Raziskovalne metode in pristopi Flashcards
Slikovne metode, nevrofiziologija, študije bolnikov
Describe two x-ray-based techniques for
visualizing the living human brain.
CONTRAST X-RAYS. Although conventional x-ray photography is not useful for visualizing the brain, contrast x-ray techniques are. Contrast x-ray techniques involve injecting into one compartment of the body a substance that absorbs x-rays either less than or more than the 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 (see Figure 5.1). 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.
COMPUTED TOMOGRAPHY. In the early 1970s, the introduction of computed tomography revolutionized the study of the living human brain. 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. During cerebral computed tomography, the
neurological patient lies with his or her head positioned in the center of a large cylinder 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 rotate rapidly 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. Then the x-ray
tube and detector are moved along the axis of the patient’s body to another level of the brain, and the process is repeated. Scans of eight or nine horizontal brain sections are typically obtained from a patient. When combined, these images provide three-dimensional representations of the brain.
Describe the positron emission tomography (PET) technique.
Positron emission tomography (PET) was the first brain imaging technique to provide images of brain activity (functional brain images) rather than images of brain structure (structural brain images). In one common version of
PET, radioactive fluorodeoxyglucose (FDG) is injected
into the patient’s carotid artery (an artery of the neck that
feeds the ipsilateral cerebral hemisphere). Because of its
similarity to glucose, the primary metabolic fuel of the
brain, fluorodeoxyglucose is rapidly taken up by active
(energy-consuming) cells. However, unlike glucose,
fluorodeoxyglucose cannot be metabolized; it therefore
accumulates in active neurons and astrocytes until it is
gradually broken down (see Zimmer et al., 2017). Each
PET scan is an image of the levels of radioactivity (indicated by
color coding) in various parts of
one horizontal level of the brain.
Thus, if a PET scan is taken of a
patient who engages in an activity such as reading for about
30 seconds after the FDG injection, the resulting scan will indicate the areas of the target brain
level that were most active during
the 30 seconds (see Figure 5.3).
Notice from Figure 5.3 that
PET scans are not really images of
the brain. Each PET scan is merely
a colored map of the amount of
radioactivity in each of the tiny
cubic voxels (volume pixels) that
compose the scan. Exactly how
each voxel maps onto a particular
brain structure can be estimated
only by superimposing the scan
on a brain image. The most significant current application of PET
technology is its use in identifying the distribution of particular molecules (e.g., neurotransmitters, receptors, transporters) in the brain (see Camardese et al., 2014). This is
accomplished by injecting volunteers with radioactively
labeled ligands (ions or molecules that bind to other molecules). Then, PET scans can document the distribution of
radioactivity in the brain.
Describe three magnetic-field-based techniques
for imaging the living human brain
MAGNETIC RESONANCE IMAGING. Magnetic resonance
imaging (MRI) is a structural brain-imaging procedure
in which high-resolution images are constructed from the
measurement of radio-frequency waves that hydrogen
atoms emit as they align with a powerful magnetic field.
Such imaging is possible because: (1) water contains two
hydrogen atoms (H2O) and (2) different brain structures
contain different amounts of water. This, in turn, means
that the number of hydrogen atoms differs between brain
structures, and, therefore, the radio-frequency waves emitted by a particular brain structure will be different from
its neighboring brain structures. MRI provides clearer
images of the brain than does CT (see Lerch et al., 2017).
A two-dimensional MRI scan of the midsagittal plane of the
brain is presented in Figure 5.4.
In addition to providing relatively high spatial
resolution (the ability to detect and represent differences
in spatial location), MRI can produce images in three dimensions. Figure 5.5 shows a three-dimensional MRI scan
of a patient with a growing tumor.
DIFFUSION TENSOR MRI. Many variations of MRI have
been developed. Arguably, one of the most innovative of these new MRI techniques has been diffusion tensor MRI.
