Flexor-Crossed ExtensorReflex(Sheridan 1900)

Painful Stimulus

Reflex CircuitsWith Inter-neurons

A Simple model or Minimum Circuit:

SensoryNeuron

InterNeuron

InterNeuron

Flexor

Extensor

MotoNeuron

MotoNeuron

Pain Stimulus

+ ++

- -

HIGHER LEVEL CONTROL

InterNeuron

InterNeuron

Flexor-crossed Extensor (Sheridan 1900)

Gaits of the cat: an informal computational model

Neural Development

CS182/CogSci110/Ling109 Spring 2007

Jerry Feldmanfeldman@icsi.berkeley.edu

Lecture Overview

Summary Overview Development from embryo Initial wiring Activity dependent fine tuning Additional information/details

Principles of Neural Science. Kandel, Schwartz, Jessell, Mcgraw-Hill (2000).

How does this happen

o Many mechanisms of human brain development remain unclear, but

o Neuroscientists are beginning to uncover some of these complex steps through studies of

o the roundworm, fruit fly, frog, zebrafish, mouse, rat, chicken, cat and monkey.

o Many initial steps in brain development are similar across species, while later steps are different.

o By studying these similarities and differences, we can learn how the human brain develops and hopefully how brain abnormalities, such as mental retardation and other brain disorders, can be prevented or treated.

Summary Overview The Amoeba (ref. book) uses

complex sensing molecules penetrating its cell membrane to trigger chemical mechanisms that cause it to move its blobby body towards food and

away from harmful substances.

Neurons are also cells and, in early development, behave somewhat like Amoeba in approaching and

avoiding various chemicals. But rather than the whole cell moving, neural growth involves the outreach of the

cell’s connecting pathway (axon) towards its downstream partner neurons.

Summary Overview

The basic layout of visual and other maps is established during development by millions of neurons each separately following a pattern of chemical markers to its pre-destined brain region and specific sub-areas within that

region.

For example, a retinal cell that responds best to red light in the upper left of the visual field will connect to cells in the

brain that are tuned to the same properties and these cells, in turn, will link to other cells that use these particular

properties giving rise to specific connectivity and cell receptive field properties (topographic maps).

Summary Overview In the course of development,

detector molecules in the growing neuron interact with guide molecules to route the connection to the right general

destination, sometimes over long distances as in the connection from the spinal cord to the knee.

This process will get neural connections to the right general area, but aligning the millions of neurons in visual and other neural maps also

involves chemical gradients, again utilizing mechanisms that are very old in

evolutionary terms.

Summary Overview

When an axon tip gets to an appropriately marked destination cell, the contact starts a process that develops rudimentary synapses.

Local competition among neural axons with similar marker profiles produces some further tuning at the destination.

Summary Overview:Activity Dependent Tuning In fact, the initial wiring is only approximate and leaves

each neuronal axon connected to several places in the neighborhood of each of its eventual partner neurons.

A second, activity dependent, mechanism is required to complete the development process.

The initial chemical wiring actually produces many more connections and somewhat more neurons than are present in adult brains.

The detailed tuning of neural connections is done by eliminating the extra links, as well as the strengthening functional synapses based on neural activity.

Summary

Neural development and learning is moving from a mystery to routine science.

We know enough to shape theories of how our brains learn skills, including language, and how we acquire and use knowledge.

Contemporary theories of learning are also heavily influenced by psychological experiments, some of which are described next week.

Lecture Overview

Summary Overview Development from embryo

Neural tube development Cell division and neuronal identity Mechanisms for cell type formation and

communication Initial wiring Activity dependent fine tuning Plasticity and Learning

Development from Embryo

The embryo has three primary layers that undergo many interactions in order to evolve into organ, bone, muscle, skin or neural tissue. The outside layer is the ectoderm

(skin, neural tissue),

the middle layer is the mesoderm (skeleton, cardiac) and

inner layer is the endoderm (digestion, respiratory).

Neural Tissue The skin and neural tissue arise from a

single layer, known as the ectoderm in response to signals provided by an adjacent

layer, known as the mesoderm. A number of molecules interact to determine

whether the ectoderm becomes neural tissue or develops in another way to become skin

Neural Tube formation

In humans, during the 3rd week, this mesoderm begins to segment. The neural plate folds to form a neural groove and folds.

The neural groove fuses dorsally to form a tube at the level of the 4th somite and "zips up” cranially and caudally and the neural crest migrates into the mesoderm (somites differentiate to form vertebrae, muscles).

Other structuresincluding heart,Skeleton, etc.

BRAIN DEVELOPMENT. The human brain and nervous system begin to develop at three weeks’ gestation as the closing neural tube (left). By four weeks, major regions of the human brain can be recognized in primitive form, including the forebrain, midbrain, hindbrain, and optic vesicle (from which the eye develops). Irregular ridges, or convolutions, are clearly seen by six months.

