Describe the main techniques that can be used to measure brain activity
in humans. Assess their relative advantages and disadvantages. Give
examples of studies that use these techniques.
Word Count: 2160
Mental activity can be defined as the activity the brain undergoes due to thought or
behaviour. This essay aims to numerate the methods invented to measure brain activity.
A brief introduction to the methods; their utility and their difference from methods that
study brain structure will be described. The essay will also illustrate each method with its
relative advantages and disadvantages, with studies which have used these methods to
provide data.
Over the last fifty years methods have been devised to view brain structure, like the MRI
(Magnetic Resonance Imaging), Contrast X- Rays and the CT (Computed Tomography)
scans. However, these only report the structure, and not the functions of each part of the
brain. Thus, the need for technology to measure the functions of the brain was felt. It is to
be noted, however, that stimulating activity in the brain measures how behaviour changes
by stimulating parts of the brain, whereas measuring mental activity is just the oppositemeasuring activity in the brain as a consequence of behaviour. Thus, functional
organization of the brain may be described as the relationships between nerve cells
(Frashowiak, 2001), which must be understood to identify neural codes of cells.
Methods that study mental activity are important as: (a) they provide valuable data about
the functions of various parts of the brain; (b) provide considerable data to compare
damaged brain with normal ones, and thus diagnose problems (Carlson, 2001); (c) guide
the planning of surgery or treatment for the brain; and (d) allow comparison of human
studies with other species and help make inferences about the evolution of neural systems
(Carlson, 2001). Thus these techniques have a wide and important applicability.
Mental activity can be measured by recording electrical, metabolic or chemical activity
(Banich, 2004). The most primitive method of measurement is lesions, by which a part of
the brain is surgically removed and the corresponding effect on behaviour is measured.
The newer, more accurate techniques are described below.
I. Positron Emission Topography (PET): In the common version of PET, the
radioactive substance 2-deoxyglucose is injected into the blood. This chemical has a short
half-life and has to be produced on-site by a technique called Cyclotron. Being similar to
glucose, 2-DG is taken up by active neurons, where it releases positrons which combine
with electrons to emit two anti-parallel gamma rays in opposite directions. These rays are
detected and their midpoint is identified as the source (Kalat, 1995). The radioactivity
(and corresponding neural activity) of the area can then be measured by the sensors that
surround the individual. Several brain levels can be scanned simultaneously as well
(Pinel, 2006).
The advantages of the PET are: (a) provides an absolute measure of the regional Cerebral
Blood Flow (rCBF) (Banich, 2004); (b) forms a clear brain image. However this method
has disadvantages which are: (a) we must assume where a particular voxel (volume pixel)
can be mapped on the brain as the image formed is not of the complete brain (Pinel,
2006); (b) ethical reasons related to the ionization radioactivity permit a person only 2-5
scans a year (Banich, 2004); (c) relies on secondary signals which are the product of
neural activity, and not the activity itself (Johansson & Windhorst, 1999); (d) the
chemicals are expensive and cumbersome as they have to be produced on-site; (e) poor
temporal resolution (time taken to form an image), makes data obtained from one person
insufficient, necessitating an average of many people.
Nonetheless, the PET scan images have a high reliability (Frashowiak, 2001) and the
method was successful in detecting when an area of the brain develops in infants. It was
found that the frontal lobes do not develop before 7 ½ months of age (Chugani & Phelps,
as cited in Kalat, 1995). It also identified that on similar tasks schizophrenia patients
make less use of their prefrontal cortex as compared to normal individuals (Berman, Zec
& Weinberger, as cited in Kalat, 1995).
II. Single Photon Emission Computed Tomography (SPECT): This technique injects
xenon, hexamethylpropylene amine oxime or other non-reactive substances into the
blood to measure radioactivity. It is quite similar to PET and uses gamma cameras which
surround the head to measure the gamma rays and form images or projections. The
patterns that form can show the rCBF. The spatial resolution is about 1-2 centimeters and
the temporal resolution is 30 minutes (Halligham, Kischka & Marshall, 2003).
SPECT as a method is less precise than PET and produces images which are not clear,
because fewer sensors are used and the isotope takes longer to decay (Banich, 2004).
However, being less costly than PET, as the gamma scanning equipment and the isotope
used are both cheaper and the isotope does not need to be produced on site, SPECT is
widely used.
These advantages have promoted its use in many studies like in a study of ObsessiveCompulsive Disorder. It was shown that there was more rCBF in OCD patients in right
superior and inferior frontal cortex and the right and left thalamus than in normal
individuals- a definite abnormality (Lacerda et al., 2003).
