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.