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Electromagnetic Fields in Biological Systems
associative learning task to disturb brain function related to memory and quick retrieval
of associative memory. The TMS disrupted associative learning of abstract patterns over
the right frontal area, which suggested that the participating cortical networks may be
lateralized in accordance with classic concepts of hemispheric specialization. This is the
first study to verify the important role of the dorsolateral prefrontal cortex of humans as
the retrieval function by TMS method.
Traditionally, stimuli are applied at various scalp positions using a latitude- and
longitude-based coordinate system referenced to Cz (central midline placement of electrodes at specific intervals along the head) in the 10–20 international system at the vertex, while the amplitude of the MEPs generated in contralateral muscles is also measured
(Ueno, Matsuda, and Fujiki 1989, 1990). This gives a “map” of the sites on the scalp
from which responses can be obtained by each reference muscle. Rothwell et al. (1987)
revealed the enormous clinical importance of TMS in motor function evaluation.
Regarding the therapeutic use of TMS, Pascual-Leone et al. (1996) studied the effects
of focal rTMS (10 Hz, produced MEP of greater than or equal to 50 μV) on the depressive
symptoms in 17 patients with medication-resistant depression of psychotic subtype. The
study was designed as a multiple crossover, randomized placebo-controlled trial. Sham
rTMS and stimulation of different cortical areas were used as controls. Left DLPFC
rTMS resulted in significant decrease in scores on the Hamilton depression rating scale
(HDRS) and the self-rated Beck questionnaire (BQ). Eleven of the total 17 patients
showed pronounced improvement that lasted for about 2 weeks after 5 days of daily
rTMS sessions. No patient experienced any significant undesirable side effects. These
findings emphasized the role of the left DLPFC in depression and suggested that rTMS
of the left DLPFC might become a safe, nonconvulsive alternative to electroconvulsive
treatment for depression.
Kujirai et al. (2006) reported that TMS pulses over the hand area of motor cortex
activate different subpopulations of synaptic connections if the direction of the induced
current in the brain is reversed from posterior–anterior (PA) direction to anterior–
posterior (AP) direction. The authors tested whether this also made a difference to the
after-effects of paired associative stimulation (PAS: ulnar nerve stimulation followed
25 milliseconds later by a TMS pulse). When 50 pairs of stimuli (0.1 Hz) were applied
using conventional suprathreshold PA-PAS in resting subjects, there was no effect on
MEPs in the FDI muscle. In contrast, when the same number of pulses were given while
subjects made a small tonic (5% maximum) contraction, MEP was facilitated and rMT
reduced when AP but not PA pulses were used. Subsequent experiments employed subthreshold TMS (95% of the aMT) during voluntary muscle contraction. Moter-evoked
potential facilitation accompanied by reduced AP threshold occurred when PAS was
given using AP pulses (AP-Sub-PAS), whereas PAS using PA pulses (PA-Sub-PAS) had
no excitatory effect. There was no facilitation if the ulnar nerve stimulus was replaced by
digital nerve stimulation. There was a tendency for short-interval intracortical inhibition (SICI) to decrease and intracortical facilitation (ICF) to increase after AP-Sub-PAS.
The authors proposed that the increased effectiveness of AP-Sub-PAS over PA-Sub-PAS
is due to the fact that AP TMS more readily activates I3 inputs to corticospinal neurons,
and hence I3 inputs are an important component of associative plasticity in the human
motor cortex.
Electromagnetic Fields in Biological Systems
associative learning task to disturb brain function related to memory and quick retrieval
of associative memory. The TMS disrupted associative learning of abstract patterns over
the right frontal area, which suggested that the participating cortical networks may be
lateralized in accordance with classic concepts of hemispheric specialization. This is the
first study to verify the important role of the dorsolateral prefrontal cortex of humans as
the retrieval function by TMS method.
Traditionally, stimuli are applied at various scalp positions using a latitude- and
longitude-based coordinate system referenced to Cz (central midline placement of electrodes at specific intervals along the head) in the 10–20 international system at the vertex, while the amplitude of the MEPs generated in contralateral muscles is also measured
(Ueno, Matsuda, and Fujiki 1989, 1990). This gives a “map” of the sites on the scalp
from which responses can be obtained by each reference muscle. Rothwell et al. (1987)
revealed the enormous clinical importance of TMS in motor function evaluation.
Regarding the therapeutic use of TMS, Pascual-Leone et al. (1996) studied the effects
of focal rTMS (10 Hz, produced MEP of greater than or equal to 50 μV) on the depressive
symptoms in 17 patients with medication-resistant depression of psychotic subtype. The
study was designed as a multiple crossover, randomized placebo-controlled trial. Sham
rTMS and stimulation of different cortical areas were used as controls. Left DLPFC
rTMS resulted in significant decrease in scores on the Hamilton depression rating scale
(HDRS) and the self-rated Beck questionnaire (BQ). Eleven of the total 17 patients
showed pronounced improvement that lasted for about 2 weeks after 5 days of daily
rTMS sessions. No patient experienced any significant undesirable side effects. These
findings emphasized the role of the left DLPFC in depression and suggested that rTMS
of the left DLPFC might become a safe, nonconvulsive alternative to electroconvulsive
treatment for depression.
Kujirai et al. (2006) reported that TMS pulses over the hand area of motor cortex
activate different subpopulations of synaptic connections if the direction of the induced
current in the brain is reversed from posterior–anterior (PA) direction to anterior–
posterior (AP) direction. The authors tested whether this also made a difference to the
after-effects of paired associative stimulation (PAS: ulnar nerve stimulation followed
25 milliseconds later by a TMS pulse). When 50 pairs of stimuli (0.1 Hz) were applied
using conventional suprathreshold PA-PAS in resting subjects, there was no effect on
MEPs in the FDI muscle. In contrast, when the same number of pulses were given while
subjects made a small tonic (5% maximum) contraction, MEP was facilitated and rMT
reduced when AP but not PA pulses were used. Subsequent experiments employed subthreshold TMS (95% of the aMT) during voluntary muscle contraction. Moter-evoked
potential facilitation accompanied by reduced AP threshold occurred when PAS was
given using AP pulses (AP-Sub-PAS), whereas PAS using PA pulses (PA-Sub-PAS) had
no excitatory effect. There was no facilitation if the ulnar nerve stimulus was replaced by
digital nerve stimulation. There was a tendency for short-interval intracortical inhibition (SICI) to decrease and intracortical facilitation (ICF) to increase after AP-Sub-PAS.
The authors proposed that the increased effectiveness of AP-Sub-PAS over PA-Sub-PAS
is due to the fact that AP TMS more readily activates I3 inputs to corticospinal neurons,
and hence I3 inputs are an important component of associative plasticity in the human
motor cortex.