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Chapter 5

PSYC 2410 Chapter Notes - Chapter 5: Transcranial Magnetic Stimulation, Magnetic Resonance Imaging, Positron Emission Tomography


Department
Psychology
Course Code
PSYC 2410
Professor
Elena Choleris
Chapter
5

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Chapter 5: The Research Methods of Biopsychology
The Ironic Case of Professor P.
5.1: Methods of Visualizing and Stimulating the Living Human Brain
Conventional X-ray are useless for viewing the living brain.
By the time an X-ray beam has passed through neural tissue, with its numerous folds of tissue with
similar ability to absorb X-Rays, there is little detail that can be seen from an x-ray
Contrast X-Rays: X-Ray techniques that involve the injection into one compartment of the body a
substance that absorbs X-ray either less than or more than the surrounding tissue.
oCerebral Angiography: Contrast X-Ray technique uses the infusion of a radio-opaque dye into
a cerebral artery to visualize the cerebral circulatory systems during X-ray photography
Most useful for locating vascular damage
Displacement of blood vessels from their usual position can indicate the presence of a
tumour
X-Ray Computed Tomography: Computer-assisted X-ray procedure that can be used to visualize
the brain and other internal structures of the human body.
oDiscovered in the 1970's
oPatient lays with their head in the center of a large cylinder
oOn one side of the cylinder is an X-ray tube that projects an X-ray beam through the head to an
X-Ray detector mounted on the other side
oBoth the emitter and the recorder rotate automatically around the patient, taking many
individual images.
oThe information from all the images are combined by the computer for a single flat image of the
brain.
oThe procedure is then repeated until about 8 or 9 flat images have been produced.
oThe images combined can offer a three dimensional view of the brain
Magnetic Resonance Imaging: A procedure in which high resolution images of structures of the
living brain are constructed from the measurement of waves that hydrogen atoms emit when they
are activated by radio-frequency waves in a magnetic field.
oMRIs provide clearer pictures of the brain than a CT
oSpatial Resolution: The ability to detect and represent differences in spacial location.
oMRIs typically have relatively high spatial resolution.
oAnother benefit of MRIs is that they are able to produce images in three dimensions.
Positron Emission Tomography: A technique for visualizing brain activity, usually by measuring
the accumulation of 2-deoxyglucose (2-DG) or radioactive water in in the various areas of the
brain.
oFirst imaging technology to provide images of brain activity (Functional images) rather than
brain structure (Structural images)
oBecause 2-deoxyglucose is so similar to glucose, it is taken up rapidly by active, energy-
consuming cells
oUnlike glucose, 2-deoxyglucose cannot be metabolized, so it accumulates in active tissues until
it is gradually broken down by the body
oThus, if a pet scan were taken of a patient who engages in an activity for about 30 seconds (ie.
Reading) the PET scan would show which areas of the brain were most active during the
performance of the activity.

