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 = contralateral I = 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 V1 Nissl stained Courtesy of David 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 cm 2 Total surface area of visual cortex: ~ 50 cm 2 ~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) * ~10 4 synapses/cell 250,000 neurons/mm 2 in V1; 100,000 neurons/mm 2 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? Function Anatomy Connectivity Topography Functional Organization of Macaque Visual Cortex

Hegdé & Felleman (2007)

Marr (1982) Model of Visual Processing Local primitives (e.g., edges) Surface-level representation Object-level representation

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:

Feed-forward Connections Develop Earlier Than Feedback Pathways Kennedy and Burkhalter (2004) Development of Visual Connectivity in the Macaque

Section III. Functional Organization

Ventral Pathway Area V4 Area AIT MT MST 7a Area V1 Dorsal 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.