Transcript Imaging the Living Brain

Cognitive Architectures
Imaging the Living Brain
Based on book Cognition, Brain and Consciousness ed. Bernard J. Baars
Janusz A. Starzyk
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Introduction
The brain imaging has been a
breakthrough technology for cognitive
neuroscience and cognitive psychology.
 Before these techniques were developed
brain study was based on experiments on
animals, and injured human beings.
 But brain injuries are imprecise, damaged areas are hard to locate, and
often observed post-mortem (as in case of Broca’s and Wernicke’s
patients).
 Brain also compensates for the damage, lesions change over time,
adaptation occurs, so that post mortem examination is very imprecise.
 Animal studies depend on presumed homologies – not very convincing.
 No other animals can speak to communicate clearly what they
experience.
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Introduction
The brain study was enhanced by imaging techniques like
electroencephalography (EEG) based on X-rays computer tomography,
positron emission tomography (PET), magnetic resonance imaging
(MRI) etc.
 We can observe functional activity of the brain
 Magnetic imaging technique known as diffusion tractography allows to
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view
white
(myelinated)
fiber
tracts
from
cortex
to
the
spinal
cord.
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Brain recording
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Individual neuron’s activities
can be recorded.
Picture shows spike counts for
a single neuron in response to
various images.
This particular neuron
responds selectively to
images of Jennifer Aniston.
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Brain imaging techniques
Electroencephalography, (EEG)
 Magnetoencephalography, (MEG)
 Arteriography or Angiography
 Computerized tomography, (CAT)
 Single Photon Emission Computer Tomography,
(SPECT)
 Positron Emission Tomography, (PET)
 Magnetic Resonance Imaging, (MRI)
 Functional MRI, (fMRI)
 Magnetic Resonance Spectroscopy, (MRS)
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Time-space tradeoff
fMRI has good spatial
resolution and poor temporal
resolution.
Magnetoencephalography
(MEG) has a good temporal
resolution but cannot locate
precisely the source of firing.
Some studies combine EEG
and fMRI
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Most popular imaging methods are compared for their time vs space
resolution.
They do not have yet resolution to track a single neuron or a cluster
of neurons.
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Single-neuron recording
 Hubel and Wiesel (1962) received Nobel price for single-neuron
activities recording in the cortex of a cat.
 Depth electrodes used in humans only in very special cases – eg.
before surgery in epileptic patients.
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Single-neuron recording
Conscious and unconscious observations
 Single neuron recording gives us only a partial information about the
brain function.
 Many scientists believe that brain processes can only be observed
on the population of neurons.
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Animal and human studies
 Until recently, studies of macaque
monkeys were dominant source of
information about vision, memory,
attention and executive function of
brain
 Their brains have similar functional
regions with minor anatomical
differences
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Electroencephalography EEG frequencies
 Delta is the lowest frequency < 4 Hz and occur in a deep sleep or
vegetative state of brain characterizing an unconscious person.
 Theta has frequency 3.5-7.5 Hz, observed during some sleep states and
during quiet focus (meditation). They are observed during memory
retrieval.
 Alpha waves are between 7.5 and 13 Hz. They originate from occipital
lobe during relaxation with eyes closed but still awake.
 Beta activity is fast irregular at low voltage 12-30 Hz. Associated with
waking consciousness, busy or anxious thinking, and active concentration.
 Gamma generally ranges between 26 and 70 Hz. Characterizes active
exchange of information between cortical and subcortical regions.
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EEG observations
 High density array of
EEG electrodes placed
on scalp at precise
locations pick up signals
from dendrites of the
outside layers of cortex.
 Fourier analysis of EEG
signal helps to classify
observed responses.
 EEG reveals patters during sleep, waking
abnormalities, even response to music.
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Magnetoencephalography (MEG)
 Measures magnetic field produced by
brain activities.
 Is has spatial resolution of few
millimeters and temporal resolution of
few milliseconds.
 MEG uses Magnetic Source Imaging
(MSI) to superimpose magnetic activities
onto brain anatomical pictures provided
by MRI.
 MSI is used before brain surgery to
locate vital parts of the brain that must
be protected during surgery.
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Magnetoencephalography (MEG)
Magnetic field produced by a neuron
Not sensitive to top or bottom
neuron activities
 Due to magnetic field properties, MEG is sensitive to dendritic
flow at the right angles to the walls of cortical folds (sulci).
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Transcranial Magnetic Stimulation
 TMS works at the
milliseconds scale so it is a
useful technique to study
contribution of specific brain
regions to cognitive process.
 In this example TMS is
applied to Brocka’s and
Wernicke’s regions in the left
hemisphere.
 TMS is safe at mild levels of
intensity and frequency.
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fMRI
 EEG and MEG measure brain
activity directly.
 Currently the most popular
techniques fMRI (functional
magnetic resonance imaging).
 fMRI measures the oxygen level in
local blood circulation technique
called BOLD (blood-oxygen level
dependent activity).
 When neurons become active,
local blood flow to those brain
regions increases, and oxygen-rich
blood occurs 2-6 sec later
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fMRI principle of operation
 Magnetic field aligns spins of oxygen atoms.
 When the field is turned off spins return to their random orientations.
 This relaxation of nuclear spin is picked up by sensitive coils and
localized in 3D.
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Positron emission tomography (PET) vs fMRI
PET scans showing
speaking, seeing,
hearing and
producing words
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PET was developed much earlier that MRI.
Provides a measure of metabolic brain activity.
It is very expensive and requires a cyclotron.
Subject must be injected with a radioactive tracer.
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Visual experiment with fMRI
 fMRI images were
obtained comparing face
objects to nonface
objects.
 Subjects were supposed
to match faces and their
location.
 Figure shows fMRI of
brain activity in two
different tasks.
 Notice that location
matching activates
different brain area than
face matching.
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Summary
Brain imaging techniques can illustrate activities of a
single neuron, large cortical structures, dynamic brain
activity, and neurons connectivity.
We learned about a number of most important methods
for brain imaging and discussed their properties.
Brain imaging transformed study of human cognition.
Combination of methods is used to enhance observation
accuracy in time and space.
New methods are constantly being produced.
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