriate” Areas

  • Functional imaging (PET) investigations of motion and colour selective visual cortical areas

  • Zeki et al.

  • Subtractive Logic

    • stimulus alternates between two scenes that differ only in the feature of interest (i.e. colour, motion, etc.)


The role of extrastriate areas3

The Role of “Extrastriate” Areas

  • Identifying colour sensitive regions

Subtract Voxel intensities during these scans…

…from voxel intensities during these scans

…etc.

Time ->


The role of extrastriate areas4

The Role of “Extrastriate” Areas

  • result

    • voxels are identified that are preferentially selective for colour

    • these tend to cluster in anterior/inferior occipital lobe


The role of extrastriate areas5

The Role of “Extrastriate” Areas

  • similar logic was used to find motion-selective areas

Subtract Voxel intensities during these scans…

…from voxel intensities during these scans

…etc.

STATIONARY

STATIONARY

MOVING

MOVING

Time ->


The role of extrastriate areas6

The Role of “Extrastriate” Areas

  • result

    • voxels are identified that are preferentially selective for motion

    • these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction

    • Akin to Human V5


The role of extrastriate areas7

The Role of “Extrastriate” Areas

  • Thus PET studies doubly-dissociate colour and motion sensitive regions


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