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Imaging the Living Brain

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Cognitive Architectures Imaging the Living Brain Based on book Cognition, Brain and Consciousness ed. Bernard J. Baars * Janusz A. Starzyk – PowerPoint PPT presentation

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Title: Imaging the Living Brain


1
Imaging the Living Brain
Cognitive Architectures
Based on book Cognition, Brain and Consciousness
ed. Bernard J. Baars
Janusz A. Starzyk
2
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 Brocas and Wernickes
    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.

3
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 view white (myelinated)
    fiber tracts from cortex to the spinal cord.

4
Brain recording
  • Individual neurons 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.

5
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)

6
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
  • 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.

7
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.

8
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.

9
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

10
Electroencephalography EEG frequencies
  • Delta is the lowest frequency lt 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.

11
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.

12
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.

13
Magnetoencephalography (MEG)
  • Due to magnetic field properties, MEG is
    sensitive to dendritic flow at the right angles
    to the walls of cortical folds (sulci).

14
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 Brockas and
    Wernickes regions in the left hemisphere.
  • TMS is safe at mild levels of intensity and
    frequency.

15
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

16
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.

17
Positron emission tomography (PET) vs fMRI
PET scans showing speaking, seeing, hearing and
producing words
  • 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.

18
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.

19
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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