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oPet scans do not show structures in the brain, and they are not really images of the brain.
oA PET scan is a conglomerate of colours showing which portions were the most active during
the time immediately following the uptake of 2-deoxyglucose
oHow well the PET scan matches up to a structural brain area can only be estimated by
superimposing the PET scan data on a structural image of the brain being observed.
Functional MRI: A magnetic resonance imaging technique for infering brain activity by measuring
increased oxygen flow into particular areas.
ofMRI has become the most influential tool used in cognitive Neuroscience, and is widely used
for medical diagnosis.
ofMRI is made possible by two qualities of oxyginated blood:
Active areas of the brain take up more oxyginated blood than necessary, meaning that
oxyginated blood accumulates in the tissues using it.
Oxyginated blood has magnetic properties, oxygen influences the effect of magnetic fields
on iron
oBOLD Signal: Signal recorded by fMRI machines. Acronym for Blood-Oxygen Level
Dependent Signal.
ofMRI has four advantages over PET technology:
Nothing has to be injected into the subject.
It provides both functional and structural information about the brain in the same image
It has improved spatial resolution compared to PET technology
It can be used to produce three dimensional images of activity over the entire brain.
oDisadvantages of fMRI
fMRI is often misunderstood because it presents so many advantages, meaning that people
will often trust studies supported by fMRI data without critically evaluating the research
involved.
fMRIs are not real-time images of neural activity. It does not capture electrical activity or
the transfer of potentials. It measures the BOLD signal, which has a complex, variable and
poorly understood relationship to neural activity in the truest sense.
fMRI is too slow to capture many neural responses. It takes between 2-3 seconds for an
image to be generated. Many neural activies, however, including action potentials happen
on the time order scale of milliseconds.
Magnetoencephalography: A technique for recording changes produced in magnetic fields on the
surface of the scalp by changes in underlying patterns of neural activity.
oTemporal Resolution: The ability to detect and represent differences in temporal location.
oThe major advantage of MEG imaging over fMRI is its temporal resolution, it's ability to detect
and record very rapid changes in neural activity.
Transcranial Magnetic Stimulation (TMS): A technique for disrupting the activity in an area of
the cortex by creating a magnetic field under a coil positioned next to the skull; The effect on
cognition is assessed to clarify the function of the affected region of the cortex.
oThe above mentioned methods for developing images of the brain are all excellent ways of
seeing what is going on inside our heads, but they all share a single common weakness:
Imaging techniques can be used to show correlation between brain activity and cognitive
activity, but it cannot prove that brain activity is the cause of cognitive activity.
5.2: Recording Human Psychophysiological Activity
There are five commonly used physiological methods of making inferences about the brain
oEEG measures brain waves

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oMuscle tension measures somatic nervous system activity
oEye movement measures somatic nervous system activity
oSkin conductance measures autonomic nervous system activity
oCardiovascular activity measures autonomic nervous system activity
Scalp Electroencephalography
oThe electroencephalagram is a measure of the overall electrical activity in the brain
oIn EEG studies of human subjects, each channel of EEG activity is recorded through an
individual electrode attached to the scalp
oEEG waves from the scalp are a mixture of neural activity, including action potentials, post-
synaptic potentials and electrical signals from the skin, muscles, blood and eyes
oThe value of EEG in research is that certain patterns of brain waves are associated with certain
states of consciousness (ie. Sleeping and waking) or particular types of cerebral pathology (ie.
Seizures)
oAlpha Waves: regular, 8- to 12-per second, high-amplitude waves that characterize relaxed
wakefulness.
oEEG waves decrease in amplitude as they move farther away from their source, wo slight
differences in amplitude strength can be used to determine the origin of particular waves.
oEvent Related Potentials: The EEG waves that typically accompany certain psychological
events.
Sensory-Evoked Potential: Commonly studied event related potential; Momentary change
in cortical EEG readings that occurs when the individual is presented with a stimulus.
Sensory-evoked potential has two components: background EEG, the portion of the
EEG which is of no interest to researchers, and the signal, the portion of the EEG which
is of interest to researchers
Often there is so much noise captured when recording a sensory-evoked potential that
the signal is difficult or impossible to detect. (The signal is masked)
Signal Averaging: A method used to reduce the noise of background EEG and isolate
the signal. A subject's EEG scalp response is recorded for the same stimulus many times.
A computer identifies the millivolt value of each of the stimulus traces at its starting
point, and averages them out. The computer then repeats this procedure for each
millisecond after the initial starting point and plots the values of these means against
time.
oThe plotted averages show a much clearer picture of the evoked potential because
the noise is cancelled out by the averaging process
oEach averaged wave is characterized by its direction and its latency
oThe direction can be either positive or negative. By convention, positive waves are
observed as downwards deflections and negative waves are observed as upwards
deflections.
oLatency reflects how long after the stimulus was presented that the wave occurred
oP300 Wave: Positive wave that occurs approximately 300 milliseconds after
presentation of a stimulus occurs that indicates the stimulus has some special
significance to the participant.
Far-Field Potentials: So-called because even though they are recorded at the scalp,
they originate far away in the sensory nuclei of the brain stem
oEEG scores high on temporal resolution, but initially did very poorly in terms of spacial
resolution.
oNew technologies have improved the spatial resolution of EEG immensely.
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