Imaging the Living Brain

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Imaging the Living Brain
Cognitive Architectures
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 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.

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

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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.

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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

• 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 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.

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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)

• 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 Brockas and
  Wernickes 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

• 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.