Diffusion tensor MRI is a method of identifying those
pathways along which water molecules rapidly diffuse (see
Jbadi et al., 2015). Because tracts (bundles of axons) are the
major routes of rapid water diffusion in the brain, diffusion tensor imaging provides an image of major tracts—see
Figure 5.6.
Most brain research focuses on the structures of the
brain. However, in order to understand how the brain
works, it is imperative to understand the connections
among those structures—the so-called connectome (see Park
& Friston, 2013; Glasser et al., 2016; Swanson & Lichtman,
2016). This is why diffusion tensor images have become a
focus of neuroscientific research. Complete descriptions of
connectomes already exist for some organisms, including
the nematode C. elegans and the mouse (see Oh et al., 2014).
Work on the so-called Human Connectome Project is well
underway.
FUNCTIONAL MRI. MRI technology has been used to
produce functional images of the brain. Indeed, functional
MRI has become the most influential tool of cognitive
neuroscience. It is often used to determine if a brain is dysfunctional, but it is also used for a variety of other purposes;
for example, to infer the content of an individual’s dreams
(see Horikawa et al., 2013; Underwood, 2013).
Functional MRI (fMRI) produces images representing the increase in oxygenated blood flow to active
areas of the brain. Functional MRI is 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 (see Hillman, 2014). Second, oxygenated blood has different
magnetic properties than does deoxygenated blood, and
this difference influences the radio-frequency waves emitted by hydrogen atoms in an MRI. The signal recorded by
fMRI is called the BOLD signal (the blood-oxygen-leveldependent signal). The BOLD signal indicates the parts of
the brain that are active or inactive during a cognitive or
behavioral test, and thus it suggests the types of analyses
the brain is performing. Because the BOLD signal is the
result of blood flow through the brain, it is important to
remember that it is not directly measuring the electrical
activity of the brain.
Functional MRI has three advantages over PET: (1)
nothing has to be injected into the volunteer; (2) it provides
both structural and functional information in the same
image; and (3) its spatial resolution is better. A functional
MRI is shown in Figure 5.7.
It is important not to be unduly swayed by the impressiveness of fMRI images and technology. The images are
often presented—particularly in the popular press or general textbooks—as if they are actual pictures of human
neural activity. They aren’t: They are images of the BOLD
signal, and the relation between the BOLD signal and neural activity is complex (see Hillman, 2014). Furthermore,
fMRI technology has poor temporal resolution, that is, it
is poor at specifying the timing of neural events. Indeed,
it takes 2 or 3 seconds to measure the BOLD signal, and many neural responses, such as action potentials, occur in
the millisecond range.
Describe an ultrasound-based technique for
imaging the living human brain.
Functional ultrasound imaging (fUS) is a new imaging
technique that uses ultrasound (sound waves of a higher
frequency than we can hear) to measure changes in blood
volume in particular brain regions. When a brain region
becomes active, blood levels increase there, and alter the
passage of ultrasound through that brain region.
As a functional brain imaging method, fUS has three
key advantages over PET and fMRI: (1) it is cheap, (2) highly
portable; and (3) can be used for imaging some individuals,
such as human infants, who cannot undergo PET or fMRI
(see Deffieux et al., 2018)
Describe three transcranial stimulation
techniques
Transcranial magnetic stimulation (TMS) is a technique that can be used to turn off an area of human
cortex by creating a magnetic field under a coil positioned
next to the skull (e.g., Candidi et al., 2015). The magnetic
stimulation temporarily turns off part of the brain while
the effects of the disruption on cognition and behavior are
assessed. Although there are still fundamental questions
about safety, depth of effect, and mechanisms of neural
disruption (see Polanía, Nitsche, & Ruff, 2018; Romei, Thut,
& Silvanto, 2016), TMS is often employed to circumvent
the difficulty that brain-imaging studies have in determining causation. Using different stimulation parameters,
TMS can also be used to “turn on” an area of cortex
(see Rossini et al., 2015).