Brain Weight

Lecture Overview

Summary Overview Development from embryo

Neural tube development Cell division and neuronal identity Cell communication

Initial wiring Activity dependent fine tuning Plasticity and Learning Development and Infant behavior

Neural cell categories

After the ectodermal tissue has acquired its neural fate,

another series of signaling interactions determine the type of neural cell to which it gives rise.

The mature nervous system contains a vast array of cell types, which can be divided into two main categories: the neurons, primarily responsible for signaling, and supporting cells called glial cells.

Factors/gradients in cell formation Researchers are finding that the destiny of neural tissue

depends on a number of factors, including position, that define the environmental signals to which the cells are exposed. For example, a key factor in spinal cord development is a

secreted protein called sonic hedgehog that is similar to a signaling protein found in flies.

The protein, initially secreted from mesodermal tissue lying beneath the developing spinal cord, marks young neural cells that are directly adjacent to become a specialized class of glial cells.

Cells further away are exposed to lower concentrations of sonic hedgehog protein, and they become the motor neurons that control muscles.

An even lower concentration promotes the formation of interneurons that relay messages to other neurons, not muscles.

Timing of Cell Differentiation

Remarkably, the final position of the neuron (its laminar position) is correlated exactly to its birthdate The birthdate is the time of final mitosis

Cells leaving later migrate past the older neurons (in deeper cortical layers) to the outermost cortex.

The layering of the cortex is thus an inside-first outside-last layering.

As the brain develops, neuronsmigrate from the inner surfaceto form the outer layers. Left:Immature neurons use fibersfrom cells called glia ashighways to carry them to theirdestinations. Right: A singleneuron, shown about 2,500times its actual size, moves ona glial fiber. (10-6 m/hr)

Illustration by Lydia Kibiuk, Copyright © 1995 Lydia Kibiuk.

Improper migration leads to diseases including childhood epilepsy, mental retardation, lack of sense of smell and possibly others.

Hiroshima Nagasaki Effects

Lecture Overview

Summary Overview Development from embryo Initial Wiring details

Axon Guidance Synapse formation

Activity dependent fine tuning Plasticity and Learning Development and Infant behavior

Axon guidance mechanisms

Axonal growth is led by growth cones Filopodia (growing from axons) are able to sense the

environment ahead for chemical markers and cues. Mechanisms are fairly old in evolutionary terms.

Intermediate chemical markers Guideposts studied in invertebrates

Short and long range cues Short range chemo-attraction and chemo-repulsion Long range chemo-attraction and chemo-repulsion

Gradient effects

Axons locate their target tissues by using chemical attractants (blue) and repellants (orange) located around or on the surface of guide cells. Left: An axon begins to grow toward target tissue. Guide cells 1 and 3 secrete attractants that cause the axon to grow toward them, while guide cell 2 secretes a repellant. Surfaces of guide cells and target tissues also display attractant molecules (blue) and repellant molecules (orange). Right: A day later, the axon has grown around only guide cells 1 and 3.

Synapse formation The two cells exchange a variety of signals.

Vesicles cluster at the pre-synaptic site Transmitter receptors cluster at the post-synaptic site.

The Synaptic Cleft forms When the growth cone contacts the target cell

(immature muscle cell in the case of a motor neuron), a cleft (basal lamina) forms.

Multiple growth cones (axons) get attracted to the cleft.

All but one axon is eliminated. A myelin sheath forms around the synaptic cleft

and the synaptic connection is made.

Overall Process

Basic Process Neurons are initially produced along the central canal in the neural

tube. These neurons then migrate from their birthplace to a final

destination in the brain. They collect together to form each of the various brain structures

and acquire specific ways of transmitting nerve messages. Their processes, or axons, grow long distances to find and connect

with appropriate partners, forming elaborate and specific circuits. Finally, sculpting action eliminates redundant or improper

connections, honing the specificity of the circuits that remain. The result is the creation of a precisely elaborated child’s network

of 100 billion neurons capable of body movement, perception, an emotion or a thought.

Nature requires Nurture

Initial wiring is genetically controlledSperry Experiment

But environmental input critical in early development Occular dominance columns

Hubel and Wiesel experiment

Sperry’s experiment

Each location in space is seen by a different location on the retina of the frog

Each different location on the retina is connected by the optic nerve to a different location in the brain

Each of these different locations in the brain causes a different movement direction.

In a normal animal, a retinal region which sees in a particular direction is connected to a tectal region which causes a movement in that direction

Innervation of the Optic tectum

Ganglion Cells in the retina map systematically to cells in the optic tectum.

The image of the external stimulus is inverted in the retina and the mapping from the retina to the optic tectum reverts to the original image.

The Nasal ganglion cells of the retina map to the posterior region of the Optic tectum and the temporal ganglion cells map to the anterior region of the tectum

Sperry’s experiment

Sperry took advantage of the fact that in amphibians, the optic nerve will regrow after it has been interrupted

Sperry cut the optic nerve and simultaneously rotated the eye 180 degrees in the eye socket.

In 'learning’ movements to catch prey, the part of the retina now looking forward (backward) should connect to the part of the brain which causes forward (backward) movement.

