Functional Neuro-anatomy of the Visual System: A Coarse Course
Jay Hegdé
How to Learn (Visual) Neuroanatomy
I. Distinguish 3-D structure from connectivity
II. Keep in mind that not all structures have (known) functions – biological structures are evolved, not designed.
III. Mind your Greek/Latin
Section I. Anatomy of Various Visual Structures
Developmental Bases of Neuroanatomy
Since the early visual system is anatomically highly ordered, visual field mapping can be highly useful in neuro-ophthalmological diagnosis.
Closer view of the Optic Chiasm
Early Visual Pathway
Clinically Important: Anatomy of Visual System Can be Highly Variable!
Diffusion Tensor Imaging (DTI) of Two Healthy Adults
Mnemonic:“On Old Olympus' Towering Top, A Friendly Viking Grew Vines And Hops”
Optic Nerve is Fundamentally Similar to Most Other Sensory Cranial Nerves
In humans, the optic nerve contains 38% of all the axons connecting to the brain.
Human Brain: Ventral View
(Tortured) Mnemonic:C-I-I-C-I-C
“See I? I See, I See”
Lateral Geniculate Nucleus (LGN)
C = contralateralI = ipsilateral
Functional Organization of Macaque Visual Cortex
Van Essen et al (1992)
Cortex has a laminar, canonical structure
Courtesy of David Hubel
A closer look at the laminae
Neocortical Layers (Total thickness ~1mm)
Opercular V1Nissl stained
Courtesy ofDavid Hubel
Probably a structure without a function
Ocular Dominance Columns in V1
Scale bar = 2 mm
Arrow: relieving cut
Area 17 of the cat / Layers 2 & 3
Cytochrome Oxidase ‘Blobs’
Another structure without a function? (Hmm…)
Some Facts and Figures about Macaque Visual Cortex
• Total cortical surface area: ~100 cm2
• Total surface area of visual cortex: ~ 50 cm2
• ~35 visual areas, ~25 primarily visual
• 323 known anatomical pathways; ~33% connectivity
• ~75-85% of visual cortical neurons are pyramidal cells* Glutamatergic (thought to be always excitatory)* ~104 synapses/cell
• 250,000 neurons/mm2 in V1; 100,000 neurons/mm2 elsewhere
• 10 billion axons in the white matter * ~10-20 million connect with nuclei outside the cortex * ~ 98.6% of the axons are intra-hemispheric * Corpus callosum contains ~100 million axons
Molecular Cognitive Science is Here Already!
Molecular pathways of plasticity in the visual cortex
Daw (2004)
Section II. Connectivity
Felleman and Van Essen (1991)
How are visual cortical areas distinguished from each other?
FunctionAnatomyConnectivityTopography
Functional Organization of Macaque Visual Cortex
Hegdé & Felleman (2007)
Image
PrimalSketch
2.5DSketch
3DSketch
Marr (1982) Model of Visual Processing
Local primitives (e.g., edges)
Surface-level representation
Object-level representation
An ImageAn Image
Early ‘Feed-forward’ Visual Pathways
Pyramidal Cell: The Workhorse of the Cerebral Cortex (‘Relay’ Neuron)
Stellate Cell: Most Common Interneuron (‘Crosstalk’ Neuron)
Inputs and Outputs of Sensory (Especially Visual) Cortex
From Crick (1995) [still largely current]
How known cortical connections join the layer 6→4 and layer 2/3 building blocks to form the entire V1/V2 laminar model.
Raizada R D S , and Grossberg S Cereb. Cortex 2003;13:100-113
Feed-forward Connections Develop Earlier Than Feedback Pathways
Kennedy and Burkhalter (2004)
Development of Visual Connectivity in the Macaque
Section III. Functional Organization
Ventral PathwayArea V4
Area AIT
MTMST
7a
Area V1Dorsa
l Pathway
Macaque visual system(Human visual system is fundamentally similar)
What Happens to the Visual Information Once It Gets to the Cerebral Cortex?
Visual Pathways in the Monkey
• A popular urban myth: The dorsal and ventral pathways are the magnocellular and parvocellular pathways, respectively. NOT TRUE!
Olshausen & Field, 2006
There is Much that We Don’t Know
This is even more true of other visual areas.