Transcranial electrical stimulation (tES) is a technique
that can be used to stimulate (“turn on”) an area of the
cortex by applying an electrical current through two electrodes placed directly on the scalp. The electrical stimulation temporarily increases activity in part of the brain while
the effects of the stimulation on cognition and behavior are
assessed (see Polanía, Nitsche, & Ruff, 2018).
The use of tES for its putative cognitive enhancement
effects has become popular, and there are many relatively
inexpensive tES systems available for purchase online (see
Bourzac, 2016). However, there is conflicting evidence about
whether tES has beneficial effects on cognition; some studies have even reported detrimental effects. Differing stimulation protocols might account for some of the discrepant
findings (see Sellers et al., 2015).
Transcranial ultrasound stiimulation (tUS) is a technique that, like tES and TMS, can be used to activate particular brain structures. However, unlike tES and TMS, which
can only be used to stimulate cortical structures, tUS can
also be used to activate subcortical structures.
To activate a brain structure using tUS, multiple sources
of low-amplitude ultrasonic sound waves are placed around
the head of the individual. Then, each of those sound
sources is directed at the target brain structure. When the
ultrasonic sound waves from each of those sources reach the
target structure they sum together, such that the amplitude
of the sound waves at the target brain structure is sufficiently large to stimulate activity in the cells there (see Tyler,
Lani, & Hwang, 2018).
The tUS technique can also be used to make small
permanent lesions to a brain structure. The procedure is
the same as that for stimulation via tUS, except that the
amplitude of each ultrasound source is larger, leading to
a larger amplitude waveform that is sufficient to create a
small (e.g., the size of a grain of rice) permanent lesion.
This tUS-based lesion method has been used to treat several conditions (e.g., lesioning a thalamic nucleus to treat
essential tremor)—all without having to make an incision.
Accordingly, the tUS lesion technique is revolutionizing
neurosurgery (see Landhuis, 2017)
Describe two psychophysiological measures of
brain activity.
SCALP ELECTROENCEPHALOGRAPHY. The electroencephalogram (EEG) is a measure of the gross electrical activity of the brain. It is recorded through large electrodes by a
device called an electroencephalograph (EEG machine), and the
technique is called electroencephalography. In EEG studies
of human participants, each channel of EEG activity is usually recorded from disk-shaped electrodes, about half the
size of a dime, which are attached to the scalp.
The scalp EEG signal reflects the sum of electrical
events throughout the head. These events include action
potentials and postsynaptic potentials as well as electrical
signals from the skin, muscles, blood, and eyes.
Thus, the utility of the scalp EEG does not lie in its
ability to provide an unclouded view of neural activity. 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
(e.g., epilepsy). For example, alpha waves are regular, 8- to
12-per-second, high-amplitude waves that are associated
with relaxed wakefulness. A few examples of EEG wave
forms and their psychological correlates are presented in
Figure 5.8.
Because EEG signals decrease in amplitude as they
spread from their source, a comparison of signals recorded
from various sites on the scalp can sometimes indicate
the origin of particular waves (see Cohen, 2017). This is
why it is usual to record EEG activity from many sites
simultaneously Psychophysiologists are often
more interested in the EEG waves
that accompany certain psychological events than in the background
EEG signal. These accompanying
EEG waves are generally referred
to as event-related potentials
(ERPs). One commonly studied
type of event-related potential is
the sensory evoked potential—
the change in the cortical EEG
signal elicited by the momentary
presentation of a sensory stimulus. As Figure 5.9 illustrates, 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 potentials is
that the noise of the background
EEG is often so great that the sensory evoked potential is masked.
