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MeasuringElectrical Activity
of the Brain
ERP Mapping inAlcohol Research
DAVID B. CHORLIAN, M.S., BERNICE PORJESZ, PH.D.,
AND HOWARD L. COHEN, PH.D.
The recording of brain electrical activity from scalp electrodes
provides a noninvasive, sensitivemeasure of brain function.
Event-related potentials (ERP's) are brain waves that are
recordedwhile the subject is exposed to a specific sensory
stimulus. Depending on experimentalconditions, ERP's are useful in
studying many brain functions, such as sensory and
informationprocessing (e.g., memory). The assessment of ERP's is
useful in studying the effects of alcoholon brain function and in
identifying people at risk for developing alcoholism.
Computerizedmapping techniques produce graphs or color-coded images
to summarize data about thegeneration of ERP's in time and space.
KEYWORDS:brain function; brain wave; evoked
potential;electrodiagnosis; diagnostic imaging; computer
technology
Rcording brain electrical activity usingscalp electrodes
provides noninva-sive, sensitive measures of brainfunction (e.g.,
cognition). These electricalrecordings consist of two phenomena:
thecontinuous electroencephalogram (EEG)and timepoint-specific
event-related brainpotentials (ERP's). Unlike positron emis-sion
tomography (PET) or magnetic reso-nance imaging (MRI) techniques,
brainmapping does not construct an image of ahidden anatomical
structure; rather, it illus-
trates spatiotemporal brain activity (i.e.,brain activity as it
occurs in both space andtime). This article presents a variety of
brain
VOL. 19, No.4, 1995
mapping techniques, focusing on the ERPcomponent P300.
--
DAVIDB. CHORL/AN,M.S., is a seniorscientific programmer in the
Neurody-namics Laboratory, State University ofNew York (SUNY)Health
Science Centerat Brooklyn, Brooklyn, New York.
RECORDING BRAINELECTRICAL ACTIVITY
--
The EEG is not linked in time to any spe-cific event but instead
reflects the activation
level of various brain regions. For example,a person in a
relaxed state will manifest agreat deal of simultaneous brain wave
ac-tivity occurring at a wave frequency between8 and 12 cycles per
second. Conversely,ERP's represent brain electrical activity in
BERNICEPORJESZ,PH.D., is an assistantprofessor in the Department
of Psychiatry,SUNY Health Science Center at Brooklyn,Brooklyn, New
York.
HOWARDL. COHEN,PH.D., is a research
scientist in the Neurodynamics Laboratory,SUNY Health Science
Center at Brooklyn,Brooklyn, New York.
315J
-
response to specific sensory or cognitiveevents occurring at a
specific time. In ad-dition, ERP's consist of characteristic,highly
reproducible wave forms that canbe measured to within a fraction of
a sec-ond, providing an immediate record of thebrain activity
associated with informationprocessing. Because ERP signals are
smalland are embedded in the ongoing EEG,statistical techniques are
used to extractthem from the background EEG.
Depending on experimental conditions,ERP's may represent
overlapping activityof many brain circuits. Therefore, they are
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I400
I600
I200
useful in studying complex brain functions,such as sensory and
information processing(e.g., memory). ERP's offer a unique
ap-proach for assessing brain function becausethey allow scientists
to observe simultane-ously electrical activity (i.e.,
electrophysi-ology) and cognition.
How Are ERP's Obtained?
To record ERP's, subjects wear a cap em-bedded with from 20 to
128 noninvasivescalp electrodes. ERP's can be measuredin response
to stimuli from any sensory
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PZ
1800
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I I I200 400 600
Time (ms)
modality (e.g., sight or sound) and alsomay be recorded while a
behavioral taskis being conducted. For example, ERP'scan be
obtained for visual stimuli using amethod in which shapes or
letters arepresented on a computer screen while thesubject performs
a simple cognitive task,such as recognizing a particular shape.The
subject may be asked to make a be-havioral response (e.g., press a
buttonwhenever a particular shape appears) orkeep count of how many
specific stimulihave been presented. Scalp signals reflect-ing
brain electrical activity are recorded
F8
Control subjects
Alcoholic subjects
-
simultansubject {:
What D
The peal<(i.e., neg;also kno'ponents,These cotheir poswell as
tlrence of Imeasurelnegativefollowinthe Nl 0(with a Iresponseof the
stithe laterpsychol(
What I!
