Other Methods
Researchers use a variety of techniques from the fields of chemistry, physiology,
psychology and anatomy.
Behavioral Neuroscience
Techniques to study the neural basis of behavior.
Method
Description
Stereotaxic
Surgery
Surgery performed using an atlas showing the location of brain areas
in 3 planes of space. Used to place recording or stimulating
electrodes or to destroy a particular part of the brain.
Lesion
Production
Destruction of a particular part of the brain. Lesions can be produced
by passing electrical current (AC or DC) through an electrode or
with chemicals (such as kainic acid or 6-hydroxydopamine) that
destroy neurons. A lesion can also be made surgically by cutting a
tract or by suction removal of part of the brain. A reversible lesion
can be made by cooling (then rewarming) part of the brain or by
injecting drugs (such as lidocaine).
Electrical Brain
Stimulation
Stimulation of a brain area by passing electrical current through an
electrode.
Microinjection
Injection of small quantities of drug or neurotransmitter into a
specific area of the brain.
Neuroanatomy
Techniques to study the structure of the nervous system.
Method
Description
Examples
Cell Body Staining
Coloring neurons to see individual
neurons (nerve cells) or groups of
neurons.
Cresyl violet stain;
neutral red stain; Golgi
stain
Tract (Myelin)
Staining
Coloring nerve fibers to see
pathways.
Weil method;
Weigert's myelin stain;
Marchi stain
Tract Tracing
Tracing the projections from one
part of the nervous system to
another part. Tracing can be
retrograde (backwards) or
anterograde (forwards).
Electron Microscopy
Electrons passed through tissue to
produce more detailed images
Immunocytochemistry
Localizing particular chemicals
(neurotransmitters, proteins) within
particular neurons.
In situ hybridization
Localizing the synthesis of proteins
or peptides in neurons.
c-Fos
c-Fos is a protein product of an
immediate-early gene and has been
used as a marker for brain areas
activated by different stimuli. To
see the c-Fos, immunocytochemical
techniques must be used.
Deoxyglucose Uptake
Neurons that are active use glucose.
By injecting deoxyglucose, the cells
that use glucose also take up the
deoxyglucose. However, the
deoxyglucose is not degraded by
enzymes in the neurons so it stays
inside the neuron. By radioactively
labeling the deoxyglucose,
neuroscientist can find out what
areas of the brain are active during
specific behaviors or events.
Horseradish
peroxidase (HRP)
method; fluorescent
microspheres;
Phaseolus vulgarisleucoagglutinin (PHAL) method; FluoroGold; Cholera B-toxin;
DiI; tritiated amino
acids
Neurophysiology
Techniques to understand the function of the nervous system.
Method
Description
Patch Clamp Technique
Recording current flow from single ion channels of a
neuron.
Intracellular Recording
Electrical recording from INSIDE of a single neuron.
Extracellular Recording
Electrical recording from outside of a single (or a few)
neuron.
Mass Unit Recording
Electrical recording from outside of a group of neurons.
Evoked Potentials
Electrical activity of the brain synchronized to an event.
Electroencephalography
(EEG)
Electrical activity of the brain recorded with scalp or brain
electrodes. The EEG can also be used to map the brain.
Neuropharmacology
Techniques to understand the chemistry of the nervous system.
Method
Microiontophoresis
Description
Injection of small quantities of chemicals (drugs,
neurotransmitter) into neural tissue by passing electrical current.
Brain Imaging
Recent technology has enabled neuroscientists to "see" inside the living brain. These
brain imaging methods help neuroscientists:
Understand the relationships between specific areas of the brain and what function
they serve.
Locate the areas of the brain that are affected by neurological disorders.
Develop new strategies to treat brain disorders.
Procedure
Method
Computed Tomography Scan
(CT Scan)
CT scans use a series of X-ray beams
passed through the head. The images are
then developed on sensitive film. This
method creates cross-sectional images of
the brain and shows only the structure of
the brain, not its function.
Image courtesy of the Yousef Mohammad, M.D., MSc; Assistant
Professor of Neurology Division of Cerebrovascular Diseases, The
Ohio State University Medical Center
Positron Emission Tomography
(PET)
Image courtesy of the National Institute on Drug Abuse
A scanner detects radioactive material
that was injected or inhaled to produce
an image of the brain. Commonly used
radioactively-labeled material includes
oxygen, fluorine, carbon and nitrogen.
When this material gets into the
bloodstream, it goes to areas of the brain
that use it. So, oxygen and glucose
accumulate in brain areas that are
metabolically active. When the
radioactive material breaks down, it
gives off a neutron and a positron. When
a positron hits an electron, both are
destroyed and two gamma rays are
released. Gamma ray detectors record
the brain area where the gamma rays are
emitted. This type of method provides a
functional view of the brain.
Advantages:
1. Provides an image of brain
activity.
Disadvantages:
1. Expensive to use.
2. Radioactive material used.
MRI uses the detection of
radiofrequency signals produced by
displaced radio waves in a magnetic
field. It provides an anatomical view of
the brain.
Advantages:
Magnetic Resonance Imaging
(MRI)
1. No X-rays or radioactive
material is used.
2. Provides detailed view of the
brain in different dimensions.
3. Safe, painless, non-invasive.
4. No special preparation (except
the removal of all metal objects)
is required from the patient.
Patients can eat or drink anything
before the procedure.
Disadvantages:
1. Expensive to use.
2. Cannot be used in patients with
metallic devices, like
pacemakers.
