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Electromagnetic Fields in Biological Systems
3.5.2 Magnetoencephalography
Magnetoencephalography is a real noninvasive method for measuring magnetic fields
as weak as 5 fT (1 fT = 10 −15 T) generated by neuronal current flow without any external
field. The magnetic signals can be detected by SQUIDs arrayed on the scalp. The measurement of MEG can be represented as brain activities with high millisecond-order
temporal resolution and high millimeter-order spatial resolution; thus, MEG is useful
for investigation of brain function in humans, including higher brain functions such as
memory and cognition. Cohen (1968) obtained an MEG for human α-waves with the
use of a SQUID and a prototype was developed by Cohen, Edelsack, and Zimmerman
(1970); as a result of these developments, the use of a whole-head MEG system for spontaneous measurement at multiple points has become practical (Squires 1991; Ahonen
et al. 1993; Vrva, Bette, and Burband 1993). In recent years, the whole-head MEG system
has been incorporated into brain functional research all over the world and this has led
to accelerated progress in research.
Application of forward and inverse problems in MEG analysis is critical to estimate
the localization of brain function and disorders. Ueno and Iramina (1991) measured the
MEG associated with short memory, cognition, and mental rotation in humans; constructed current-dipole and distributed intracerebral electrical source models; and carried out estimations for the localization of various brain functions during the processing
of information. The electrical source of a visually evoked reaction with 150 milliseconds
at latency localized in the primary visual cortex was described in a current-dipole model
relatively well, whereas a distributed intracerebral electrical source model was more useful in estimating the electrical source incident to a mental rotation with 180 milliseconds
or higher at latency. In the distributed electrical source model, a chronological transition of electrical source groups from the occipital lobe area to the posterior temporal
lobe area was captured. The MEG is a tool not only for basic brain functional research
but also for medical research. Clinical applications of MEG include detection of epileptic spikes, measurement of slow waves associated with brain tumors and cerebrovascular diseases, and detection of cerebroelectric activity of ELFs induced by event-related
potentials.
Studies on MEG have revealed enhancement of neural activity of the N1m response of
auditory evoked fields in well-trained musicians, reflecting neuroplastic modification of
the representation of the auditory cortex (Pantev et al. 1998). In contrast, the amplitude
of the P2 response of auditory evoked potentials is modified by musical experience, with
no alteration of N1 (Shahin et al. 2003). Kuriki, Kanda, and Hirata (2006) performed a
comprehensive MEG study using stimulation of successive musical instrument tones to
examine how the neural activities of different MEG responses were modified in experienced musicians who had commenced musical lessons at the age of 5 years and had
continued to practice. The dipole moment of the P2m response occurring at 160–180
milliseconds was significantly enlarged in musicians compared with that in individuals
who had not received any musical lessons. The enlargement was found for the dipole
moment of N1m occurring at 100–120 milliseconds in a restricted condition but not for
the moment of P1m at 50–60 milliseconds. Furthermore, the dipole moment of P2m for
successive stimuli, normalized by the moment for the first stimulus, was significantly
Electromagnetic Fields in Biological Systems
3.5.2 Magnetoencephalography
Magnetoencephalography is a real noninvasive method for measuring magnetic fields
as weak as 5 fT (1 fT = 10 −15 T) generated by neuronal current flow without any external
field. The magnetic signals can be detected by SQUIDs arrayed on the scalp. The measurement of MEG can be represented as brain activities with high millisecond-order
temporal resolution and high millimeter-order spatial resolution; thus, MEG is useful
for investigation of brain function in humans, including higher brain functions such as
memory and cognition. Cohen (1968) obtained an MEG for human α-waves with the
use of a SQUID and a prototype was developed by Cohen, Edelsack, and Zimmerman
(1970); as a result of these developments, the use of a whole-head MEG system for spontaneous measurement at multiple points has become practical (Squires 1991; Ahonen
et al. 1993; Vrva, Bette, and Burband 1993). In recent years, the whole-head MEG system
has been incorporated into brain functional research all over the world and this has led
to accelerated progress in research.
Application of forward and inverse problems in MEG analysis is critical to estimate
the localization of brain function and disorders. Ueno and Iramina (1991) measured the
MEG associated with short memory, cognition, and mental rotation in humans; constructed current-dipole and distributed intracerebral electrical source models; and carried out estimations for the localization of various brain functions during the processing
of information. The electrical source of a visually evoked reaction with 150 milliseconds
at latency localized in the primary visual cortex was described in a current-dipole model
relatively well, whereas a distributed intracerebral electrical source model was more useful in estimating the electrical source incident to a mental rotation with 180 milliseconds
or higher at latency. In the distributed electrical source model, a chronological transition of electrical source groups from the occipital lobe area to the posterior temporal
lobe area was captured. The MEG is a tool not only for basic brain functional research
but also for medical research. Clinical applications of MEG include detection of epileptic spikes, measurement of slow waves associated with brain tumors and cerebrovascular diseases, and detection of cerebroelectric activity of ELFs induced by event-related
potentials.
Studies on MEG have revealed enhancement of neural activity of the N1m response of
auditory evoked fields in well-trained musicians, reflecting neuroplastic modification of
the representation of the auditory cortex (Pantev et al. 1998). In contrast, the amplitude
of the P2 response of auditory evoked potentials is modified by musical experience, with
no alteration of N1 (Shahin et al. 2003). Kuriki, Kanda, and Hirata (2006) performed a
comprehensive MEG study using stimulation of successive musical instrument tones to
examine how the neural activities of different MEG responses were modified in experienced musicians who had commenced musical lessons at the age of 5 years and had
continued to practice. The dipole moment of the P2m response occurring at 160–180
milliseconds was significantly enlarged in musicians compared with that in individuals
who had not received any musical lessons. The enlargement was found for the dipole
moment of N1m occurring at 100–120 milliseconds in a restricted condition but not for
the moment of P1m at 50–60 milliseconds. Furthermore, the dipole moment of P2m for
successive stimuli, normalized by the moment for the first stimulus, was significantly