Sperry’s findings

After regeneration, his animals responded to prey items in

front by turning around and to prey items behind by moving forward.

and kept doing this even though they never

succeeded in reaching the prey.

Conclusion from experiment

The conclusion from this (and some supporting experiments) is

that the pattern of connections between retina and tectum, and the movement information represented is not based on experience.

It is innate based on the initial distribution of chemical markers in the brain.

Lecture Overview

Summary Overview Development from embryo Initial wiring Activity dependent fine tuning

The role of the environment

The development of ocular dominance columns Cat and later monkey (Hubel and Wiesel)

Retinal input connects to the LGN (Thalamus) LGN is composed of layers. Each layer receives input (axons)

from a single eye LGN connects to layer IV of the visual cortex

The visual cortex develops ocular dominance columns Cells that are connected to similar layers in the LGN get stacked

together in columns forming stripes.

http://neuro.med.harvard.edu/site/dh/

VISUAL CORTEX

LGN

Monocular deprivation critical period Hubel and Wiesel deprived one of the eyes of

the cat (later macaque monkey) at various times 1 week – 12 weeks (in the monkey case) 4 weeks – 4 months (for the cat).

The found that the ocular dominance cell formation was most severely degraded if deprivation occurred at 1 – 9 weeks after birth.

Deprivation after the plastic period had no long-term effect.

C I

1

C I

7

C I

5

C I

6

C I

3

C I

4

C I

2

Cat Striate Cortex Layer IV

Monkey Striate Cortex Area 17 (V1) Layer IV

CLOSED EYE

OPEN EYE

Critical Periods in Development

There are critical periods in development (pre and post-natal) where stimulation is essential for fine tuning of brain connections.

Other examples of columnsOrientation columns

Pre-Natal Tuning: Internally generated tuning signals But in the womb, what provides the feedback to establish which

neural circuits are the right ones to strengthen? Not a problem for motor circuits - the feedback and control networks for

basic physical actions can be refined as the infant moves its limbs and indeed, this is what happens.

But there is no vision in the womb. Recent research shows that systematic moving patterns of activity are spontaneously generated pre-natally in the retina. A predictable pattern, changing over time, provides excellent training data for tuning the connections between visual maps.

The pre-natal development of the auditory system is also interesting and is directly relevant to our story. Research indicates that infants, immediately after birth, preferentially

recognize the sounds of their native language over others. The assumption is that similar activity-dependent tuning mechanisms work with speech signals perceived in the womb.

Post-natal environmental tuning

The pre-natal tuning of neural connections using simulated activity can work quite well – a newborn colt or calf is essentially functional at birth. This is necessary because the herd is always on the move. Many animals, including people, do much of their development after

birth and activity-dependent mechanisms can exploit experience in the real world.

In fact, such experience is absolutely necessary for normal development.

As we saw, early experiments with kittens showed that there are fairly short critical periods during which animals deprived of visual input could lose forever their ability to see motion, vertical lines, etc. For a similar reason, if a human child has one weak eye, the doctor

will sometimes place a patch over the stronger one, forcing the weaker eye to gain experience.

Adult Plasticity and Regeneration

The brain has an ability to reorganize itself through new pathways and connections.

• Through Practice:• London cab drivers, motor regions for the skilled

• After damage or injury • Release from inhibition• Undamaged neurons make new connections and take

over functionality or establish new functions• But requires stimulation (phantom limb sensations)

• Stimulation standard technique for stroke victim rehabilitation

When nerve stimulation changes, as with amputation, the brain reorganizes. In one theory, signals from a finger and thumb of an uninjured person travel independantly to separate regions in the brain's thalamus (left). After amputation, however, neurons that formerly responded to signals from the finger respond to signals from the thumb (right).

Possible explanation for the recovery mechanism The initial pruning of connections leaves some

redundant connections that are inhibited by the more active neural tissue.

When there is damage to an area, the lateral inhibition is removed and the redundant connections become active

The then can undergo activity based tuning based on stimulation.

Great area for research.

Phantom Limb Phenomena

Hand movement observation by individuals born without hands: phantom limb experience constrains

visual limb perception.Funk M, Shiffrar M, Brugger P.

We investigated the visual experiences of two persons born without arms, one with and the other without phantom sensations.

Normally-limbed observers perceived rate-dependent paths of apparent human movement .

The individual with phantom experiences showed the same perceptual pattern as control participants, the other did not.

Neural systems matching action observation, action execution and motor imagery are likely contribute to the definition of body schema in profound ways.

Summary

Both genetic factors and activity dependent factors play a role in developing the brain architecture and circuitry. There are critical developmental periods where

nurture is essential, but there is also a great ability for the adult brain to regenerate.

Next lecture: What computational models satisfy some of the biological constraints.

Question: What is the relevance of development and learning in language and thought?

5 levels of Neural Theory of Language

Cognition and Language

Computation

Structured Connectionism

Computational Neurobiology

Biology

MidtermQuiz Finals

Neural Development

Triangle Nodes

Neural Net

Spatial Relation

Motor Control Metaphor

SHRUTI

Grammar

abst

ract

ion