Transcript Cell Structure: From an Information Processing View

Cognitive Neuroscience
What are we doing with our
brains right now?
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Feeling your chair
Squirming (moving)
Watching
Listening
Remembering
Paying attention
Sleeping
Feeling anxious
Feeling hungry
What happens when you ask a
question?
The Brain
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Approx 3lbs
1 trillion cells
100 billion neurons
½ of neurons are in cerebellum
Each neuron has 5000 – 10,000 synapses
Males and females differ, but only slightly
Neurons transmit signals at speed of about 2 m/s
There are more possible connections in your brain
than there are atoms in the universe
Cognitive Neurosccience
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The cell
Cortex
Mid-Brain
Hindbrain
Cell Structure:
From an Information Processing View
Dendrites receive
signals.
Signals travel along
the Soma or Cell
Body and Summate at
the Axon Hillock
The signal must be
above a Threshold for
the signal to travel
down the Axon
Cell Structure:
From an Information Processing View
Dendrites
– Receive in Modulate signals Omnidirectionally
– They are graded potentials – they decay as they travel
Soma
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Where the signals are summed
The amplitude and the timing (concurrence) are important
The signal strength must be greater than the resistance at the axon hillock
The threshold can shift
The soma has a baseline
• Baseline indicates all is normal
• Indicates cell is alive
– Firing is not perfect, Noise: can fire for no reason, or not fire when it
“should”
Cell Structure:
From an Information Processing View
Axons
– Transmit signals in one direction and are ballistic
(all or nothing)
– Have myelin sheaths which sustain the signal
strengths (do not decay)
– Their axon buttons or knobs release
neurotransmitters
Functional
classification
Afferent or sensory neuron: conduct impulses from the
sensory organs to the central nervous system (brain and spinal cord)
Efferent or motor neuron: conduct impulses from the
central nervous system to the effector organs (such as muscles and
glands) are called motor (or efferent) neurons
Interneurons (also known as connector neurons or association
neurons): connect sensory neurons to motor neurons.
Quizz
The dendrite is part of the neuron which
a) Is covered in myelin sheath
b) Propagates signals in one direction only
c) Receives modulated signals
d) Releases neurotransmitters
Functional
classification
The glial cells
• maintain the ionic environment
• modulate nerve signal propagation
• controls the uptake of
neurotransmitters
• Protect by surrounding and buffering
• Speed transmission by forming myelin
sheaths
Quizz
Glial cells have numerous functions. Which one of
these is NOT one of its functions.
a) To produce myelin sheathing
b) Protect by surrounding and buffering
c) Summing signals to trigger an action potentials
d) A and C
Think About This
• Each cell has a criteria for firing
• criteria shift
• No one neuron firing is sufficient
information, the meaning is in the average
rate in a given population.
Thinking Machines
Pinker, How the Mind Works
• How do you get neurons
to represent simple
logical functions?
• By setting the threshold
• This requires pre-wiring
• But cannot pre-wire for
every thing in the world
like mouse or justice.
Thinking Machines
Pinker, How the Mind Works
• Instead of oneneuron-onesymbol the brain
uses distributed
representation
• Many categories
combined (e.g.,
crispy, green,
edible) sum up to
create one value
(e.g., celery)
Thinking Machines
Pinker, How the Mind Works
• Turn the network
upside down, and you
can represent the
fuzzy quality of logic
where tokens can be
more or less good
examples of a
category.
• Here the item tomato
only lights up some of
the qualities for fruit
Thinking Machines
Pinker, How the Mind Works
• The neuron at the top is
removed and the correlations
amongst the nodes is preserved.
• The everything-connects-toeverything is called autoassociator, and has five features
in common with human pattern
recognition.
Thinking Machines
Pinker, How the Mind Works
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Five features of Auto-associators
1. Reconstructive and content addressable memory
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Specifying an item in memory automatically lights up a copy or
version of that memory anywhere else. If you light one part of
the network, if the weights are strong enough, the parts will light
up. So you can recognize a word that has ‘mudsplashes’ on it.
2. Graceful degradation. Do not discard the whole percept
because of one faulty piece of information (e.g., PRITN,
HELF)
3. Constraint satisfaction.
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Sinned a pin does not make sense.
But send a pen does even though the sounds are very similar.
With logic you have to test each possibility. With auto-associator
the context is intrinsic to the network, and the most meaningful
evaluation emerges. Ambiguities are allowed: Necker Cube.
Thinking Machines
Pinker, How the Mind Works
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Five features of Auto-associators
4. Generalizes automatically.
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E.g., bottom row is distributed pattern for an animal
(parrot). The top row are the features of the category
the animal belongs to (feathers, beak, flies). The
relationship amongst the features of a category have
an intrinsic correlation.
5. Learn from examples, where learning is a change
in the weights.
White Matter and Grey Matter
What is the Order of Intelligence? Why?
Cat
Raccoon
Rabbit
Kangaroo
Camel
Baboon
Monkey
Human
Forebrain
Function of Limbic System
Hindbrain
Lobes of the Cerebral
Cortex
• Frontal
– Reasoning & Planning
• Parietal
– Touch, Temperature, Pain, &
Pressure
• Temporal
– Auditory & Perceptual processing
• Occipital
– Visual processing
The Cortex
“The Bark”
Prefrontal
Cortex
• Recall the order of
things
• Provides the brakes on
impulse control
– Damage can produce
Tourette’s like symptoms
of tic or exclaiming
obscenities
Story of Phineas Gage: Damage to the
Prefrontal cortex
Gage was fitful, irreverent, indulging at times in the grossest
profanity (which was not previously his custom), manifesting
but little deference for his fellows, impatient of restraint or
advice when it conflicts with his desires, at times
pertinaciously obstinate, yet capricious and vacillating,
devising many plans of future operations, which are no sooner
arranged than they are abandoned in turn for others
appearing more feasible. A child in his intellectual capacity
and manifestations, he has the animal passions of a strong
man. Previous to his injury, although untrained in the schools,
he possessed a well-balanced mind, and was looked upon by
those who knew him as a shrewd, smart businessman, very
energetic and persistent in executing all his plans of operation.
In this regard his mind was radically changed, so decidedly
that his friends and acquaintances said he was 'no longer
Gage.’
Testing the Prefrontal cortex: The
Wisconsin Card Sorting task
• The patient is told to sort the
cards, but they are not told
how to sort.
• But they are told if they
matched the cards correctly.
• They must use their memory
for the order of things to
figure out the rules of the
sorting.
• It took an entire manual to
score the test, nowadays it is
done on the computer.
Primary Somatosensory and Motorsensory Cortex