An Introduction to Magnetic Source Imaging
The advent of modern functional imaging techniques
has raised expectations that scientists will be able
to describe the brain mechanisms responsible for sensory, motor,
and cognitive functions.
What are functional brain imaging techniques?
Each functional imaging technique measures one aspect of brain
activity in the form of electromagnetic signals. Three-dimensional
(spatial) maps of this activity are then reconstructed using special
algorithms. These maps are subsequently used to derive information
regarding the degree to which neurophysiological activity (i.e.,
neuronal signaling) increases above certain “baseline”
levels as participants engage in experimental tasks, each is designed to evoke a particular cognitive function. Such changes
in regional activity are viewed as evidence that the “activated”
brain region participates in the performance of the cognitive
function(s) that the experimental task requires.
How is MSI unique? MSI is unique
among other functional imaging techniques for its ability to provide
spatiotemporal brain activation profiles that reflect not only
where activity occurs in the brain but also when this activity
occurs in relation to the presentation of an external stimulus.
In this way, we elicit information about the participation of
brain areas for a particular sensory, motor, cognitive,
or linguistic function.
What type of machine is used in MSI?
The neuromagnetometer (BTi, Magnes 2500) is housed in a magnetically
shielded chamber designed for reducing environmental
noise that interferes with the recordings of physiological signals.
A typical recording session has
a duration of approximately 40 minutes. During this period of
time, repeated measurements for the purpose of establishing the
reliability of the results are feasible.
How does MSI work? The principles
underlying the ability of MSI to identify brain areas that show
event-related increases in local neuronal activity may be described
briefly as follows: External stimuli are known to evoke neurophysiological
activity as soon as they impinge upon sensory receptors. One basic
aspect of such activity is the intra-and extracellular flow of
ions associated with electrical currents and magnetic
flux, that can be recorded from the head surface in the form
of evoked potentials (EPs) and evoked fields (EFs). Application
of a train of similar stimuli results in the repeated evocation
of such activity which is recorded and averaged to improve signal
quality. The resulting average EFs consist of early
(30-150 ms. poststimulus) and late (150-1000 ms poststimulus)
components. To identify the intracranial origin of either type
of component, the magnetic field distribution that had been recorded
simultaneously over the entire head surface at successive points
(4 ms apart) is analyzed. The analysis consists of the application
of a mathematical model which considers the intracranial activity
sources (sets of active cells) as equivalent to physical current
dipoles and provides estimates of the location and strength of
these sources, the activity of which produces the recorded magnetic
fields at that point in time. The location estimates of each “dipolar”
source are specified with reference to a Cartesian coordinate
system, anchored on three fiducial points on the head (the nasion
and the external meatus of each ear). The fiducial points
enabling the superimposition
of the precise location of each dipolar source on the subject’s
(or patient’s) MRI.
Thus, the dipolar sources that account for a particular
EF component projected onto the MRI identify the brain areas activated
during that time interval in response to the stimulus. The degree
of activation of a particular area (or the total duration of its
activation) following a stimulus is estimated by the total number
of successive dipoles that account for the EF components. The
validity of this estimate is not based on any theoretical considerations,
but is empirically derived. Namely, among all other possible
indices of the degree of activation of an area (e.g. mean or median
amperage of all dipolar sources), the number of sources was the
one which resulted in the most consistent mapping results (Breier
et al., 1999; Papanicolaou et al., 1999; Simos et al., 1998).
