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 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 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 Pathways

W. W. Norton

• Primary cortex maintains distinct pathways – functional segregation

• M and P pathways synapse in different layers

The Role of “Extrastriate” Areas

• Different visual cortex regions contain cells with different tuning properties

The Role of “Extrastriate” Areas

• Consider two plausible models:

1. 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

2. System is analytic and parallel:– different areas elaborate on different features of the input

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” Areas

• Identifying colour sensitive regions

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

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” Areas

• similar logic was used to find motion-selective areas

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

MOVING STATIONARY MOVING STATIONARY

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” Areas

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