Much at!componctive com500 ms ;of P300a rear (i.
1 r800' 0
I200
I600
1800
I400
Figure 1 Electroencephalographic (Le., brain wave) tracings of
the waveform P300 obtained from nonalcoholic (Le., control)
andalcoholic subjects in response to a visual stimulus. Each
tracing represents a different location on the scalp. The
hori-zontal scales represent time in milliseconds (ms). The
vertical scales represent wave amplitude measured in
microvolts(!lV) of electrical potential. The data are statistically
derived from 197 subjects and 61 scalp electrodes.
316 ALCOHOL HEALTH & RESEARCH WORLD
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VOL.19,
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,0 simultaneously from all electrodes as thesubject processes
the stimuli.lsk
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What Do ERP's Indicate?
thelsk,
The peaks (i.e., positive waves) and troughs
(i.e., negative waves) of the ERP wave form,also known as
positive and negative com-
ponents, are measured in microvolts (~V).These components are
named according to
their positive (P) or negative (N) polarity aswell as their
latency (i.e., the time of occur-rence of the peak wave after the
stimulus,measured in milliseconds [ms]). Thus, a
negative ERP wave occurring 100 ms
following the stimulus would be calledthe NI or N100 wave. Early
components
(with a latency of less than 100 ms) areresponses to the
physical characteristics
of the stimulus (e.g., intensity), whereasthe later components
are influenced bypsychological factors (e.g., cognition).
)e-Iormliflect -led
What Is a P300?
Much attention has focused on the P300
component of the ERP, a prominent posi-tive component peaking
between 300 and500 ms after a stimulus. Scalp recordingsof P300 are
strongest at the parietal area,a rear (i.e., posterior) brain
region. The
ERP task most commonly used to elicitthe P300 is the so-called
"oddball" task,in which subjects are asked to attend
toand/orrespondto a rare stimuluspresent-ed in a series of other
stimuli. For exam-ple, in an auditory paradigm, the subjectlistens
to frequent "boops" and rare "beeps"(targets) in a random stream of
tone burstspresented about 1.5 seconds apart. The sub-ject may be
askedto press a button or counteach time a target occurs (for a
review, seeRegan 1989). P300 results from auditoryparadigms,
however, are not as consistentas those from visual paradigms.
Conse-quently, the P300 data discussed here arefrom a study using a
visual paradigm inwhich alcoholics, high-risk subjects, andcontrol
subjects pressed a button in responseto the rare occurrence of the
target letter"X" embedded in a sequence of other vis-ual
shapes.
Advantages of ERP Measurementin Alcoholism Research
ERP's provide sensitive measures of brainfunctions. Unlike other
imaging techniques,ERP's reflect subtle, dynamic,
real-time,millisecond-to-millisecond transactionsthat are elicited
while the brain is chal-lenged and are therefore highly
sensitive
Measuring the Brain's Electrical Activity
to specific brain processes. Most sensoryactivity occurs within
100 ms after a stim-ulus is presented, and most cognitive ac-tivity
takes place within 500 ms. Althoughimages obtained using PET also
offer afunctional measure of brain processes, eIec-trophysiological
measures link images ofbrain activity to specific timepoints
(Le.,provide a temporal resolution) with a pre-cision far greater
than that of PET's. Otherimaging methods, such as MRI and com-puted
tomography (CT) scans,portray grossbrain structure but do not
reflect ongoingelectrical activity. Thus, they cannot pro-vide
direct measures of brain function. ERPabnormalities, however, can
be observedeven when MRI and CT images reveal noanatomical changes.
In addition,many EEGand ERP characteristicsare extremely sen-sitive
to acute and chronic effects of alcoholon the brain and are
responsive to intoxi-cation, tolerance, withdrawal, and the
ef-fects of prolonged abstinence.
Electrophysiological features also maybe hereditary markers of
risk for alcoholism.Evidence indicates that the characteristicsof
both EEG's and ERP's are geneticallydetermined and that P300
attributes gen-erally differ between alcoholics and non-alcoholics
(Porjesz and Begleiter 1995). Arecent twin study (O'Connor et aI.
