Methods In Cognitive Neuroscience

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Methods In Cognitive Neuroscience
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Basic Principles

• When an individual uses a part of the brain,
  neurons in that area change their firing rate
• Can be excitatory or inhibitory
• This change in firing rate is accompanied by
  increased glucose and oxygen consumption by
  active cells
• Although the precise mechanism is still unknown,
  increased glucose utilization appears to produce
  a local increase in blood perfusion, known as the
  hemodynamic response

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Basic Principles

• Electrical activity can be measured
• From cells directly
• In animals
• In neurosurgery patients
• From the scalp
• Electroencephalogram (EEG)
• Event-related potentials (ERPs)
• From outside the head
• Magnetoencephalogram (MEG)
• Metabolic activity can be measured
• With Positron Emission Tomography (PET)
• Inject the subject with a radioactive isotope
  that binds to glucose or water, and use a PET
  camera to localize the isotope in the brain
• With functional Magnetic Resonance Imaging (fMRI)
• Relies on the magnetic properties of oxygenated
  versus deoxygenated blood

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Event-Related Potentials (ERPs)A Primer

• Electrical current flow in the brain can be
  measured at the surface of the scalp (the
  electroencephalogram EEG).
• Measurable changes in current flow are
  predominantly
• produced by post-
• synaptic potentials
• on the apical den-
• drites of large
• groups of Layer V
• pyramidal cells.
• This generates
• current in a way
• similar to an
• electical dipole.

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Event-Related Potentials
I think Ill go to the pub later.
Does this chin strap make me look dumb?
Ooh, a red light just flashed on the left!
Ooh, a red light just flashed on the left!
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The Inverse Problem

• A given set of electrical sources in a volume
  will produce exactly one pattern of voltage at
  the volumes surface
• Forward Solution
• A given pattern of voltage at a volumes surface
  can be produced by a theoretically infinite
  number of possible sources
• Inverse Problem
• Attempts at solving the inverse problem generally
  constrain the allowed solutions in some way
• Restrict to brain (or cortex)
• Constrain based on fMRI/PET data

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Positron Emission Tomography
Subject is injected with a radioactive isotope,
and their head is placed inside the PET camera.
Before the introduction of fMRI, 18FDG
(FluoroDeoxyGlucose) and, later, 15H2O where
commonly used to measure brain activation.
Now, the most common research use of PET is to
measure levels of specific neurotransmitters
(e.g. 11C-raclopride).
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Positron Emission Tomography
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Introduction to MRI Slides by Martin Lindquist

• The subject is placed into the field of a large
  electromagnet.
• The magnet has a very strong magnetic field
  (1.5 - 7.0 Tesla)
• 1 Tesla 10,000 Gauss
• Earths magnetic field 0.5 Gauss
• 3 Tesla is 60,000 times stronger than the
  Earths magnetic field

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Be careful with metal objects
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The protons can be viewed as positively charged
spheres which are always spinning. They give rise
to a net magnetic moment along the axis of the
spins.
M0
In the absence of an external magnetic field, the
nuclear magnetic moments are randomly oriented.
No net magnetization.
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When placed in a strong magnetic field, the
nuclei will align with the field. (Either in a
parallel or anti-parallel state)
A slight majority of nuclei will be in the
parallel state giving rise to a net magnetization
in the direction of the magnetic field.
At 1.5 Tesla only 10 nuclei in 1,000,000
contribute to the net magnetic moment. The rest
cancel each other out.
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Perturb the spins by applying an oscillating RF
magnetic field, B1, that is transverse to the
main field and oscillates with the Larmor
frequency.
This will tip the net magnetization vector so
that a component lies in the transverse xy-plane.
M
Mz
Mxy
As the nuclei attempt to return to equilibrium,
the changing magnetic field generates a current
in a coil surrounding the subject
This is the raw signal detected by the MR scanner.
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The sample being imaged is split into a number of
equally sized volume elements, or voxels.
Each voxel contains a different amount of
Hydrogen atoms and give rise to a different
amount of signal.
The measured signal combines signal over the
whole sample.
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Fourier Transform/Decomposition(An aside)
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xkcd.com
That cat has some serious periodic components
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K-space
The set of measurements (k-space) is made up of
the Fourier transform of the image we would like
to view.
k-space
Image space
IFT
y
ky
x
kx
FT
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2 Dimensions
K-space
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Spatial Resolution
32x32 image
1024 points sampled in k-space
64x64 image
4096 points sampled in k-space
128x128 image
16,384 points sampled in k-space
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Hemoglobin exists in two different states,
oxyfied and deoxyfied, each of which has
different magnetic properties and produces
different local magnetic fields. (Pauling
1936) Oxyhemoglobin - diamagnetic Deoxyhemoglobin
- paramagnetic
Deoxyhemoglobin has the effect of suppressing the
MR signal. As the concentration of
deoxyhemoglobin decreases the signal increases.
Blood Oxygenation Level Dependent (BOLD) effect
(Ogawa).
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Hemodynamic Response function
The hemodynamic response function represents the
changes in the MR signal triggered by neuronal
activity.
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Statistical Parameteric Map
The results of statistical tests at each voxel
are typically shown in a statistical parametric
map.
Each voxel is color-coded according to the size
of its corresponding p-value.
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Example
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Example
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