The Brain and the Mind
Fall 1999 (Lecture 1)
Imaging the Brain (and the Mind?)
Emanuel Donchin
Sequence of lectures
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Monday
Wed.
Friday.
Monday
Wed.
Friday
October 18
October 20
October 22
October 25
October 27
October 29
Neuroimaging - methods
The Body and the Mind
The Recalling Brain
The Erring Brain
The Preparing Brain
Brain Computer Interfaces
The “Higher Mental Functions”
• The next 6 lectures
– Focus on Neuroimaging.
– Observing the brain in action
– In awake behaving human subjects
– This allows the study of brain process
– Associated with the “higher mental functions”
Neuroimaging...
• Allows direct observation on brain activity in humans?
– Which signals are available?
– How are they affected by the “mind”
– What can we learn from this relationships about the brain and
the mind?
The Two Varieties of Neuroimaging
• Hematological
– Measures changes in blood flow in the brain
• PET and fMRI
• Electrophysiological
– Measures the electrical activity of the brain
• EEG and ERP
Blood Flow and the Mind
• Angelo Mosso, 1881, studied adult who suffered head injury.
• Recorded pulsations of the brain through skull defect.
• Mosso believed pulsations reflect blood flow
Mosso’s data
(A) Forearm, (C) Brain
• Subject resting
– stable pulsations
• Room clock strikes 12. Church bells sound.
• Asked Question
• 8 x 12?
– Answer
Implications of Mosso’s observations.
• There is an increase in blood flow to the frontal cortex with
mental activity.
• Hence, measurement of blood flow can be used to identify
active areas.
• Mosso had no direct evidence that the pulsations are due
to arrival of blood.
Roy and Sherrington , 1890
• Based on studies with lab animals suggest:
– There exists an automatic mechanism that regulates blood
supply
– When there is an increase in the oxygen needs of brain tissue…
– There will be an increase in blood supply to that part of the brain.
Walter K’s Noisy Brain.
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Patient studied in 1928 by Fulton
Complained of “humming in the head”
Blood vessel abnormalities created noisy blood flow
Different activities modulated blood flow.
Half a century later:
PET Camera
• Same as Mosso, but oh! What a difference….
• System measures changes in blood flow within 5mm accuracies
• Nuclear medicine version of Mosso’s device.
Principle of operation
• Unstable O, emits protons.
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• Proton meets electron and they annihilate each other
• Two photons moving in opposite directions are generated.
Principle of operation (continue)
• “Camera” consists of photon detectors
• Wired to identify photons that coincide in opposite detectors
• This identifies location of 15O proton.
The Data
The Experimental Paradigm
Does it work? A test based on visual anatomy
• The upper visual field is connected to the lower part of the visual
area
• The lower visual field is connected to the upper part of the visual
area
Indeed it works...
• Visual stimulus to upper field activated area below calcarine fissure
• Visual stimulus to lower field activated are above calcarine fissure
Thus PET provides
• A method for monitoring the regulation of the blood supply
in the brain as a function of activity
• Excellent spatial resolution
• Requires about a minute for the signal to accumulate
• Depends on subtraction technique
More recently developed: MRI
• Based on response of brain to magnetic field which is modulated
by blood flow.
• Provides excellent structural images
• Precise functional information.
• More later
We got “where”
What about “when”?
• The blood flow techniques provide excellent functional
neuroanatomy
• We also need information on the sequence of activity.
– When do activities occur?
– What happens before what?
– How does the timing of brain activity relate to the timing of
processing?
A Tool for Timing: The EEG
The fortunate topography of the cortex
Need proper neuronal organization
• Neurons must be aligned in parallel
• Or electrical fields will not be large enough to be recordable from
the scalp
Event-related Brain Potentials
Averaging: From EEG to ERP
The structure of the Signal
• An ERP is a sequence of components.
• A component is assumed to represent specific intracranial activity
invoked to serve a specific processing function.
The Component Vocabulary.
Terminology
• Event Related Brain Potentials (ERP)
• Components often labeled:
– Letter
• P for positive (down)
• N for negative (up)
– Number
• number of milliseconds since stimulus (Latency)
Very precise timing:
The Brain Stem Potentials
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The first 9 msec after a tone.
Note the fifth large peak
If delayed by 200 microseconds - brain stem damage
Note variation with body temperature
Latency varies with body temperature.
• The response is very sensitive to conduction speed
• Record of potentials over 24 hours shows large variation in latency
This “sensory” components
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Display passage of information along the sensory channels
Very sensitive to physical nature of the stimulus
Very sensitive to subject’s conditions
Not very sensitive to “Psychology”
These are EXOGENOUS components.
Consider the following sentences:
The pizza was too hot to eat
The pizza was too hot to drink
The pizza was too hot to cry
An Endogenous component
• Here are the ERPs elicited by the last word in the sentence.
• Not that the most deviant word elicits large N400.
• Why is “drink” of a lower amplitude?
Distance in “semantic space”
• Even though no one ever “drinks a pizza”
• And “drinking a pizza” is as silly as “crying a pizza”
• “Eat” and “Drink” are closer in meaning to each other,
while “cry” is no relation.
Energy vs. Information
• The exogenous components reflect which form of physical
energy (light, sound, touch…) was presented to which
sensory organ.
• But not all stimuli (physical energy) deliver information
• “Information is the resolution of uncertainty…” (Shannon)
The discovery of the P300
• Sutton et al 1965
– Subjects presented with random or known sequences of tones.
– Asked to guess the next tone
– Measured the effect of uncertainty on the ERP
Absence makes the brain respond!
• Subject gets:
– Loud
– Soft
– Loud. Loud
– Soft. Soft
• Subject reports:
– Loud or soft
– Single or double
• When is information delivered?
The response to a missing stimulus
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When judging loudness first stimulus gives all information
When judging pairs must wait for second stimulus
Information delivered by absence of second event
I.e. P300 elicited by missing event
The P300 is an ENDOGENOUS component
• Appearance depends on processing of stimulus, not only
on its physical nature
• It manifests (reveals) the activity of modules in the
information processing system
• Does not even require a stimulus
Thank you for your attention