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					Cognitive Neuroscience
and Embodied Intelligence


 Imaging the Living Brain
         Based on book Cognition, Brain and Consciousness ed. Bernard J. Baars




                                                                          1
  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
   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 recording
   Individual neuron’s recording is
    seldom possible in humans due
    to ethical reasons.
   Instead deep electrode
    recordings can be performed on
    some primates like macaque
    monkey.
   This is used to test working
    memory shown here in Delayed
    Match To Sample task.
   The second set is displayed 16
    sec later – during this time
    monkey must remember the
    sample to have a correct match.
   Firing of the single neuron during
    this 16 sec is responsible for
    working memory.                      5
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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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   Time-space tradeoff




                                                 A coronal slice

 Brain Navigator software shows various slices through the brain.
 It shows precise x,y,z, coordinates into brain locations

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   Time-space tradeoff




                                            A mid-sagittal slice

 A horizontal slice through the eyeballs
  as indicated on the top of the image
 Notice they white matter inside and the
  gray outer layer                                           9
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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.
 More recent work recoded activities in medial temporal lobes.
 Depth electrodes used in humans only in very special cases – eg.
  before surgery in epileptic patients.
 The implants can help to determine location of the seizure onset.   10
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    Single-neuron recording



                     Conscious and unconscious observations




 Single neuron recording gives us only a partial information about the
  brain function.
 Other observations like subcellular processes, non-classical cells
  and synapses, glial cells participate in neural processing.
 Many scientists believe that brain processes can only be observed
  on the population of neurons.                                        11
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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                            12
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    Electroencephalography (EEG)
 Brain’s large scale electrical activities
  can be recorded through the scalp on
  the surface of the cortex.
 EEG picks up electrical field
  generated by group neurons
  activities.
 EEG was discovered in 1929 by Hans
  Berger.
 EEG has extremely good temporal
  resolution.
 A disadvantage is that the surface
  neurons are observed better than
  activities of deeper neurons.
                                     Averaged EEG over a number of
                                      experiments yields regular waveforms
                                      that are easier to interpret.
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  EEG frequencies and their functions
 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 – arise from synchronous firing of
  many neurons, 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 – arise from synchronous
  firing of large groups of neurons. They originate from occipital lobe
  during relaxation with eyes closed but still awake.

 Beta activity is fast irregular at low voltage. 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. Seen during the conscious waking state and REM dreams
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  (Rapid Eye Movement sleep). Overlaps with beta activity.
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   EEG frequencies and their functions




 Regular waveforms from different brain regions: alpha indicates alert
  state, theta reflects interactions between hippocampus and cortex,
  and gamma indicates conscious perception.                          15
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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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  EEG response to evoked potential

                                                      Red color – higher
                                                       voltage.

                                                      Explicit recognition
                                                       shows stronger
                                                       signals that a
                                                       ‘feeling of knowing’.

                                                      Activities are not
                                                       very well localized.



 EEG responds to changes of the evoked potential (ERP) averaging
  it over many trials.
 ERP can illustrate, for instance, brain patterns in visual processing.
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  EEG response to evoked potential




 Evoked potential (ERP) in response to music.
 Regular musical sequences are perceived as more pleasant and
  show a larger evoked potential.




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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)
  MEG is entirely silent
   (unlike MRI) and
   noninvasive so it is
   attractive for use with
   children and
   vulnerable people


  Pictures at the bottom
   show the vector fields
   of MEG over the head
   of the subject




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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).    21
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  Stimulating the brain
                        Penfield’s
                        map of
                        language
                        perception
                        and
                        production


 Noninvasive imaging techniques (fMRI, MEG, EEG) help to
  observe brain activities.
 Equally useful information may come from evoking neural activities
  directly by applying electrical stimuli.
 Wilder Penfield stimulated brains of his epileptic patients during
  surgery.                                                           22
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  Transcranial Magnetic Stimulation




 Today it is possible to simulate brain lesions in healthy patients without
  surgery using transcranial magnetic stimulation (TMS).
 Brief magnetic pulses either excite or inhibit a small region of cortex.
 For instance, if a hand area in motor cortex is stimulated, the subject’s
  hand will suddenly move.
 Applying inhibitory pulse over the same region makes movement of the
                                                                         23
  hand difficult.
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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 and PET
 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).




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   fMRI and PET
 After burst of neurons
  activity there is a drop in
  oxygen and glucose.

 BOLD curve shows the
  level of oxygen in the
  local region.

 Then and upswing in the
  BOLD curve reflects a
  wave of a new blood-
  curried nutrients to the
  active region.
 This wave is used up and the oxygen level returns to normal within few
  seconds.

 Because of this time delay the fMRI temporal resolution is not so good.
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   fMRI principle of operation




 Magnetic field aligns spins of oxygen atoms.
 When the field is turned off spins relax returning to their normal random
  orientations.
 This relaxation of nuclear spin is picked up as a signal by sensitive 27
                                                                        coils
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  and localized in 3D.
    PET vs fMRI

                                          PET scans showing
                                          speaking, seeing, hearing
                                          and producing words




 Positron emission tomography is another indirect brain imaging technique.
 It 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.
 PET is still important in medical research since different tracers can be
  used to trace different molecules.                                    28
 For other investigations MRI and fMRI are preferred.
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   Co-registration




 MRI shows structure but not function.
 fMRI shows function but its spatial resolution is not well defined.
 Typically the two images are superimposed in the process called co-
  registration to relate brain activity to its anatomical features.
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    Co-registration
 Brain is active all the times.

 To identify task specific
  activities a common method is
  to subtract two fMRI images
  comparing stimulated activity
  and control activity.

 Subtraction can have
  unwanted consequences when
  the important processing goes
  on in both conditions.

 Another approach is known as
  parametric variation in which
  variance of each variable is
  estimated.
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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.                  31
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    Cognitive functions and brain
 There is an evidence that using specific
  cognitive functions changes not only
  the brain efficiency of doing them but
  also size of the bran part responsible
  for such processing.
 For instance hippocampus is
  responsible for navigation and memory
  of places and routs.
 Birds and animals that burry or hide
  their food have bigger hippocampus
  than non-storing animals.
 A study compared size of hippocampus
  between London taxi drivers and
  reference population and showed that
  their hippocampus is bigger.

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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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posted:9/1/2011
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