Download What is memory? How does the brain perceive the outside

How the brain perceives
the outside world
Saturday Morning Physics
December 4, 2004
Presenter: Rhonda Dzakpasu
Brain blunders
Kanizsa’s
Triangle
x
“Peripheral drift” by Akiyoshi Kitaoka
Taken from Nationalgeographic.com
W.E. Hill
Central Nervous System:
Brain and spinal cord
• The Brain
– Three pound wrinkled
mass
– Fits into cranial cavity
surrounded by
cerebrospinal fluid
cushioning
– Consists of multiple
areas
– Localized regions are
task specific
Courtesy of Morphonix LLC, Sausalito, CA
Central Nervous System:
Brain and spinal cord
• Spinal cord
– Carries fibers that relay
information from brain to
rest of body and from
the body to the brain
– Encased in bone
fortress known as the
vertebrae
– Surrounded by
cerebrospinal fluid
Courtesy of Morphonix LLC, Sausalito, CA
Peripheral Nervous System:
nerves to rest of body
• Somatic nerves
– Fibers that relay
commands from brain
to all muscle groups
below the neck and
controls
somatosensory
function such as touch,
pain, temperature and
joint position
Courtesy of Morphonix LLC, Sausalito, CA
Peripheral Nervous System:
nerves to rest of body
• Autonomic nerves
– Fiber carrying information
from brain to all organs in
the body
– And all this is done
automatically
• Cranial nerves
– 12 nerves connecting the
brain to the organs in the
head; also have sensory
and motor functions
Courtesy of Morphonix LLC, Sausalito, CA
The “Sense”ational Organs

Receives “raw” data from around us

Each organ handles single function
Converts external stimuli to language
of the brain

Seeing is believing: The Eye
Retina is initial
visual processing
center
Has three basic
layers of cells:
Light sensitive
photoreceptors
Bipolar cells
Ganglion cells

Courtesy of Morphonix LLC, Sausalito, CA
Seeing is believing: The Retina
Color is coded by
photoreceptor rods
and cones

Intensity is coded by
frequency of firing of
ganglion cells

Courtesy of Morphonix LLC, Sausalito, CA
Can you hear me now?
The ear: Wide dynamic
range: 20 Hz – 20000 Hz

Three major components:
Outer ear
Middle ear
Inner ear

Courtesy of Morphonix LLC, Sausalito, CA
Can you hear me now?




Cochlea: neurosensory
component for hearing
Hair cells convert sound
pressure to “brain
language”
Outermost end – high
frequencies
Innermost end – low
frequencies
Courtesy of Morphonix LLC, Sausalito, CA
Olfaction and Taste

Smell and taste are related
(artificially separated?)



Both are chemical senses
Odors bind to receptors
which send electrical signals
to olfactory bulb
Tastes are dissolved in
mucous fluid in oral cavity to
be carried to taste receptors
Courtesy of Morphonix LLC, Sausalito, CA
Like minded ordering

Cells of similar type group according to their
function
Located in well defined layers

Advantage: facilitates ease of communicaton


Disadvantage: disease or injury can wipe out
entire function
Brain Structure
whyfiles.org
The Cerebral Cortex
Purves et al., Life: The Science of Biology,
4th Edition
Cortical Parallel Processing

Visual system


Auditory system


Must decode shape,color, position and
movement
Must decode many aspects of sound, e.g.,
loudness, pitch, harmonics, timing and
location of multiple sounds
Olfactory System

Odors are composed of different chemical
groups that bind to different receptors
The Father of Modern
Neuroscience
Santiago Ramon y Cajal,
1852-1934
Outlined fundamental architecture
of CNS

Demonstrated basic changes
neurons undergo as they function
in the CNS

First to isolate neurons near
surface of the brain

Courtesy: The Nobel Foundation
The Father of Modern
Neuroscience
Santiago Ramon y Cajal,
1852-1934
Courtesy: The Nobel Foundation
Who are the main players?

Neuron
– Generate and propagate electrical activity
– Responsible for our thinking, feeling,
hearing, seeing, moving, loving and hating!

Glial cells
– Supporting cells
The Neuron
There are ~ 100 billion

Small enough for 30000
to fit on head of a pin

Connects with thousands
of other neurons

Generally do not reproduce

Courtesy of Morphonix LLC, Sausalito, CA
The Neuron
Three major components:

Dendrites:

Receives information from
other neurons
Courtesy of Morphonix LLC, Sausalito, CA
The Neuron
Soma:

Cell body containing the
nucleus, the brain of the neuron
Courtesy of Morphonix LLC, Sausalito, CA
The Neuron
Axon:

Connects with other cells to
transmit information
Can be up to several feet long!
Covered in myelin allowing for
fast information transfer

Courtesy of Morphonix LLC, Sausalito, CA
The Neuron
Always active!