III. Functional Magnetic Resonance Imaging (fMRI): When tasks are performed and
neurons fire, the blood in nearby arterioles changes in oxygen level. The fMRI provides a
non-invasive technique of studying brain activation by measuring the change in blood
oxygenation level. Thus, it is Blood Oxygen Level Dependent (Yan, Hu, Zhou & Liu,
2004). The basic concept behind the fMRI is that oxygenated blood is diamagnetic (has
magnetic properties), and deoxygenated blood is paramagnetic (has no magnetic
properties). Thus signals are picked up from the relative proportion of oxygenated to
deoxygenated blood (Banich, 2004), by high powered, oscillating magnetic field
gradients. A Statistical Parametric Map (SPM) is produced, mapping active areas (Ou &
Golland, 2005). It must be noted that fMRI needs baseline scans determining the normal
functioning of the brain, before event related scans can be made.
The fMRI method has many advantages: (a) as compared to PET, no substance is injected
and a person may be scanned multiple times; (b) has sub-millimeter spatial resolutions,
providing clear, three dimensional images of the entire brain (Cabeza & Nyberg, as cited
in Pinel, 2006). Despite these, some problems faced with the fMRI are: (a) poor temporal
resolution because the BOLD signal has a half life of several seconds (Frashowiak,
2001); (b) like PET, it relies on secondary signals, not measuring neural activity directly
(Johansson & Windhorst, 1999); (c) requires a person to remain still which may not be
practical, specially in trauma situations; (d) may take time to obtain scans for diagnosis.
Even so, the fMRI has proved to be effective and reliable in most situations. For example,
a study for content specific processes during text comprehension using fMRI found, that
the left anterior temporal lobe is important during emotional stories, and spatial
information activated the collateral sulci and the posterior cingulate cortex, which are
areas important for visuo-spatial cognition (Ferstl & Yves von Cramon, 2007). Again, in
people with anti-social personality disorders and their response to monetary rewards, an
absence of prefrontal responses and reduced BOLD signal in the sub-cortical reward
system was found during positive reinforcement, by fMRI (Vollm et al., 2007). Thus
fMRI has a vast and varied applicability in research.
IV. Electroencephalography: Electroencephalography means recording brain activity
from
the
scalp
(Johansson
&
Windhorst,
1999).
This
technique
uses
an
electroencephalogram (EEG) - the recording of the gross electrical activity in the brain.
The post-synaptic potentials produced in neurons during a period of activity are
measured. Activity can be recorded for a single neuron using a microelectrode or an
entire region using macroelectrodes. In such a case, electrodes are attached to the scalp
and one electrode is attached to an electrically inactive area to provide a baseline. EEG
signals decrease in amplitude across space, so their source can be detected. About 100200 recordings are taken and averaged to identify the relevant signal relative to the
unnecessary ‘noise’. These recordings are displayed on an occillograph called the
polygraph. There are specific EEG wave formations for cognitive functions,
abnormalities in which can help in diagnosis of a disorder (Kalat, 1995). EEG recordings
for specific events, Event Related Potentials (ERP) may also be measured. However a
person’s EEG traits must not be considered permanent, as body temperature, stress,
menstrual cycles and such phenomena often create variations in the EEG (Niedermeyer &
Lopes da Silva, 2005). Earlier the EEG only had good temporal resolution, but recently
signals can be recorded and plotted on a colour coded MRI scan, which has improved its
spatial resolution (Gevins et al., as cited in Pinel, 2006).
Thus, as is evident the EEG has advantages such as: (a) a good temporal and relatively
good spatial resolution; (b) measures neural activity directly, not through secondary
signals; (c) can detect radially oriented cells in the brain (Banich, 2004). The
disadvantages are: (a) is often painful, so not preferred; (b) is a reference measurement
and needs a reference electrode (Niedermeyer & Lopes da Silva, 2005); (c) is not always
clear due to background ‘noise’ that distorts recordings.
EEG has successfully been used in research over the years. An example is a study
conducted to find interactions between the working memory and visual perception which
found, that the larger the amplitude of the EEG recording during presentation of stimuli,
the more accurate the imitation of the motion sequence was (Agam & Sekuler, 2007). In
another study, it was shown that EEG waves or in effect, neural activity, of different
frequency ranges induced different stages of memory formation (Axmacher et al., 2006).
