2. WHAT IS MAGNETOENCEPHALOGRAPHY (MEG)?
• Magnetoencephalography (MEG) is a non-invasive neurophysiological technique that
measures the magnetic fields generated by neuronal activity of the brain
• The spatial distributions of the magnetic fields are analyzed to localize the sources of the
activity within the brain, and the locations of the sources are superimposed on anatomical
images, such as MRI, to provide information about both the structure and function of the
brain
3. IS THE SIGNAL ARTIFICIAL OR NATURAL ?
• Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping
brain activity by recording magnetic fields produced by electrical currents occurring
naturally in the brain, using very sensitive magnetometers.
• Since magnetic fields are recorded from the naturally occurring electrical currents in the
brain. It is an Natural Signal
4.
5. DETERMINISTIC OR NON-DETERMINISTIC SIGNAL?
• Determinism is a special property of some systems and is defined by its
state-space behavior in which the trajectories in time never intersect.
• Whether or not determinism exists in brain activities is a question that may
be resolved by analysis of the dynamical properties of the
magnetoencephalogram (MEG)
• Due to the strong nonlinear character of the brain dynamics standard linear
methods are frequently not applicable for the characterization of normal or
pathologic brain states.
• This is because spectral properties of brain oscillations are very individual
and may vary in time and space.
• Though brain signals (MEG) looks very irregular, it is possible that these are
generated by deterministic and mathematically simple nonlinear systems
6. HOW DOES MEG WORK?
• At the cellular level, individual neurons in the brain have electrochemical
properties that result in the flow of electrically charged ions through a cell.
• Electromagnetic fields are generated by the net effect of this slow ionic
current flow.
• While the magnitude of fields associated with an individual neuron is
negligible, the effect of multiple neurons (for example, 50,000 – 100,000)
excited together in a specific area generates a measureable magnetic field
outside the head.
• These neuron magnetic signals generated by the brain are extremely
small—a billionth of the strength of the earth’s magnetic field. Therefore,
MEG scanners require superconducting sensors SQUID
7. PRINCIPLE FEATURES OF MEG
• MEG is a direct measure of brain function, unlike functional measures such
as fMRI, PET and SPECT that are secondary measures of brain function
reflecting brain metabolism.
• MEG has a very high temporal resolution & excellent spatial resolution
• MEG is completely non-invasive. It does not require the injection of isotopes
or exposure to X-rays or magnetic fields
• MEG is complementary to other modalities, the information provided by each
modality adds to the full picture.
8. WHAT KIND OF INFORMATION DOES SIGNAL
CONTAINS?
• Magnetoencephalography (MEG) data are available to the neuroimaging community.
The Open MEG Archive (OMEGA a centralized repository in which to regroup MEG
data in raw and processed form, for open dissemination.
• This free-access continuously expanding repository also contains anatomical MRI
volumes, demographic and questionnaire information, and eventually will feature
other forms of electrophysiological data (e.g., EEG, field and cell recordings).
OMEGA features the technological framework for multi-centric data aggregation, and
is amongst the largest freely available resting-state MEG datasets presently
available.
9. WHAT IS THE PARAMETER WHICH CONTAINS THIS
INFORMATION?
• MEG is based on SQUID technology. The superconducting quantum interference device
(SQUID), is a sensitive detector of magnetic flux. Today's whole-head MEG systems
contain a large number of SQUIDs (between 100 to 300) connected to sensor coils in a
configuration roughly following the curvature of the head
• Since the environmental magnetic noise level due to traffic, elevators etc is several
orders of magnitude higher than the neuromagnetic signals, the MEG system needs to be
placed in a magnetic shielded room.
• The magnetic field measured by MEG is produced directly by electrical neuronal activity, it
is possible to detect signals from the brain on a sub-millisecond time scale.
10. CAN FOURIER TRANSFORMATION BE USED TO
EXTRACT THE SIGNAL INFORMATION
• We explored the ability of several spectral based measures to summarize the information
of the power spectral density (PSD) function from spontaneous magnetoencephalographic
(MEG)
• The method is based on a linear decomposition of recordings: it maximizes the signal
power at a peak frequency while simultaneously minimizing it at the neighboring,
surrounding frequency bins. Such procedure leads to the optimization of signal-to-noise
ratio and allows extraction of components with a characteristic “peaky” spectral profile,
which is typical for oscillatory processes. We refer to this method as spatio-spectral
decomposition (SSD).
• Hence the signal information can be extracted using Fourier series
11. ADVANTAGES
• MEG provides timing as well as spatial information about brain
activity.
• MEG signals are obtained directly from neuronal electrical
activity.
• MEG signals are able to show absolute neuronal activity
• MEG does not make any operational noise
• Adaptable to mapping many functions- sensory, motor,
language, memory cortex
• High precision- millimeter resolutions