Measuring a sensory evoked
potential can be like detecting a whisper at a rock concert. A method used to reduce the noise of the background
EEG is signal averaging. First, a subject’s response to
a stimulus, such as a click, is recorded many—let’s say
1,000—times. Then, a computer identifies the millivolt
value of each of the 1,000 traces at its starting point
(i.e., at the click) and calculates the mean of these 1,000
scores. Next, it considers the value of each of the 1,000
traces 1 millisecond (msec) from its start, for example,
and calculates the mean of these values. It repeats this
process at the 2-msec mark, the 3-msec 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. See
Figure 5.9, which illustrates the averaging of an auditory
evoked potential.
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. For example, the P300 wave illustrated in
Figure 5.10 is the positive wave that occurs about
300 milliseconds after a momentary stimulus that has
meaning for the participant (e.g., a stimulus to which the participant must respond). In contrast, the portions of an
evoked potential recorded in the first few milliseconds
after a stimulus are not influenced by the meaning of the
stimulus for the participant. These small waves are called
far-field potentials because, although they are recorded
from the scalp, they originate far away in the sensory
nuclei of the brain stem.
MAGNETOENCEPHALOGRAPHY. Another technique
used to monitor brain activity from the scalp of human subjects is magnetoencephalography (MEG). MEG measures
changes in magnetic fields on the surface of the scalp that are
produced by changes in underlying patterns of neural activity. Because the magnetic signals induced by neural activity
are so small, only those induced near the surface of the brain
can be recorded from the scalp (see Hari & Parkkonen, 2015).
MEG has two major advantages over EEG. First, it
has much better spatial resolution than EEG; that is, it can
localize changes in electrical activity in the cortex with
greater precision. Second, MEG can be used to localize subcortical activity with greater reliability than EEG (Baillet,
2017). Some downsides to the use of MEG include its high
price, the large size of the MEG machines (see Figure 5.11),
and the requirement that participants remain very still during recordings (Baillet, 2017; but see Boto et al., 2018).
Describe two psychophysiological measures of
somatic nervous system activity
MUSCLE TENSION. Each skeletal muscle is composed
of millions of threadlike muscle fibers. Each muscle fiber
contracts in an all-or-none fashion when activated by the
evoked potential recorded in the first few milliseconds
after a stimulus are not influenced by the meaning of the
stimulus for the participant. These small waves are called
far-field potentials because, although they are recorded
from the scalp, they originate far away in the sensory
nuclei of the brain stem.
MAGNETOENCEPHALOGRAPHY. Another technique
used to monitor brain activity from the scalp of human subjects is magnetoencephalography (MEG). MEG measures
changes in magnetic fields on the surface of the scalp that are
produced by changes in underlying patterns of neural activity. Because the magnetic signals induced by neural activity
are so small, only those induced near the surface of the brain
can be recorded from the scalp (see Hari & Parkkonen, 2015).
MEG has two major advantages over EEG. First, it
has much better spatial resolution than EEG; that is, it can
localize changes in electrical activity in the cortex with
greater precision. Second, MEG can be used to localize subcortical activity with greater reliability than EEG (Baillet,
2017). Some downsides to the use of MEG include its high
price, the large size of the MEG machines (see Figure 5.11),
and the requirement that participants remain very still during recordings (Baillet, 2017; but see Boto et al., 2018).
Psychophysiological Measures of
Somatic Nervous System Activity
LO 5.7 Describe two psychophysiological measures of
somatic nervous system activity.
MUSCLE TENSION. Each skeletal muscle is composed
of millions of threadlike muscle fibers. Each muscle fiber
contracts in an all-or-none fashion when activated by the
Figure 5.10 An average auditory evoked potential. Notice
the P300 wave. This wave occurs only if the stimulus has
meaning for the participant; in this case, the ‘click’ sound
signals the imminent delivery of a reward. By convention, positive EEG waves are always shown as downward
deflections.
Time (milliseconds)
Meaningful click 200 400 600
Far-field
potentials
P300
motor neuron that innervates it. At any given time, a few
fibers in each resting muscle are likely to be contracting,
thus maintaining the overall tone (tension) of the muscle.