3. Cannot be used with
uncooperative patients because
the patient must lie still.
4. Cannot be used with patients
who are claustrophobic (afraid of
small places). However, new
MRI systems with a more open
design are now available.
Functional Magnetic Resonance
Imaging (fMRI)
Figure 1
Functional MRI detects changes in blood
flow to particular areas of the brain. It
provides both an anatomical and a
functional view of the brain.
A BOLD-based functional map
during right finger and right
toe movements, combined with
head sketch. Since an oblique
slice was selected along the left
central sulcus, the left
hemisphere is shown
predominantly. An arrow
indicates the left central sulcus.
Yellow represents functional
areas activated only during the
finger movements; red, only
during the toe movements; and
green, during both tasks. (Kim
et al., 1994a).
Angiography involves a series of X-rays
after dye is injected into the blood. This
method provides an image of the blood
vessels of the brain.
Angiography
Here are some examples of using a combination of PET and MRI techniques:
Thalamus
Cortex
These 2 images show the averaged data from 14 subjects who received a painful
injection of the chemical capsaicin into the upper arm. The colored part of the
images show increased blood flow (the PET) to the thalamus and primary
somatosensory cortex after the injection. The gray areas of the images (the MRI)
show the brain anatomy. So using this method can identify the areas of the brain
that are active during specific conditions. This technique could be used to study just
about any other cognitive function.
More re. Imaging
Tome is Greek for slice. The standard slice orientation in most brain imaging is transaxial
or "axial". Left is shown at right. Note that, like the "lower organs", we look up to the
brain.
Other standard planes of view are coronal
and sagittal.
Non-tomographic images represent "projections" from a single point of view and include
bolus contrast x-ray angiograms and MR angiograms.
Tomographic images are made up of little squares called "pixels" (picture elements), each
of which takes a gray-scale value from 1 (black) to 256 (white). Each pixel represents
brain tissue which is about 1 mm. on each of two sides. The thickness of the slice is often
3 or 5 mm, thus creating a three-dimensional volume element, or "voxel", which is
shaped like a shoe box. Pixel intensity represents an average from tissue within the voxel.
Image types
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CT (roentgen-ray computed tomography) A beam of x-rays is shot straight
through the brain. As it comes out the other side, the beam is blunted slightly
because it has hit dense living tissues on the way through. Blunting or
"attenuation" of the x-ray comes from the density of the tissue encountered along
the way. Very dense tissue like bone blocks lots of x-rays; gray matter blocks
some and fluid even less. X-ray detectors positioned around the circumference of
the scanner collect attenuation readings from multiple angles. A computerized
algorithm reconstructs an image of each slice.
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MRI (magnetic resonance imaging) When protons (here brain protons) are
placed in a magnetic field, they become capable of receiving and then
transmitting electromagnetic energy. The strength of the transmitted energy is
proportional to the number of protons in the tissue. Signal strength is modified by
properties of each proton's microenvironment, such as its mobility and the local
homogeneity of the magnetic field. MR signal can be "weighted" to accentuate
some properties and not others.
When an additional magnetic field is superimposed, one which is carefully varied
in strength at different points in space, each point in space has a unique radio
frequency at which the signal is received and transmitted. This makes
constructing an image possible. It represents the spatial encoding of frequency,
just like a piano.
MR signal sources
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When protons are placed in a magnetic field, they oscillate.
The frequency at which they oscillate depends on the strength of the magnetic
field.
Protons are capable of absorbing energy if exposed to electromagnetic energy at
the frequency of oscillation. After they absorb energy, the nuclei release or
reradiate this energy so that they return to their initial state of equilibrium. This
reradiation or transmission of energy by the nuclei as they return to their initial
state is what is observed as the MRI signal.
The return of the nuclei to their equilibrium state does not take place
instantaneously, but rather takes place over some time.
The return of the nuclei to their initial state is governed by two physical
processes:
o the relaxation back to equilibrium of the component of the nuclear
magnetization which is parallel to the magnetic field, and
o the relaxation back to equilibrium of the component of the nuclear
magnetization which is perpendicular to the magnetic field.
The time that it takes for these two relaxation processes to take place is roughly
equal to:
o time T1 for the first process, and
o time T2 for the second process.
The strength of the MRI signal depends primarily on three parameters.
o Density of protons in a tissue: The greater the density of protons, the
larger the signal will be.
o T1
o T2
The contrast between brain tissues is dependent upon how these 3 parameters
differ between tissues.
For most "soft" tissues in the body, the proton density is very homogeneous and
therefore does not contribute in a major way to signal differences seen in a image.
However, T1 and T2 can be dramatically different for different soft tissues, and
these parameters are responsible for the major contrast between soft tissues.
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T1 and T2 are strongly influenced by the viscosity or rigidity of a tissue.
Generally speaking, the greater the viscosity and rigidity, the smaller the value for
T1 and T2.
It is possible to manipulate the MR signal by changing the way in which the
nuclei are initially subjected to electromagnetic energy. This manipulation can
change the dependence of the observed signal on the three parameters: proton
density, T1 and T2. Hence, one has a number of different MR imaging techniques
("weightings") to choose from, which accentuate some properties and not others.
SPECT/PET (single photon/positron emission computed tomography) When
radiolabeled compounds are injected in tracer amounts, their photon emissions
can be detected much like x-rays in CT. The images made represent the
accumulation of the labeled compound. The compound may reflect, for example,
blood flow, oxygen or glucose metabolism, or dopamine transporter
concentration. Often these images are shown with a color scale.