1994)
B. Potential Map for Alcoholic Subjects
o
18.016.014.012.010.08.06.04.02.00.0
-2.0-4.0-6.0-8.0
-10.0
250 500 750
Time (ms)
1,000
Figure 2 Spatiotemporal (Le., space-time) potential maps
displaying electroencephalographic (Le., brain wave) tracings of
thewave form P300 obtained from (A) nonalcoholic (Le., control) and
(B) alcoholic subjects in response to a visual stimulus.This same
data are shown in figure 1. The horizontal scales represent time in
milliseconds (ms). The color-coded verticalscales represent changes
in electrical potential measured in microvolts (IJV) in a spatial
continuum along a scalp line fromfront to rear. Several such maps
are needed to demonstrate the spatiotemporal changes at all
electrodes. The maps showthat although large amplitude differences
exist over time in P300 between the nonalcoholic and alcoholic
subjects, spatialdistributions between the two groups are
similar.
VORLD VOL. 19, No.4, 1995 317
A. Potential Map for Control Subjects
18.0
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10 I I OJ10.0 OJ
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_"If j ',rj' f\ro 6.0 rou u. CZ 4.0 CZ0 I I OJ 2.0 OJiiJ 0.0
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-
Figure 3 Topographic maps of (A) nonalcoholic (Le., control) and
(B) alcoholic subjects representing electrical
potentials(microvolts) recorded 430 milliseconds after exposure to
a target stimulus (when P300 is at its maximum). Because themaps
indicate fewer electrodes than points, mathematical models must be
used to estimate values for the intermediatepoints. These two maps
show little variation in potential.
reported that P300 amplitude is highly heri-table; similar
findings on the heritability ofP300 are being obtained by
researchers inthe Collaborative Study on the Genetics ofAlcoholism
(COGA) through a large fam-ily study (Porjesz et al. in press).
METHODS OF ERP ANALYSISAND MAPPING
Amplitude and Latency Measures
Researchers have used various auditory andvisual paradigms to
study the P300 compo-nent and have found the P300 amplitude tobe
smaller in alcoholics than in control sub-
jects (figure I) (for a review, see Porjeszand Begleiter 1993).
Researchers previouslythought that these low P300 voltages
re-sulted from neurotoxic effects of alcohol onthe brain. Low
P300's, however, do not re-cover with prolonged abstinence and
canoccur in subjects at risk for alcoholism priorto alcohol
exposure. Thus, low P300 ampli-tudes may characterize populations
at risk fordeveloping alcoholism (Polich et al. 1994).
Topographic Representations
Although amplitude and latency measuresprovide important
information about brainfunction, they do not address the spatial
dis-tribution ofERP's across the scalp. Spatial
318
distributions provide information aboutbrain areas possibly
involved in generatingERP activity.Displaying ERP's from multi-ple
electrodes does not show values acrossthe entire scalp surface but
only at the sitesof the electrodes. Mathematical techniquesenable
researchers to assign values to thepoints that have not been
directly meas-ured; maps can then be created by calcu-lating the
values between electrode points.Figures 2 and 3 show two types of
maps-spatiotemporal and topographic-basedon this approach (Fein et
al. 1992).
Topographic maps are frozen at a sin-gle point in time, thereby
losing the dy-namic aspect of the recorded data. Withthe advent of
computer technology, thesespatial maps can be presented as a
seriesof rapidly changing, computer-generatedanimations that
describe how the topo-graphic distribution of the recorded
activitychanges over time.
the radially oriented current density at apoint on the scalp.
The CSD values de-note current sources and "sinks" (i.e.,locations
where the current becomes un-detectable). Researchers create CSD
mapsby taking simultaneous data from all theelectrodes and
converting them into spatialdata. A CSD topographic map or
spatio-temporal map (figures 4 and 5) may dis-play results in a
more useful fashion thana topographic map based on ERP's (figures2
and 3).