Movement results when a large number of fibers contract
at the same time.
In everyday language, anxious people are commonly
referred to as “tense.” This usage acknowledges the fact that
anxious or otherwise aroused individuals typically display
high resting levels of tension in their muscles. This is why
psychophysiologists are interested in this measure; they use
it as an indicator of psychological arousal.
Electromyography is the usual procedure for measuring
muscle tension. The resulting record is called an electromyogram (EMG). EMG activity is usually recorded between two
electrodes taped to the surface of the skin over the muscle of
interest. An EMG record is presented in Figure 5.12. You will
notice from this figure that the main correlate of an increase
in muscle contraction is an increase in the amplitude of the
raw EMG signal, which reflects the number of muscle fibers
contracting at any one time.
Most psychophysiologists do not work with raw EMG
signals; they convert them to a more workable form. The
raw signal is fed into a computer that calculates the total
amount of EMG spiking per unit of time—in consecutive
0.1-second intervals, for example. The integrated signal
(i.e., the total EMG activity per unit of time) is then plotted.
The result is a smooth curve, the amplitude of which is a
simple, continuous measure of the level of muscle tension
(see Figure 5.12).
EYE MOVEMENT. The electrophysiological technique for
recording eye movements is called electrooculography,
and the resulting record is called an electrooculogram (EOG).
Electrooculography is based on the fact that a steady
potential difference exists between the front (positive) and back (negative) of the eyeball. Because of this steady potential, when the eye moves, a change in the electrical potential
between electrodes placed around the eye can be recorded.
It is usual to record EOG activity between two electrodes
placed on each side of the eye to measure its horizontal
movements and between two electrodes placed above
and below the eye to measure its vertical movements (see
Figure 5.13).
Describe three approaches to
neuropsychological testing
The nature of neuropsychological testing has changed
radically since the 1950s. Indeed, the dominant approach
to psychological testing has evolved through three distinct phases: the single-test approach, the standardized-testbattery approach, and the modern customized-test-battery
approach.
THE SINGLE-TEST APPROACH. Before the 1950s, the
few existing neuropsychological tests were designed to
detect the presence of brain damage; in particular, the goal
of these early tests was to discriminate between patients
with psychological problems resulting from structural brain
damage and those with psychological problems resulting
from functional, rather than structural, changes to the brain.
This approach proved unsuccessful, in large part because
no single test could be developed that would be sensitive
to all the varied and complex psychological symptoms that
could potentially occur in a brain-damaged patient.
THE STANDARDIZED-TEST-BATTERY APPROACH. The
standardized-test-battery approach to neuropsychological
testing grew out of the failures of the single-test approach,
and by the 1960s, it became predominant in North America.
The objective stayed the same—to identify brain-damaged
patients—but the testing involved standardized batteries
(sets) of tests rather than a single test. The most widely
used standardized test battery has been the Halstead-Reitan
Neuropsychological Test Battery. The Halstead-Reitan is a set
of tests that tend to be performed poorly by brain-damaged patients in relation to other patients or healthy controls;
the scores on each test are added together to form a single aggregate score. An aggregate score below the designated cutoff leads to a diagnosis of brain damage. The
standardized-test-battery approach proved only marginally successful; standardized test batteries discriminate
effectively between neurological patients and healthy individuals, but they are not so good at discriminating between
neurological patients and psychiatric patients.
THE CUSTOMIZED-TEST-BATTERY APPROACH. The
customized-test-battery approach—an approach
largely developed by Luria and other Soviet Union
neuropsychologists (see Ardila, 1992; Luria & Majovski,
1977)—began to be used routinely in a few American neuropsychological research institutions in the 1960s. This
approach proved highly successful in research, and it soon
spread to clinical practice. It now predominates in both the
research laboratory and the neurological ward.
