Psychology 2220A/B Chapter Notes - Chapter 5: Default Mode Network, Dichotic Listening Test, Pipette

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METHODS OF VISUALIZING AND STIMULATING THE LIVING HUMAN BRAIN
Prior to the early 1970s, biopsychological research was impeded by the inability to
obtain images of the organ of primary interest: the living brain
Conventional X-ray photography is next to useless for this purpose
When an X-ray photograph is taken, an X-ray beam is passed through an object and
then onto a photographic plate
Each of the molecules through which the beam passes absorbs some of the radiation;
thus, only the unabsorbed portions of the beam reach the photographic plate
X-ray photography is therefor effective in characterizing internal structures that differ
substantially from their surroundings in the degree to which they absorb X-rays
However, by the time an X-ray beam has passed through the numerous overlapping
structures of the brain
It carries little information about the structures through which it has passed
Contrast X-Rays:
Contrast X-ray techniques involve injecting into one compartment of the body a
substance that absorbs X-rays either less than or more than surrounding tissue
The injected substance then heightens the contrast between the compartment and
the surrounding tissue during X-ray photography
One contrast X-ray technique, cerebral angiography, uses the infusion of a radio-
opaque dye into a cerebral artery to visualize the cerebral circulatory system
during X-ray photography
Cerebral angiograms are most useful for localizing vascular damage, but the
displacement of blood vessels from their normal position also can indicate the
location of a tumor
X-Ray Computed Tomography:
Computed tomography (CT) is a computer assisted X-ray procedure that can be
used to visualize the brain and other internal structures of the living body
On 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
The X-ray tube and detector automatically rotate around the head of the patient at
one level of the brain, taking many individual X-ray photographs as they rotate
The meager information in each X-ray photograph is combined by a computer to
generate a CT scan of one horizontal section of the brain
Magnetic Resonance Imaging:
MRI is a procedure in which high-resolution images are constructed from the
measurement of waves that hydrogen atoms emit when they are activated by
radio-frequency waves in a magnetic field
Provides clearer images of the brain than does CT
In addition to providing relatively high spatial resolution (the ability to detect and
represent differences in spatial location), MRI can produce images in 3D
Positron Emission Tomography:
PET provide images of brain activity (functional brain images) rather than images
of brain structure (structural brain images)
In one common version of PET, radioactive 2-deoxyglucose (2-DG) is injected into
the patients carotid artery
2-DG is rapidly taken up by active cells
Chapter 5: The Research Methods of Biopsychology
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However, cannot be metabolized, therefore it accumulates in active neurons - or in
associated astrocytes - until it is gradually broken down
Each PET scan is an image of the levels of radioactivity in various parts of one
horizontal level of the brain
Functional MRI
fMRI produce images representing the increase in oxygen flow in the blood to
active areas of the brain
Possible because of two attributes of oxygenated blood
First, active areas of the brain take up more oxygenated blood than they need
for their energy requirements, and thus oxygenated blood accumulates in active
areas of the brain
Second, oxygenated blood has magnetic properties
The signal recorded by fMRI is called the BOLD signal (the blood-oxygen-level-
dependent signal)
fMRI has four advantages over PET:
1. Nothing has to be injected into the subject
2. It provides both structural and functional information in the same image
3. Its spatial resolution is better
4. It can be used to produce 3D images of activity over the entire brain
fMRI technology is too slow to capture many neural responses - it takes 2 or 3
seconds to create an fMRI image, and many neural responses, such as action
potentials, occur in milliseconds
Magnetoencephalography:
MEG measures changes in magnetic fields on the surface of the scalp that are
produce by changes in underlying patterns of neural activity
Its major advantage over fMRI is its temporal resolution; it can record fast changes
in neural activity
Transcranial Magnetic Stimulation:
PET, fMRI, and MEG all have the same weakness: They can be used to show a
correlation between brain activity and cognitive activity, but they can’t prove that
the brain activity caused the cognitive activity
TMS is a techniques for affecting the activity in an area of the cortex by creating a
magnetic girls under a coil positioned next to the skull
In effect, the magnetic stimulation temporarily turns off part of the brain while the
effects of the disruption on cognition and behavior are assessed
TMS is often employed to circumvent the difficulty that brain imaging studies have
in determining causation
RECORDING HUMAN PSYCHOPHYSIOLOGICAL ACTIVITY
Scalp Electroencephalography:
The EEG is a measure of the gross electrical activity of the brain
It is recorded through large electrodes by a device called and
electroencephalograph (EEG machine), and the technique is called
electroencephalography
The scalp EEG signal reflects the sum of electrical events throughout the head
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These events include action potential and postsynaptic potentials, as well as
electrical signals from the skin, muscles, blood, and eyes
Its value as a research and diagnostic tool rests on the fact that some EEG wave
forms are associated with particular states of consciousness or particular types of
cerebral pathology
Because EEG signals decrease in amplitude as they spread from their source, a
comparison of signals recorded form various sites on the scalp can sometimes
indicate the origin of particular waves
Psychophysiologists are often more interested in the EEG waves that accompany
certain psychological events than they are in the background EEG signal
These accompanying EEG waves are generally referred to as event-related
potentials (ERPs)
Sensory evoked potential - the change in the cortical EEG signal that is elicited by
the momentary presentation of a sensory stimulus
The cortical EEG that follows a sensory stimulus has two components: the
response to the stimulus (the signal) and the ongoing background EEG activity (the
noise)
The signal is the part of any recording that is of interest; the noise is the part that
isn’t
The problem in recording sensory evoked background EEG is often so great that
the sensory evoked potential is masked
A method used to reduce the noise of the background EEG is signal averaging
First, a subjects response to a stimulus is recorded many times
Then a computer identifies the millivolt values of each of the traces at its starting
point and calculates the mean of the scores
Next, it considers the value of each of the traces 1-millisecond from its start and
calculates the mean of these values
It repeats this process at the 2-msec mark, the 3-sec mark and so on
When these averages are plotted, the average response evoked by the click is
more apparent, because the random background EEG is canceled out by the
averaging
The analysis of average evoked potentials (AEPs) focuses on the various waves in
the averaged signal
Each wave is characterized by its direction, positive, or negative, and by its latency
The P300 wave is the positive wave that occurs about 300 milliseconds after a
momentary stimulus that has meaning for the subject
The proportions of an evoked potential recored in the first few milliseconds after a
stimulus are not influenced by the meaning of the stimulus for the subject called
farfield potentials
The spatial resolution of these techniques is sufficiet to enable the amplitude of
evoked EEG signals recored on the cortex to be color coded and plotted on the
surface of a 3D MRI
Muscle Tension:
Muscle fiber contracts in an all-or-none fashion when activated by the motor
neuron that innervates it
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