In the P300 experiment mentionedearlier, for example, the peak
of the P300component (occurring at 430 ms) from 61scalp electrodes
is transformed to CSD. Inhealthy control subjects, these maps
indi-cate both a large posterior focus of activityas well as front
(i.e., anterior) foci (figure4). These brain-mapping techniques aid
inpinpointing possible sites of brain dysfunc-tion in alcoholics
and in subjects at riskfor developing alcoholism.
A comparison of CSD maps obtainedfrom alcoholics and
nonalcoholics showsweaker activity in the alcoholics' brains.Figure
5 displays the distribution of P300activity across the alcoholics'
scalps. Com-pared with control subjects, alcoholics canappear to
have a weaker posterior focusof activity and no clear frontal
focus. There-fore, control subjects and alcoholics ex-hibit not
only differences in amplitudebut differences in spatial
distributions,particularly in the brain's frontal regions.
MAPPING OF CURRENTSOURCE DENSITY
Because voltages measured at a point onthe scalp result from
both local (i.e., di-rectly under the electrode) and remotesources
by mathematically transformingthe ERP data, researchers can
emphasizelocal sources. The data transformationproduces a quantity
known as the currentsource density (CSD), which represents
ALCOHOL HEALTH & RESEARCH WORLD
-
heite
vat a; de-,e.,~sun-
D mapsill the
) spatialpatio-y dis-In than
(figures
1ede P300'rom61:':SD.In,s indi-
activity(figure:said inIysfunc-t risk
,tainedshows
brains.)f P300s.Com-,lics canfocus. There-cs
ex-tudeIons,egions.
WORLD
Measuring the Brain's Electrical Activity
A. Current Density Map for Control Subjects
FZ(f)c:.21iiu.3 CZa.
-
performs a task. The results are interpret-ed as reflecting the
activity of activenetworks in the cerebral cortex and can be
applied to various ERP paradigms.
Structural Findings
To determine the relationship betweenbrain structure and
function, researchershave investigated the neuroanatomicalorigins
of P300 using various techniques.Evidence from recordings obtained
fromelectrodes implanted in the human brainimplicates both the
medial temporal lobe'as well as source(s) within the frontal lobeas
contributing to P300 generation. Thesefindings, coupled with the
rather small ef-fect that temporal lobe removal has on scalpP300
during auditory oddball tasks, sug-gest that multiple brain sites
contribute tothe P300 (for a review, see Regan 1989).
Researchers have used noninvasiveimaging techniques to assess
brain struc-ture. In a recent study using P300 ampli-tude and MRI,
Ford and colleagues (1994)reported that reduced P300's recorded
dur-ing both automatic and effortful attentiontasks (i.e., tasks
requiring subjects to focusintently) correlated with frontal and
pari-etal gray-matter volumes. These findingsare consistent with
the CSD maps of P300described earlier in which both parietal
andfrontal foci of activity were found. The re-duced P300
amplitudes in alcoholics andtheir CSD maps may be manifestations
offrontal lobe damage (see Oscar-Bermanand Hutner 1993).
'For a definition of this and other technical terms
used in this anicle. see central glossary, pp. 293-295.
CONCLUSIONS
Electrophysiological recordings of ERP'sreflect the activation
of neural circuits in-volved in the mediation of sensory and
cog-nitive processes. The assessment ofERP'shas proven extremely
useful in studyingthe effects of alcohol(ism) on brain func-tion
and in identifying people at risk fordeveloping alcoholism. In
contrast, con-ventional imaging techniques have helpedidentify
brain structural changes resultingfrom
alcoholism.Electrophysiologicalbrainmapping is the representation
of brain func-tioning in its spatiotemporal dimensions.These
electrophysiological techniques pro-vide exquisite temporal
resolution of brainprocesses and have now been enhanced toprovide
spatial information about possiblebrain generators of this
activity. Thesenovel techniques not only potentially pro-vide a
window into the brain's function butalso may contribute simple
clinical toolsfor identifying both the adverse effects ofalcohol
consumption on the brain and peo-ple who may be at risk for
alcoholism.
ACKNOWLEDGMENT
The authors would like to thank DouglasHarsh for developing
computer graphicssoftware to create brain images.
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320 ALCOHOL HEALTH & RESEARCH WORLD
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