Transcript Neurons and the BOLD response

Chapter 4: Methods.
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To accompany Baars & Gage Chapter 2
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The brain generates an electromagnetic field.
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Very large arrays of neurons in the brain generate
significant electrical and magnetic fields, which can
be detected fairly easily. That is how the EEG was
discovered in 1929, simply by placing an electrode
on the scalp and amplifying the electrical signal.
Above, the first alpha rhythms at 7-12 Hz, discovered by Hans
Berger in 1929 by placing an electrode over the back of the
scalp --- the occipital lobe. The regular waves below are
artificially generated to provide a timing trace at 10 Hz.
To accompany Baars & Gage Chapter 2
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The brain generates an electromagnetic field.
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Today, EEG is recorded by an array of electrodes
placed on the scalp. Medical EEG makes use of
about 20 electrodes, but research EEG machines
may have as many as 256, to get the highest spatial
resolution possible. (Left, bottom)
Delta waves, regular, high-voltage
- <2.5 Hz
Unconscious non-REM sleep.
Fast, complex, irregular lowvoltage waves - waking state and
REM dreams.
Alpha waves, regular - 8-12 Hz,
working memory, visual imagery,
and eye closure.
To accompany Baars & Gage Chapter 2
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MEG picks up the magnetic component of the electromagnetic field
around the brain.
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MEG has much better spatial
resolution than EEG. Both
MEG and EEG have
instantaneous temporal
resolution.
To accompany Baars & Gage Chapter 2
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Single-cell studies:
A small needle electrode, or a grid of tiny electrodes, can pick up
electrical activity from cells (or small sets of cells) inside the brain.
Electrodes in a monkey brain. The spiking graph is
shown on the right, stimulated by a visual slanted bar.
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EEG and MEG signals reflect tens of billions of
neurons, generally near the surface of cortex. To
look more closely at specific neurons or clusters of
neurons anywhere in the brain, needle electrodes
(or tiny electrode grids) are placed in the brain itself.
Single-cell studies are fundamental in cognitive
neuroscience. They often show large-scale
functions at the smallest level of analysis.
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Most single-cell recording are done in animals, like
the monkey (upper left). These implants are not
painful, since the brain itself lacks pain receptors.
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However, epileptic patients are often studied with
implanted electrodes, to determine the location of
the epileptic "focus" (which triggers seizures) to be
removed by surgery. It is also vital to avoid cutting
areas of the brain that are needed for normal
functioning, like language cortex.
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Scientific studies can sometimes be done under
ethical guidelines in medical patients with electrode
implants.
Typical placement of depth electrodes in humans.
To accompany Baars & Gage Chapter 2
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Averaged evoked potentials (EPs) allow us to simplify massive, and
highly complex brain activity.
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EPs (also called Event-Related Potentials, or ERPs)
occur when a fast, intense stimulus is presented,
like a loud clap, a light flash, or an electrical shock
to the hand.
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The result is a series of massive waves going
through the entire cortex over a one-second period,
like a tsunami; so large that it can be isolated from
the background wave activity of the brain. These
voltages are evoked, hence "evoked potentials."
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When EPs are averaged over multiple trials, a
beautifully regular pattern emerges out of the noisy
complexity of spontaneous EEG. (MIddle left) The
peaks and valleys are labeled "Negative 1" or
"Positive 2," by convention.
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EPs are highly sensitive to personal or cognitive
significance. E.g., compare the subject's own name
vs. other names on the lower left. Notice that both
the height and timing of the large wave wave has
shifted.
To accompany Baars & Gage Chapter 2
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Complex EEG or MEG can also be analyzed into component sine waves.
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With the advent of high-powered
computers, it also became possible
to analyze very complex EEG/MEG
into its components --- sine waves.
(Using Fourier Analysis).
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The top graph shows raw EEG; the
middle shows different colors for
different frequencies; and the bottom
shows the distribution of the
electrical power in each frequency
range.
Regular rhythms can be extracted from the
back, front and middle of the head. These
appear to have important cognitive functions.
For example, theta is believed to be involved in
episodic memory retrieval.
To accompany Baars & Gage Chapter 2
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We can also use electrical or magnetic energy to stimulate neurons
in the brain.
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Electrical stimulation of a human
brain during surgery. Electrodes
stimulated points in the crosshatched regions. (Penfield &
Roberts, 1957).
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TMS (Transcranial Magnetic
Stimulation) of the intact human
head stimulates specific brain
regions. TMS is generally safe, and
may have medical applications as
well. Notice the number-8-shaped
magnetic field generator. It
stimulates a small patch of cortex.
(Lower left).
To accompany Baars & Gage Chapter 2
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EEG, MEG and single-neuron recording are direct measurers of electromagnetic
activity in the brain. MRI, fMRI, and PET are indirect measures of brain metabolic
activity or blood-flow.