Constantly integrating and
generating information

Hotbed of activity! Neurons
don’t hibernate!

Courtesy of Morphonix LLC, Sausalito, CA
How is information
transmitted?!
The Resting Potential


Primarily due to concentration
of K+ions inside and outside
cell membrane
Typically between -80 mV to –
55 mV
Action Potential:
Generation



Excitatory input from synapses to
dendrites depolarizes cell
Voltage activated Na+ channels open
Inside of cell becomes positive,
~20mV
Action Potential: Propagation



Change in voltage creates current that
depolarized adjacent region in axon
Impulse travels down axon
Lasts ~2 ms
Neuronal Communications

Neurons are like
islands: no physical
contacts
Chemical signals are
converted to electrical
ones

The supporting cells


Glial cells
Outnumber neurons by
10:1 margin
Two types:
Astrocytes
Oligodendrites and
Schwann cells

The supporting cells
Astrocytes:
Provide physical support
by forming a mesh around
neurons
Formation of blood-brain
barrier
Remove dead cells
Transport of nutrients
Destroy
neurotransmitters in extra
cellular space

The supporting cells
Oligodendrites (CNS) and
Schwann cells (PNS):
Prevents abnormal
communication between
neurons
Contain myelin and wraps
around neuronal axons to
boost communications, up to
200 mph!

Where’s the Physics?!
The “big” questions?


How do high-level psychological
processes come about from basic
neurophysiological effects?
How does the complex organization of
brain cells, give rise to behavior?
Where’s the Physics?!

Computational models of neuronal behavior



How can neural activity be defined?
What types of coding mechanisms are involved?
Models from single ion channel and synaptic
models to “black-box” models describing
psychological phenomena
The “Standard” Model
• Hodgkin-Huxley (1950’s)
– First model of action potential
propagation
e
CV   0.1(V  67)  100m3 h(V  50)  80n 4 (V  100)  I syn
0.32(54  V )
0.28(V  27)
(1  m) 
m,
1  exp( 0.25(V  54))
exp( 0.2(V  27))  1
4
h  0.128 exp( (50  V ) / 18)(1  h) 
h,
1  exp( 0.2(V  27))
0.032(V  52)
n 
(1  n)  0.5 exp( (57  V ) / 40)n,
1  exp( 0.2(V  52))
m 
Hodgkin-Huxley Model
• Simulates action potential from giant
axon of squid
Hodgkin, A. L. and Huxley, A. F. (1952) Journal of Physiology 117: 500-544
Where’s the Physics?!

Study of spatio-temporal patterns

What new types of behaviors occur in
large-scale models?



Synchronization
Oscillations
Are they important to brain function?
Patterns of Emergence

Large scale
behavior of
networks of
neurons
Makeig, S., et. al., Science, 295, 690, 2002
Patterns of Emergence

Temporal and
spatial changes
in networks
Seidemann, E., et. al., Science, 295, 862, 2002
Where’s the Physics?!

Imaging techniques

How can we “see” inside the brain?
 Structural methods
 Functional methods
Imaging Techniques


Clinical application
– PET scans
– CT scans
– MRI
Research applications
– Fluorescence based imaging
– Nuclear magnetic resonance (NMR)
imaging
Why should you care that we
want to understand how the
brain works???
What can we do when the brain
fails?
Hybrid Brain Machine
Interfaces*


Real-time direct interfaces between
brain, electronics and mechanical
devices
Can be used to restore lost or impaired
sensory and motor function
* Nicolelis, M., Nature, 409, 403,2001
Prosthetic Ear:
Cochlear Implants




Converts acoustic signal
to electrical stimuli
Couples to array of
implanted electrodes to
auditory nerves
Stimulates cochlea:
High frequencies – basal
Low frequencies – apical
Mimics normal auditory
processing
Reprinted with permission from Rauschecker, J.P.,
and Shannon, R.V., Science, 295, 1025-1029,
(2002) Copyright 2002, AAAS
A Bionic Eye?
Retinal Prostheses
Reprinted with permission from Zrenner, E., Science, 295, 1022-1025, (2002) Copyright 2002, AAAS
A Bionic Eye?
Retinal Prostheses
Type 1: subretinal implant
Replaces photoreceptors with
microphotodiodes and electrodes

Type 2: epiretinal implant
No light-sensitive areas
Uses camera and processor outside of body
Implanted electrodes stimulate axons of
ganglion cells

It’s all Monkey Business!
Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003
It’s all Monkey Business!
Monkeys learn to produce complex
hand movements
Implanted microwire arrays record
activity of hundreds of neurons

Linear and non-linear
models are used to
extract motor
control signals


Outputs used to
control movement
of the robot arm
Reprinted with permission, Nicolelis, M., Nature Reviews, 4, 417, 2003
Stay tuned!
Next week’s lecture
• Imaging techniques
– Clinical methods
– Research methods