Neu 257; Winter 2010In Celebration of Darwin

Natural Selection and Conservation of Brain Circuits:

Alternate Migration and the Origins of the Mammalian Neocortex

Harvey J. Karten

University of California San Diego

Evolution of Sensory Projections Upon “Cortex”

Sensory Representations on Cortex

• Olfactory

• Visual

• Auditory

• Somatosensory

Theories of Telencephalic Organization Prior to 1960’s

• 1) Telencephalon as a derivative of the Olfactory System – Archi/Paleo Cortex

• 2) Birds/Reptiles: Mostly striatum• 3) “Neocortex” as a uniquely mammalian

expansion• 4) The brain rewires in the course of evolution

(Out with the Old and In with the New)

• WRONG, WRONG, WRONG & WRONG!

Yet Another Myth

• When you find “profound” similarities in brain nuclei, circuits and behaviors in different classes of vertebrates, it shows the wonder of Convergence/Parallelism!

• This logic does not pertain to the genome, and if the genome controls the wiring of the brain, why should be so quick to adopt a Crypto-Theological view of brain evolution?

Did “Neocortex” Arise As A Novel Product of Mammalian Evolution?

• Where did all those specific neurons of brainstem, thalamus and cortex come from?

Darwin (1809-1882)

The Origin of Species (24 November, 1859)

“I have called this principle, by whicheach slight variation, if useful, is preserved,

by the term Natural Selection.”

Visual Cortex

Rat Brain

Basal Ganglia

Cortex

Pigeon Brain: Lateral View

Studying Evolution of the Nervous System

• 1) What has evolved? Are there any evident precursors?

• 2) What are the molecular determinants of change?

• 3) What are the physiological and behavioral manifestation of those changes?

• 4) What are the driving forces of the change?

Proposition: Cortex did not evolve de novo with the appearance of

mammals.

•Cortical evolution can be analyzed as three related problems:

–1) The history of the constituent neurons and connections of the cortex–2) The laminar apposition of these populations. –3) Microcircuitry underlying information processing

•What are the molecular mechanisms of each of these events?

Rat Brain

Basal Ganglia

Cortex

Where is the "real" striatum?

Evolution of the Mammalian Sensory Systems &Cortical

Pathways• How does the mammalian auditory & visual system evolve?

•This can only be addressed by first understanding what may be the ancient pattern of organization of e.g., the vertebrate auditory & visual systems, and then identifying those cell and circuit properties that may be unique and novel amongst mammals.

The Nature of Mammalian Neocortex

•Peripheral sensory inputs reach the thalamus, and thalamus projects upon the cortex

•=========

•A simple model:

•Thalamic recipient neurons (e.g., Layer IV)

•Interneurons (e.g., Layers II-III)

•Cortical Descending Efferents (e.g., Layers V-VI)

Transposition/Migration during Development

Karten 1969, 1970

Karten 1969, Nauta & Karten 1970

Transposition/Migration during Development

Molecular Markers & the DVR Hypothesis

Recent studies of molecular markers reveal extensive similarities between cells of the DVR and those of individual layers of the mammalian ‘neocortex’ (Dugas Ford and Ragsdale)

In addition to EAG, RORbeta, Type III Tubulin, the DVR and the Cortex both express Ngn 1 and 2 as well as many other regulatory genes.

Layer 4 Markers:

Eag2/Kcnh5

Layer 4 Marker: RORbeta

Field LLayer 4 Markers:

Eag2/Kcnh5

Layer 4 Marker: RORbeta

Computational Implications

•What are the advantages, if any, of lamination?

•What unique properties does lamination confer upon those animals?

•Have we overstated the virtues of laminar structures?

Properties of Cortex

•Laminar populations

•Specific Afferentation and Efferentation

•Differential morphology and molecular properties

•Radial/columnar organization

•Recurrent loops

•Re-entrants

The Radial Hypothesis: Columns

The Radial Hypothesis Revisited I.

•The concept of columns and modules proved of great heuristic value in physiolgical investigation of the mammalian cortex. (Lorente de No, Mountcastle, Hubel & Wiesel)•Rakic postulated a radial unit hypothesis to explain the development of columns and modules•However, the evolutionary origins of the column and module in the forebrain were unknown.

