Zotero Chunk RAG

than those oriented tangentially. Scalp potentials will be
measured only when a sufficient number of cells are active
synchronously, and this synchrony is facilitated by the
columnar organization of the cortex. MEG is similar to
EEG but measures more the intracellular currents. The
sources of MEG may be better localized than with EEG
because MEG is not distorted by the skull and scalp, but
MEG is blind to radial sources. PET and fMRI are tech-
niques for functional neuroimaging and have good spatial
localization but less temporal resolution. PET using O-15
water measures regional cerebral blood flow, and this
is a reasonable measure since synaptic activity increases
local metabolism and stimulates changes in perfusion.
fMRI most commonly uses the BOLD technique, which
measures the oxidation state of hemoglobin in the blood.
Since with metabolism, blood flow increases more than
oxygen extraction, blood becomes more oxygenated. This
is a rather indirect measure of neuronal activity, but it
does correlate with perfusion measures, and like EEG and
MEG, it is most closely correlated with synaptic activity.
One example of how the techniques show different
views of a physiological process is what happens in the
motor cortex in the no-go trials in a go/no-go experiment.
The go/no-go experiment is a two-choice reaction time
experiment, to either move or not move, depending on
ability prior to the movement, and in the no-go trials, there
was inhibition (Leocani et al., 2000a; Waldvogel et al.,
2000). There is a potential problem, however, in under-
standing what is happening with a suppression of the
MEP amplitude after TMS. There could be simple with-
drawal of excitation, or there could be active inhibition.
This issue can be addressed with a study of SICI in the
reaction time period, and it does turn out that there is
Figure 7. Technique of Paired Associative Stimulation
The method is illustrated in the middle part where 90 pairs of median
nerve stimulation and TMS are given with an interstimulus interval
(ISI) of 25 ms. The post-test MEP (on the right) has become larger
than the pre-test MEP (on the left). From Stefan et al. (2000), with
permission.
Excitation and Inhibition
Method Excitatory Mode
Inhibitory Mode
rTMS
high frequency, R5 Hz low frequency, 0.2–1 Hz
TBS
intermittent
continuous
tDCS
anodal
cathodal
PAS
synchronous
heterosynaptic
stimulation
asynchronous
heterosynaptic
stimulation