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measure the brain's electrical activity directly, whereas comparable
approaches monitor only changes in blood fl ow (e.g., SPECT,
fMRI) or metabolic activity (e.g., PET), and thus only provide
indirect measures of brain electrical activity. Finally, EEG recording
in human subjects is a noninvasive and economical way to monitor
the brain's electrical activity.
However, EEG studies do have several limitations. Most
important limitation is the poor spatial resolution. EEGs are most
sensitive to a very particular set of postsynaptic potentials, those
being generated in the superfi cial layers of the cortex, on the crests
of gyri directly abutting the skull and radial to the skull. Dendrites
which are deeper in the cortex, those within sulci, the midline, or
deep structures (such as the cingulated gyrus or hippocampus), or
those producing currents which are tangential to the skull provide
far smaller contributions to EEG signals. In addition, it is mathe-
matically impossible to reconstruct a unique intracranial current
source for any given EEG signal. This conundrum is referred to as
the inverse problem.
5.2. Evoked Potentials
EP recordings are direct and noninvasive measures of brain func-
tion which provide temporal sequence of waves. EP recordings can
map the temporal sequence of the stages of information process-
ing, and this temporal information is unique. EP recordings are
also sensitive enough to brain pathology to detect areas of brain
damage before clinically obvious symptoms appear. Furthermore,
EP recordings can accurately track disease progression and recov-
ery of brain function. There are two general limitations of EP
recordings: (1) lack normative standards since the characteristics of
EP waveforms are affected by amplifi ers, digitized rate, signal fi lter,
and other technical factors and (2) EP signals are not able to pro-
vide directly association to specifi c areas of brain damage.
5.3. Extracellular
Single-Unit Recording
Extracellular recoding is advantageous because the recordings only
refl ect the activity of one particular neuron and usually only one
electrode is used. It is the only tool which makes it possible to
monitor neuron activities in real time while the animal is conscious,
freely performing, and performing specifi c behaviors. However, this
technique can prove limiting because it is neither easy to identify
the recorded neuron nor to study its neuronal interconnections by
labeling dye injection. Some of these disadvantages can be overcome
by intracellular recording as describing in the following chapter.
References
1. Mirsattari SM, Ives JR, Leung LS, Menon RS
(2007) EEG monitoring during functional MRI
in animal models. Epilepsia 48(Suppl 4):37-46
2. Hughes JR (1996) A review of the usefulness of
the standard EEG in psychiatry. Clin
Electroencephalogr 27:35-39
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