Radial and Columnar Organization in the Avian DVR

• Auditory pathways to the telencephalon:– Nucleus ovoidalis thalami (Karten 1967)– Field L (Karten 1968) – Field L as homologue

of Layer IV of auditory cortex

• Tonotopic representation in Ovoidalis & Field L (Konishi and Zaretsky)

• Laminar subdivisions of Field L (L1, L2, L3, HV) (Bonke and Scheich, 1979)

Tonotopic & Columnar Organization of Field L

Evolutionary Origins of Radial Units

•Studies of the Auditory region of Field L with 2-DG, showed columnar monotonic organization (Bonke & Scheich 1979)

•Does columnar/radial organization exist in the Avian/reptilian DVR?

Granular cells in L2aOther cell types

in L2a

20 micrometer

Mammalian AI

I

II

III

IV

V

VI

Non-auditory inputsICc ICx

MGvNon-auditory inputs

MGmPINMGd

Lemniscal pathway

Nonlemniscal pathway

Amygdale

Auditory inputAuditory input Non-auditory inputsMLd ICo

OvNon-auditory inputs

Avian auditory cortex

L2

Nd & Ai

L3

HV

L1

Lemniscal pathway

Nonlemniscal pathway

“Ov Shell”

Descending projections Descending projections

FunctionalModular

FunctionalModular

Amygdale

Evolutionary Origins of Radial Units

•Contrary to our long held beliefs, radial and columnar units exist in the DVR of nonmammalian vertebrates.

•These discoveries imply that graded degrees of lamination and radial-columnar interactions arose at an early stage of telencephalic evolution.

•Homologous computational components of cortical-equivalent neurons is most likely a property common to all amniotes. •HJK – May 2005

What’s Next?

•What are the molecular events that determine these highly conserved properties of cells and circuits? What are the molecular events that mediate the modifications in alignment of these cells that lead to the evolution and development of the mammalian type of cortex?

What is the significance for computational modeling of cortical processing?

• We still do not have an accurate map of the circuitry of the cortex

• What is the implication for first order processing?

• What is the ratio of stellates to granule cells? (1:10 or greater?)

Evolution, Connectomics & Natural Selection

Circuitry responsible for specialized functions is highly conserved.

Once established and operational, only minor changes generally appear.

The more detailed the similarities, the greater the likelihood of common origin

Natural Selection narrows the prospects for major changes in circuitry and function

Connectomics and Genomics

How are connections, transmitter/receptors, cell morphology, ion channels specified during embryogenesis?

How are the vast numbers of specific neurons and connections encoded in the genome

To what extent are they subject to environmental modification and/or differences between individuals of a species?

Stability, Species Changes, Individuality, and Evolution of

Circuits• Neural circuits underlie all operations of the brain• Neural circuits are both highly conserved, but also the

target of vast expansion (and loss) in the course of evolution and individual aging.

• Specific circuits are often unique to individual species

• Many circuits are common to virtually all vertebrates

Genomics, Connectomics and Evolution

• The paradox of neural circuitry – conservation and change.

• Enormous variation within a highly conserved core set of properties

• The conserved properties imply genetic regulation• How is this encoded in the genome?• “Inherited musical or mathematical abilities” – is

this “in the genes” that regulate neural circuitry

A Few Postulates About Evolution and Circuitry

• Primary Sensory and Motor Processing is highly conserved (initial stages of cognitive processing)

• Homologous circuits operate in similar manner• Homologous circuits mediate similar “core” behaviors• Closely similar behaviors have high probability that they are

mediated by homologous circuits• This is true for central visual, auditory, somatosensory

pathways from periphery to telencephalon.• Equally true for descending projections from telencephalon• At this time, we have few ideas about how neural cell

typology and connections are encoded in the genome.

Connectomics and Genomics

• How are connections, transmitter/receptors, cell morphology, ion channels specified during embryogenesis?

• To what extent are they subject to environmental modification

Henry Beston’s Outermost House

“We need another and a wiser and perhaps a more mystical concept of animals. Remote from universal nature, and living

by complicated artifice, man in civilization surveys the creature through the glass of his knowledge and sees thereby a feature magnified and the whole image in distortion. We

patronize them for their incompleteness, for their tragic fate of having taken form so far below ourselves. And therein we err, and greatly err. For the animal shall not be measured by

man. In a world older and more complete than ours they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we

shall never hear. They are not brethren, they are not underlings; they are other nations, caught with ourselves in

the net of life and time, fellow prisoners of the splendour and travail of the earth.”

Nothing in Biology Makes Sense Except in the Light of Evolution

Theodosius Dobzhansky, 1973

William Hodos

Walle Nauta

Anton Reiner

Nicholas Brecha

Yuan Wang

J. Martin Wild

Agnieszka Prechtl

Jorge Mpodozis

Harald Luksch

Jaap Dubbeldam

Locations:

WRAIR

MIT

SUNY

UCSD