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 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