137
Static, Low-Frequency, and Pulsed Magnetic Fields
visual field in which the memory probe was presented. Although it seems paradoxical,
the finding of rTMS-induced improvement in task performance has a precedent and is
consistent with the idea that regions associated with spatial sensory-motor processing
make necessary contributions to the short-term retention of this information.
The same research group further investigated the neurophysiological effects of rTMS
(10 Hz) delivered during the delay period of a visual working memory task by simultaneously recording brain activity with electroencephalography (EEG) (Hamidi et al.
2009). Subjects performed visual working memory tests for locations or for shapes,
and in half the trials rTMS was delivered to the SPL or a control brain area. The wide
range of individual differences in the effects of rTMS on task accuracy, from improvement to impairment, was predicted by individual differences in the effect of rTMS on
power in the α-band of the EEG (10 Hz): A decrease in α-band power corresponded
with improved performance, whereas an increase in α-band power corresponded with
the opposite. The EEG effect was localized to cortical sources encompassing the FEFs
and the IPS, and was specific to task (location, but not object memory) and to the rTMS
target (SPL, not control area). Furthermore, for the same task condition rTMS-induced
changes in cross-frequency phase synchrony between α- and γ-band (>40 Hz) oscillations predicted changes in behavior. These results suggest that α-band oscillations play
an active role in cognitive processes and do not simply reflect the absence of processing.
Furthermore, this study shows that the complex effects of rTMS on behavior can result
from biasing endogenous patterns of network-level oscillations.
Regarding rTMS influence on concurrent task performance, Johnson, Hamidi, and
Postle (2010) revealed the effects of rTMS on power in the α-band and on α:γ-phase
synchrony, each predicting its effect on behavior in a combined rTMS/EEG study that
investigated a complex set of relations between rTMS, EEG activity, and behavioral performance. These findings suggest that rTMS influences performance by biasing endogenous task-related oscillatory dynamics, rather than by creating a “virtual lesion.” To
further differentiate between these two alternatives, the authors compared the effects of
a 10 Hz rTMS on neural activity with the results of an experiment in which rTMS was
replaced with a 10 Hz luminance flicker. The authors reasoned that the 10 Hz flicker
would produce widespread entrainment of neural activity to the flicker frequency and
comparison of these EEG results with those from the rTMS study would shed light on
whether the latter also reflected entrainment to an exogenous stimulus. Results revealed
pronounced evidence for “entrainment noise,” produced by 10 Hz flicker-increased
oscillatory power and intertrial coherence (ITC) at the driving frequency, and increased
α:γ-phase synchronization that were nonetheless largely uncorrelated with behavior.
This contrasts markedly with 10 Hz rTMS for which the only evidence for stimulationinduced noise, elevated ITC at 30 Hz, differed qualitatively from the flicker results. The
authors suggested that simultaneous recording of the EEG offers an important means of
directly testing assumptions about how rTMS exerts its effects on behavior.
3.3.1.7 Musculoskeletal System
Abdelmelek et al. (2006) investigated the noradrenergic system and skeletal muscle
HSP72 response to an SMF (≤128 mT, 1 h/day for 5 days) in male rats. At thermoneutrality
Static, Low-Frequency, and Pulsed Magnetic Fields
visual field in which the memory probe was presented. Although it seems paradoxical,
the finding of rTMS-induced improvement in task performance has a precedent and is
consistent with the idea that regions associated with spatial sensory-motor processing
make necessary contributions to the short-term retention of this information.
The same research group further investigated the neurophysiological effects of rTMS
(10 Hz) delivered during the delay period of a visual working memory task by simultaneously recording brain activity with electroencephalography (EEG) (Hamidi et al.
2009). Subjects performed visual working memory tests for locations or for shapes,
and in half the trials rTMS was delivered to the SPL or a control brain area. The wide
range of individual differences in the effects of rTMS on task accuracy, from improvement to impairment, was predicted by individual differences in the effect of rTMS on
power in the α-band of the EEG (10 Hz): A decrease in α-band power corresponded
with improved performance, whereas an increase in α-band power corresponded with
the opposite. The EEG effect was localized to cortical sources encompassing the FEFs
and the IPS, and was specific to task (location, but not object memory) and to the rTMS
target (SPL, not control area). Furthermore, for the same task condition rTMS-induced
changes in cross-frequency phase synchrony between α- and γ-band (>40 Hz) oscillations predicted changes in behavior. These results suggest that α-band oscillations play
an active role in cognitive processes and do not simply reflect the absence of processing.
Furthermore, this study shows that the complex effects of rTMS on behavior can result
from biasing endogenous patterns of network-level oscillations.
Regarding rTMS influence on concurrent task performance, Johnson, Hamidi, and
Postle (2010) revealed the effects of rTMS on power in the α-band and on α:γ-phase
synchrony, each predicting its effect on behavior in a combined rTMS/EEG study that
investigated a complex set of relations between rTMS, EEG activity, and behavioral performance. These findings suggest that rTMS influences performance by biasing endogenous task-related oscillatory dynamics, rather than by creating a “virtual lesion.” To
further differentiate between these two alternatives, the authors compared the effects of
a 10 Hz rTMS on neural activity with the results of an experiment in which rTMS was
replaced with a 10 Hz luminance flicker. The authors reasoned that the 10 Hz flicker
would produce widespread entrainment of neural activity to the flicker frequency and
comparison of these EEG results with those from the rTMS study would shed light on
whether the latter also reflected entrainment to an exogenous stimulus. Results revealed
pronounced evidence for “entrainment noise,” produced by 10 Hz flicker-increased
oscillatory power and intertrial coherence (ITC) at the driving frequency, and increased
α:γ-phase synchronization that were nonetheless largely uncorrelated with behavior.
This contrasts markedly with 10 Hz rTMS for which the only evidence for stimulationinduced noise, elevated ITC at 30 Hz, differed qualitatively from the flicker results. The
authors suggested that simultaneous recording of the EEG offers an important means of
directly testing assumptions about how rTMS exerts its effects on behavior.
3.3.1.7 Musculoskeletal System
Abdelmelek et al. (2006) investigated the noradrenergic system and skeletal muscle
HSP72 response to an SMF (≤128 mT, 1 h/day for 5 days) in male rats. At thermoneutrality