The objective of current neuropsychological testing
is not merely to identify patients with brain damage; the
objective is to characterize the nature of the psychological
deficits of each brain-damaged patient. So how does the
customized-test-battery approach to neuropsychological testing work? It usually begins in the same way for all
patients: with a common battery of tests selected by the neuropsychologist to provide an indication of the general nature
of the neuropsychological symptoms. Then, depending on
the results of the common test battery, the neuropsychologist selects a series of tests customized to each patient in an
effort to characterize in more detail the general symptoms
revealed by the common battery. For example, if the results
of the test battery indicated that a patient had a memory
problem, subsequent tests would include those designed to
reveal the specific nature of the memory problem.
The tests used in the customized-test-battery approach
differ in three respects from earlier approaches. First, the
newer tests are specifically designed to measure aspects
of psychological function that have been spotlighted by
modern theories and data. For example, modern theories,
and the evidence on which they are based, suggest that
the mechanisms of short-term and long-term memory are
totally different; thus, the testing of patients with memory
problems virtually always involves specific tests of both
short-term and long-term memory. Second, the interpretation of the test results often does not rest entirely on how
well the patient does; unlike early neuropsychological tests,
currently used tests often require the neuropsychologist
to assess the cognitive strategy that the patient employs
in performing the test. Third, the customized-test-battery
approach requires more skill and knowledge on the part
of the neuropsychologist to select just the right battery of
tests to expose a particular patient’s deficits and to identify
qualitative differences in cognitive strategy
Describe those tests that are often
administered as part of an initial common
neuropsychological test battery.
Because the customized-test-battery approach to neuropsychological testing typically involves two phases—a battery
of general tests given to all patients followed by a series
of specific tests customized to each patient—we’ll cover
examples of these neuropsychological tests in two sections.
In this section, we’ll look at some tests that are often administered as part of the initial common test battery.
INTELLIGENCE. Although the overall intelligence quotient
(IQ) is a notoriously poor measure of brain damage, a test of
general intelligence is nearly always included in the battery of
neuropsychological tests routinely given to all patients. Many
neuropsychological assessments begin with the Wechsler
Adult Intelligence Scale (WAIS), first published in 1955 and
standardized in 1981 on a sample of 1,880 U.S. citizens between
16 and 71. The WAIS is composed of many subtests and is
often the first test administered, because knowing a patient’s
IQ can help a neuropsychologist interpret the results of subsequent tests. Also, a skilled neuropsychologist can sometimes
draw inferences about a patient’s neuropsychological dysfunction from the pattern of deficits on the subtests of the WAIS.
For example, low scores on subtests of verbal comprehension
tend to be associated with left hemisphere damage.
MEMORY. One weakness of the WAIS is that it often fails
to detect memory deficits, despite including subtests specifically designed to test memory function. For example,
the information subtest of the WAIS assesses memory for
general knowledge (e.g., “Who is Queen Elizabeth?”), and
the digit span subtest (the most widely used test of shortterm memory) identifies the longest sequence of random
digits that a patient can repeat correctly 50 percent of the
time; most people have a digit span of 7. However, these
two forms of memory are among the least likely to be disrupted by brain damage—patients with seriously disturbed
memory function often show no deficits on either the information or the digit span subtest. Be that as it may, memory
problems rarely escape unnoticed because they are usually
reported by the patient or the family of the patient.
LANGUAGE. If a neuropsychological patient has taken
the WAIS, deficits in the use of language can be inferred
from a low aggregate score on the verbal subtests. A patient
who has not taken the WAIS can be quickly screened for
language-related deficits with the token test. Twenty tokens
of two different shapes (squares and circles), two different
sizes (large and small), and five different colors (white, black,
yellow, green, and red) are placed on a table in front of the
patient. The test begins with the examiner reading simple instructions—for example, “Touch a red square”—and the
patient trying to follow them. Then the test progresses to
more difficult instructions, such as “Touch the small, red
circle and then the large, green square.” Finally, the patient
is asked to read the instructions aloud and follow them.
