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Electromagnetic Fields in Biological Systems
Sadleir, Grant, and Woo (2010) attempted to theoretically analyze neural function
imaging using magnetic resonance electrical impedance tomography (MREIT). The
authors found that phase noise in a candidate 17.6-T MRI system should be sufficiently
low to detect phase signal differences between active and resting membrane states at
resolutions around 1 mm 3 . The authors further delineated the broad dependencies of
signal-to-noise ratio on activity frequency, current application time, and active tissue
fractions and outlined strategies that can be used to lower phase noise below that presently observed in conventional MREIT techniques. The authors also proposed the idea
of using MREIT as an alternative means of studying neuromodulation.
Once high-quality current-distribution MRI of the detailed distribution of electric
source incident to brain neural activities becomes available, comparison of the results
of MRI and fMRI will show the relationship between brain neural activities associated with BOLD effects and neural current distributions, which may lead to various
new observations of dynamics in brain function localizations. Baumann et al. (2010)
mapped the BOLD signal time course in the inferior colliculus (IC), in medial geniculate
body (MGB), and in tonotopically defined fields in the auditory cortex of two macaques.
The results showed little differences in the BOLD signal time courses within the auditory pathway. However, the authors observed systematic differences in the magnitude
of change in the BOLD signal with significantly stronger signal changes in field A1
of the auditory cortex compared with field R. The measured time course of the signal
was in good agreement with similar studies in human auditory cortex, but it showed
considerable differences with data reported from macaque visual cortex. Consistent
with the studies in humans the authors measured a peak in the BOLD response around
4 seconds after the onset of 2-second broadband noise stimuli, whereas previous studies
(Logothetis et al. 2001) recorded from the primary visual cortex of the macaque monkey
had reported the earliest peaks to short visual stimuli several seconds later. The comparison of the results with those of previous studies does not support differences in
haemodynamic responses within the auditory system between human and nonhuman
primates. Furthermore, the authors assumed that the data will aid optimal design of
future auditory fMRI studies in nonhuman primates.
Robertson et al. (2010) investigated the dose-response relationship (sham and 100,
200, and 1000 μT) between an ELF-PEMF (≤300 Hz) and acute thermal pain on the
dominant right hand using fMRI. The authors found significant correlations between
applied field strength and change in BOLD activity in the anterior cingulate and the
ipsilateral insula, indicating that there might be either a dose response or a threshold
effect for the applied PEMF.
It is known that fMRI and MEG are sensitive to the frontal and temporal language
functions, respectively. Kamada et al. (2007) established combined use of fMRI and
MEG to make reliable identification of global language dominance in pathological brain
conditions. The authors investigated 117 patients with brain lesions whose language
dominance was successfully confirmed by the Wada test. All patients were asked to generate verbs related to acoustically presented nouns (verb generation) for fMRI and to
read three-letter words for fMRI and MEG. The fMRI typically showed prominent activations in the inferior and middle frontal gyri, whereas calculated dipoles on MEG typically clustered in the superior temporal region and the fusiform gyrus of the dominant
Electromagnetic Fields in Biological Systems
Sadleir, Grant, and Woo (2010) attempted to theoretically analyze neural function
imaging using magnetic resonance electrical impedance tomography (MREIT). The
authors found that phase noise in a candidate 17.6-T MRI system should be sufficiently
low to detect phase signal differences between active and resting membrane states at
resolutions around 1 mm 3 . The authors further delineated the broad dependencies of
signal-to-noise ratio on activity frequency, current application time, and active tissue
fractions and outlined strategies that can be used to lower phase noise below that presently observed in conventional MREIT techniques. The authors also proposed the idea
of using MREIT as an alternative means of studying neuromodulation.
Once high-quality current-distribution MRI of the detailed distribution of electric
source incident to brain neural activities becomes available, comparison of the results
of MRI and fMRI will show the relationship between brain neural activities associated with BOLD effects and neural current distributions, which may lead to various
new observations of dynamics in brain function localizations. Baumann et al. (2010)
mapped the BOLD signal time course in the inferior colliculus (IC), in medial geniculate
body (MGB), and in tonotopically defined fields in the auditory cortex of two macaques.
The results showed little differences in the BOLD signal time courses within the auditory pathway. However, the authors observed systematic differences in the magnitude
of change in the BOLD signal with significantly stronger signal changes in field A1
of the auditory cortex compared with field R. The measured time course of the signal
was in good agreement with similar studies in human auditory cortex, but it showed
considerable differences with data reported from macaque visual cortex. Consistent
with the studies in humans the authors measured a peak in the BOLD response around
4 seconds after the onset of 2-second broadband noise stimuli, whereas previous studies
(Logothetis et al. 2001) recorded from the primary visual cortex of the macaque monkey
had reported the earliest peaks to short visual stimuli several seconds later. The comparison of the results with those of previous studies does not support differences in
haemodynamic responses within the auditory system between human and nonhuman
primates. Furthermore, the authors assumed that the data will aid optimal design of
future auditory fMRI studies in nonhuman primates.
Robertson et al. (2010) investigated the dose-response relationship (sham and 100,
200, and 1000 μT) between an ELF-PEMF (≤300 Hz) and acute thermal pain on the
dominant right hand using fMRI. The authors found significant correlations between
applied field strength and change in BOLD activity in the anterior cingulate and the
ipsilateral insula, indicating that there might be either a dose response or a threshold
effect for the applied PEMF.
It is known that fMRI and MEG are sensitive to the frontal and temporal language
functions, respectively. Kamada et al. (2007) established combined use of fMRI and
MEG to make reliable identification of global language dominance in pathological brain
conditions. The authors investigated 117 patients with brain lesions whose language
dominance was successfully confirmed by the Wada test. All patients were asked to generate verbs related to acoustically presented nouns (verb generation) for fMRI and to
read three-letter words for fMRI and MEG. The fMRI typically showed prominent activations in the inferior and middle frontal gyri, whereas calculated dipoles on MEG typically clustered in the superior temporal region and the fusiform gyrus of the dominant