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Electromagnetic Fields in Biological Systems
The MRI of electrical phenomena in living bodies is potentially useful for quantitative evaluations of the biological effects of EMFs and for direct detection of neuronal
electrical activities in the brain. Magnetic fields in an object cause a shift in the resonant
frequency (Manassen, Shalev, and Navon 1988; Sekino, Matsumoto, et al. 2004) and a
change in the phase of MR signals (Joy, Scott, and Henkelman 1989). Spatial distributions of an externally applied magnetic field and electrical current can be estimated
from these changes in MR signals. These methods have use in certain medical applications, such as the imaging of current distributions in electrical defibrillation (Yoon
et al. 2003).
The fMRI developed by Ogawa, Tank, and Menon (1992) utilizes a technology that
reflects various magnetic features of hemoglobin in blood on MR signal patterns.
Tomograms of brain function can be obtained from information on localized blood
flow in the brain. The fMRI utilizes a BOLD effect of localized blood flow on brain activation for indirect imaging of brain activities. However, no information on electrical
conditions in vivo can be obtained with the current MRI and fMRI systems.
Tomatsu et al. (2008) showed the possible brain sites that have activity equivalent to
the motor commands with fMRI. The authors hypothesized that short-term temporal
patterns of movements or stimuli are reflected in BOLD responses and they searched
for regions representing the response. Participants performed two temporal patterns
of tapping and/or listened to the same patterns of auditory stimuli in a 3-T fMRI. All
patterns were designed to have the same number (11) of events and the same duration
but different temporal distributions of events. The 11 events were divided into two parts
(10 repetitive taps and one stand-alone tap) and the interval between the two parts was
three seconds. The two patterns had reverse order of the two parts. The results revealed
that different temporal patterns of auditory stimuli were represented in different temporal features of BOLD responses in the bilateral auditory cortex, whereas different temporal patterns of tapping were reflected in contralateral primary motor cortex and the
ipsilateral anterior cerebellum. In bilateral premotor cortex, SMA, visual cortex, and
posterior cerebellum, task-related BOLD responses were exhibited, but their responses
did not reflect the temporal patterns of the movement and/or stimuli. One possible
explanation is that the neuronal activities were similar for the two patterns in these
regions. The sensitivity of BOLD response to temporal patterns reflects local differences
in functional contributions to the tasks. The authors proposed that this experimental
design and analysis may be useful in revealing particular brain regions that participate
in multiple functions.
Using fMRI, Postle and Hamidi (2007) demonstrated that short-term retention of the
identity or location of visually presented stimuli is disrupted by nonvisual secondary
tasks performed in passive listening in the dark to nouns or endogenous generation of
saccades, respectively. This indicates that the short-term retention of visual information
relies on multiple mental codes, some of them nonvisual. Event-related fMRI reveals
the neural correlates of these interference effects to be more complex and more regionally specific than previously described. Although nonspecific dual-task effects produce
a generalized decrease in task-evoked fMRI response across many brain regions, the
interference-specific effect is a relative increase of activity localized to regions associated with the secondary task in question: left hemisphere perisylvian cortex in the case
Electromagnetic Fields in Biological Systems
The MRI of electrical phenomena in living bodies is potentially useful for quantitative evaluations of the biological effects of EMFs and for direct detection of neuronal
electrical activities in the brain. Magnetic fields in an object cause a shift in the resonant
frequency (Manassen, Shalev, and Navon 1988; Sekino, Matsumoto, et al. 2004) and a
change in the phase of MR signals (Joy, Scott, and Henkelman 1989). Spatial distributions of an externally applied magnetic field and electrical current can be estimated
from these changes in MR signals. These methods have use in certain medical applications, such as the imaging of current distributions in electrical defibrillation (Yoon
et al. 2003).
The fMRI developed by Ogawa, Tank, and Menon (1992) utilizes a technology that
reflects various magnetic features of hemoglobin in blood on MR signal patterns.
Tomograms of brain function can be obtained from information on localized blood
flow in the brain. The fMRI utilizes a BOLD effect of localized blood flow on brain activation for indirect imaging of brain activities. However, no information on electrical
conditions in vivo can be obtained with the current MRI and fMRI systems.
Tomatsu et al. (2008) showed the possible brain sites that have activity equivalent to
the motor commands with fMRI. The authors hypothesized that short-term temporal
patterns of movements or stimuli are reflected in BOLD responses and they searched
for regions representing the response. Participants performed two temporal patterns
of tapping and/or listened to the same patterns of auditory stimuli in a 3-T fMRI. All
patterns were designed to have the same number (11) of events and the same duration
but different temporal distributions of events. The 11 events were divided into two parts
(10 repetitive taps and one stand-alone tap) and the interval between the two parts was
three seconds. The two patterns had reverse order of the two parts. The results revealed
that different temporal patterns of auditory stimuli were represented in different temporal features of BOLD responses in the bilateral auditory cortex, whereas different temporal patterns of tapping were reflected in contralateral primary motor cortex and the
ipsilateral anterior cerebellum. In bilateral premotor cortex, SMA, visual cortex, and
posterior cerebellum, task-related BOLD responses were exhibited, but their responses
did not reflect the temporal patterns of the movement and/or stimuli. One possible
explanation is that the neuronal activities were similar for the two patterns in these
regions. The sensitivity of BOLD response to temporal patterns reflects local differences
in functional contributions to the tasks. The authors proposed that this experimental
design and analysis may be useful in revealing particular brain regions that participate
in multiple functions.
Using fMRI, Postle and Hamidi (2007) demonstrated that short-term retention of the
identity or location of visually presented stimuli is disrupted by nonvisual secondary
tasks performed in passive listening in the dark to nouns or endogenous generation of
saccades, respectively. This indicates that the short-term retention of visual information
relies on multiple mental codes, some of them nonvisual. Event-related fMRI reveals
the neural correlates of these interference effects to be more complex and more regionally specific than previously described. Although nonspecific dual-task effects produce
a generalized decrease in task-evoked fMRI response across many brain regions, the
interference-specific effect is a relative increase of activity localized to regions associated with the secondary task in question: left hemisphere perisylvian cortex in the case