LANGUAGE LATERALIZATION. It is usual for one
hemisphere to participate more than the other in languagerelated activities. In most people, the left hemisphere is
dominant for language, but in some, the right hemisphere is
dominant. A test of language lateralization is often included
in the common test battery because knowing which hemisphere is dominant for language is often useful in interpreting the results of other tests. Furthermore, a test of language
lateralization is virtually always given to patients before
any surgery that might encroach on the cortical language
areas. The results are used to plan the surgery, trying to
avoid the language areas if possible.
There are two widely used tests of language lateralization. The sodium amytal test (Wada, 1949) is one and the
dichotic listening test (Kimura, 1973) is the other.
The sodium amytal test involves injecting the anesthetic sodium amytal into either the left or right carotid
artery in the neck. This temporarily anesthetizes the ipsilateral (same-side) hemisphere while leaving the contralateral
(opposite-side) hemisphere largely unaffected. Several tests
of language function are quickly administered while the
ipsilateral hemisphere is anesthetized. Later, the process is
repeated for the other side of the brain. When the injection is
on the side dominant for language, the patient is completely
mute for about 2 minutes. When the injection is on the nondominant side, there are only a few minor speech problems.
Because the sodium amytal test is invasive, it can be administered only for medical reasons—-usually to determine the
dominant language hemisphere prior to brain surgery.
In the standard version of the dichotic listening test,
sequences of spoken digits are presented to volunteers
through stereo headphones. Three digits are presented to
one ear at the same time that three different digits are presented to the other ear. Then, they are asked to report as
many of the six digits as they can. Kimura (1973) found that
patients correctly report more of the digits heard by the ear
contralateral to their dominant hemisphere for language, as
determined by the sodium amytal test.
Describe tests that might be used by a
neuropsychologist to investigate in more
depth general problems revealed by a common
neuropsychological test battery.
Following analysis of the results of a neuropsychological patient’s performance on a common test battery, the europsychologist selects a series of specific tests to clarify
the nature of the general problems exposed by the common battery. There are thousands of tests that might be
selected from. This section describes a few of them and
mentions some of the considerations that might influence
their selection. EMORY. Following the discovery of memory impairment by the common test battery, at least four fundamental questions about the memory impairment must be
answered (see Chapter 11): (1) Does the memory impairment involve short-term memory, long-term memory, or
both? (2) Are any deficits in long-term memory anterograde
(affecting the retention of things learned after the damage),
retrograde (affecting the retention of things learned before
the damage), or both? (3) Do any deficits in long-term
memory involve semantic memory (memory for knowledge
of the world) or episodic memory (memory for personal experiences)? (4) Are any deficits in long-term memory deficits
of explicit memory (memories of which the patient is aware
and can thus express verbally), implicit memory (memories
demonstrated by the improved performance of the patient
without the patient being conscious of them), or both?
Many amnesic patients display severe deficits in
explicit memory with no deficits at all in implicit memory
(see Squire & Dede, 2015). Repetition priming tests have
proven instrumental in the assessment and study of this
pattern. Patients are first shown a list of words and asked
to study them; they are not asked to remember them. Then,
at a later time, they are asked to complete a list of word
fragments, many of which are fragments of words from the
initial list. For example, if “purple” had been in the initial
test, “pu_p_ _” could be one of the test word fragments.
Amnesic patients often complete the fragments as accurately as healthy control subjects. But—and this is the really
important part—they often have no conscious memory of
any of the words in the initial list or even of ever having
seen the list. In other words, they display good implicit
memory of experiences without explicit memories of them.
LANGUAGE. If a neuropsychological patient turns out
to have language-related deficits on the common test battery, a complex series of tests is administered to clarify the
nature of the problem. For example, if a patient has a speech
problem, it may be one of three fundamentally different
problems: problems of phonology (the rules governing the
sounds of the language), problems of syntax (the grammar
of the language), or problems of semantics (the meaning of
the language). Because brain-damaged patients may have one of these problems but not the others, it is imperative that
the testing of all neuropsychological patients with speech
problems include tests of each of these three capacities.