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PET was the first indirect brain imaging
method to become practical. A radioactive
glucose or oxygen "tag" is injected into
the bloodstream, and as nutrients are
metabolized in the brain, safe levels of
radiation are released. The are recorded
by radiation detectors situated around the
head, and processed by computer to
reconstructed slices of the brain.
(Slice=tomos, PET = Positron emission
tomography).
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A great deal of important work has been
done using PET. (Left) However, it is
expensive, invasive (because of the
tracer injection) and slow (exposure times
are about 20-30 minutes). A more popular
method currently is MRI (and fMRI),
based on somewhat different principles.
Q uickTim e™ and a
TI FF ( Uncom pr essed) decom pr essor
ar e needed t o see t his pict ur e.
To accompany Baars & Gage Chapter 2
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MRI physics.
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(a and b) Protons in water --- the
H's in H2O--- have random
magnetic spin. Under a high
magnetic field, protons all line up
with the same "spin" orientation.
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(c, d and e) When a radio-frequency
pulse is provided and the proton
spin is "relaxed," the spin
orientations become random again
and the target emits an
electromagnetic signal.
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The signal is picked up by the donut
of detectors surrounding the
patient's head.
To accompany Baars & Gage Chapter 2
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MRI measures brain structure; fMRI measures brain function.
(MRI is tuned to proton spin; fMRI is tuned to the oxygen spin in hemoglobin.)
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A famous MRI study: Brain structure
in London taxi drivers. Notice that
we are not looking at brain activity
directly, but at the relative size of a
part of the brain, the hippocampus,
which is known to be involved in
spatial navigation. Notice the
computer-constructed brain slices in
the upper right-hand corner.
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An fMRI study: Looking at left-hand
stimulation. We are looking at brain
activity due to neurons firing in the
right cortical region for the hand
(somatosensory cortex). Notice that
Rest and Task periods alternate
every forty seconds or so. We can
see the results of brain activity (but
not as fast as the activity itself.)
To accompany Baars & Gage Chapter 2
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MRI and fMRI measure the atomic spin of molecules in a high
magnetic field. (Magnetic Resonance Imaging)
(A)
Neuronal firing
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(A) Neurons and the BOLD response. Spiking
neuron trace in the upper graph shows fast
electrochemical firing several seconds before the
blood oxygen signal begins to change (lower
graph).
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BOLD reflects a population of neurons. The
BOLD (blood-oxygen-dependent) signal occurs
when oxygen is used by active neurons (small
dip), followed by a surge of new oxygen supply to
the region (large peak), a momentary overshoot
(second small dip), and a return to homeostatic
balance. Notice that the BOLD response takes
place over a period of several seconds, while the
neuronal firing burst occurs much sooner. That is
why fMRI is said to be an indirect measure of
neuronal activity.
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(B) Another view of the same process. Red dots
indicate oxygenated blood cells, with high levels
BOLD response
(B)
of oxygenated hemoglobin.
To accompany Baars & Gage Chapter 2
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Signal subtraction is needed to get meaningful results in PET/MRI/fMRI.
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Because the experimental effects typically
constitute a small signal in a great deal of
background activity, PET and fMRI use signalaveraging at each point in space. Two very
similar experimental conditions are used,
differing in only one crucial feature. Notice that
the yellow brain scans (upper left) look very
similar, but when one is subtracted from the
other, a very clean signal remains. In this case
it shows significant activity in the occipital
lobe.
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Notice that we are looking down on a
horizontal (or axial) slice of the brain. (Below)
To accompany Baars & Gage Chapter 2
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A typical fMRI design: Faces vs. non-faces.
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Notice the similarities
between the faces and
non-faces conditions in
this experimental
comparison.
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The results are shown
in the yellow and red
regions of activation in
the brain diagrams
below, which include
the face recognition
areas of the inferior
temporal lobe.
To accompany Baars & Gage Chapter 2
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Tractography: White matter tracts (bundles of axons) are traced by
water flow.
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Since the axons that make up
neural pathways are tiny tubes,
water molecules tend to flow in the
direction of the tube.
MRI is tuned to the hydrogen
molecules in water, and the H2O
flow is transformed into artificially
colored patterns, as above.
These beautiful tractography scans
reveal an immense highway system
inside the white matter of the brain.
To accompany Baars & Gage Chapter 2
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Spatial scale
Summary: Picking up both direct and indirect brain
signals.
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Direct brain signals are usually
electromagnetic. They include
EEG, MEG, and single-cell
electrical recording. We can
also pump in electromagnetic
energy to stimulate neurons in
the brain.
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Indirect signals are produced by
brain metabolism and blood
flow (glucose and oxygen for
MRI/fMRI), and radioactive
tracers (PET).
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All brain recording methods
have their pros and cons. (Left)
To accompany Baars & Gage Chapter 2
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