V. Magneto Encephalography (MEG): This technique measures changes in the
magnetic field on the scalp produced by underlying neural activity. It can measure
strength and localization of the source of activity, along with the direction of current
densities (Niedermeyer & Lopes da Silva, 2005). The signals are recorded by a
Superconducting Quantum Inference Device (SQUID), a highly sensitive device (Kolb &
Whishaw, 2001).
The advantages of MEG are: (a) needs no reference points and is not attached to the
scalp as magnetic fields exist in free space; (b) has tighter and more focal patterns than
the EEG (Niedermeyer & Lopes da Silva, 2005); (c) has a temporal resolution of less
than 1 second which is better than fMRI (Pinel, 2006);(d) the strength of the signal does
not vary with structures in the brain (Banich, 2004); (e) is preferred over the EEG as it is
less painful. The disadvantages are: (a) it cannot detect radially aligned cells; (b) though
it can detect intracellular currents very accurately it cannot do the same for extracellular
currents (Niedermeyer & Lopes da Silva, 2005).
MEG can detect brain activity well, which is illustrated in a study where MEG found that
the M100 wave, which indicates perceptual memory for auditory stimuli, has an atypical
location in paranoid schizophrenics (Banich, 2004). This helps infer that schizophrenic
hallucinations may be due to an error in the auditory perceptual memory.
VI. Autoradiography: This is an invasive procedure, where 2-deoxyglucose is inserted
and taken up similar to PET. Consequently, the patient is killed; the brain is removed and
sliced; photographic emulsion is put on it and it is developed like film. The dark spots
that appear are the 2-DG rich areas. As is imaginable, this technique is very rarely used
due to ethical reasons.
VII. Cerebral Dialysis: This method measures the extracellular concentration of
neurochemicals to determine neural activity (Robinson & Justice, as cited in Pinel, 2006).
A fine tube with a semi-permeable section is implanted in the brain, and extracellular
fluids diffuse into the tube, which are then tested in a chromatograph (device for testing
constituents of liquids). However, ethical constraints discourage dialysis in humans
(Carlson, 2001).
VIII. Optical Imaging: In this technique a laser source of infrared light (wavelength of
700-1000 nm) is shone on the scalp. The detectors, made of optic fibre bundles, sense the
alterations in the path of the light. They detect two signals- a slow signal indicating
absorption of light (related to chemical concentration of the brain area), and a fast signal
indicating scattering of light (related to swelling of glial cells or neurons, due to firing).
Another sub-type of optical imaging is the Near Infrared Spectroscopy (NIRS), which
can detect changes by this method in blood hemoglobin levels, which are associated with
neural activity. Event Related Optical Signals (EROS) can also be obtained when the
technique is applied to specific events (Banich, 2004).
There are some advantages of this method: (a) has a very good temporal resolution of 20
milliseconds; (b) can be applied where PET/fMRI cannot, as the patient need not keep
still during Optical Imaging (Gratton et al.; Villringer & Chance; as cited in Rosenzweig,
Breedlove, & Leiman, 2002); (c) is relatively less expensive. However, its main
disadvantage is that it does not detect activity in sub-cortical regions as light gets
absorbed in brain structures above.
Optical imaging, specifically NIRS, helped study schizophrenic and normal patients in a
verbal fluency test and corresponding activation in their frontal lobes and found lower
activation in schizophrenic patients confirming ‘functional deficits within the frontal
lobe’ in schizophrenics (Ehlis et al., 2007).
Thus, as described above, most functional neuro-imaging techniques have contributed
vastly to our knowledge of the brain and mind. A method that could be developed in this
field as well, is measuring the proteins released from neurons during activity (Carlson,
2001). Forming archives of brain images to compare findings with previous results has
also been suggested (Rolland et al., as cited in Pinel, 2006). Precision is vital in this field
and attempts are being made to combine EEG/MEG signals with fMRI/PET studies to
acquire accurate data (Frashowiak, 2001). Recently, it has also been proposed that
neuroelectrical activity may interact with a spin-polarized population to obtain ‘Direct
Neural Imaging’ (DNI) using MRI methods. DNI would complement fMRI, EEG and
MEG studies, providing a tomographic method to measure neural activities directly
(Kraus, Volegov, Matlachov & Espy, 2007).
The techniques to study brain function have been outlined above with their general
characteristics, benefits, drawbacks and examples of studies which have used them. We
must remember that these techniques in no way compete with psychological analysis,
instead complement and support them (Gabrieli, as cited in Rosenzweig, Breedlove, &
Leiman, 2002). The study of brain function or mental activity is a very promising
approach to a full understanding of the brain’s organization.
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