Reading aloud can be disrupted in different ways by
brain damage, and follow-up tests must be employed that
can differentiate between the different patterns of disruption. Some dyslexic patients (those with reading problems)
remember the rules of pronunciation but have difficulties
pronouncing words that do not follow these rules, words
such as come and tongue, whose pronunciation must be
remembered. Other dyslexic patients pronounce simple
familiar words based on memory but have lost the ability
to apply the rules of pronunciation—they cannot pronounce
nonwords such as trapple or fleeming.
Describe the paired-image subtraction
technique.
ith the central role played by PET and fMRI in cognitive neuroscience research, the paired-image subtraction
technique has become one of the key behavioral research
methods in such research (see Kriegeskorte, 2010; Posner &
Raichle, 1994). Let us illustrate this technique with the classic PET study of single-word processing by Petersen and
colleagues (1988). Petersen and his colleagues were interested in locating the parts of the brain that enable a person
to make a word association (to respond to a printed word
by saying a related word). You might think this would be
an easy task to accomplish by having a volunteer perform a
word-association task while a PET image of the volunteer’s
brain is recorded. The problem with this approach is that
many parts of the brain that would be active during the test
period would have nothing to do with the constituent cognitive process of forming a word association; much of the
activity recorded would be associated with other processes
such as seeing the words, reading the words, and speaking.
The paired-image subtraction technique was developed to
deal with this problem.
The paired-image subtraction technique involves
obtaining functional brain images during several different
cognitive tasks. Ideally, the tasks are designed so that pairs
of them differ from each other in terms of only a single
constituent cognitive process. Then the brain activity associated with that process can be estimated by subtracting the
activity in the image associated with one of the two tasks
from the activity in the image associated with the other.
For example, in one of the tasks in the study by Petersen
and colleagues, volunteers spent a minute reading aloud
printed nouns as they appeared on a screen; in another,
they observed the same nouns on the screen but responded
to each of them by saying aloud an associated verb (e.g.,
truck—drive). Then Petersen and his colleagues subtracted
the activity in the images they recorded during the two
tasks to obtain a difference image. The difference image illustrated the areas of the brain specifically involved in
the constituent cognitive process of forming the word association; the activity associated with fixating on the screen,
seeing the nouns, saying the words, and so on, was eliminated by the subtraction.
Understand the default mode network and
know the structures that are part of that
network.
Interpretation of difference images is complicated by the
fact that there is substantial brain activity when humans sit
quietly and let their minds wander—this level of activity
has been termed the brain’s default mode (Raichle, 2010).
Brain structures typically active in the default mode but
less active during cognitive or behavioral tasks are collectively referred to as the default mode network, and
their pattern of activity is known as the resting state-fMRI
(R-fMRI). The default mode network comprises many
structures (see Fox et al., 2015) including the following
four cortical areas: medial parietal cortex, lateral parietal
cortex, medial prefrontal cortex, and lateral temporal cortex. See Figure 5.22.
Explain the concept of functional connectivity.
In addition to being interested in which brain regions are
active during particular cognitive tasks, cognitive neuroscientists are also eager to understand how network activity across multiple brain regions is related to a particular
cognitive task. This approach is referred to as the study of
functional connectivity (FC). To measure functional connectivity, a cognitive neuroscientist examines which brain
regions have parallel patterns of activity over time.
When a cognitive neuroscientist studies changes in FC
with the presentation of a stimulus, or during the performance of a task, they are studying extrinsic FC. This is in
contrast to intrinsic FC, which is FC that is present during
the R-fMRI (see Kelly & Castellanos, 2014). Collectively, the
task of characterizing the FC associated with each behavior and cognitive process is known as the study of the
functional connectome (see Matthews & Hampshire, 2016)
