Biomedical Engineering Reference
In-Depth Information
the depth of the tissue. Sub-cortical recordings can in fact be made from awake patients because the brain
has no sensory neurons.
In EEG recordings we are not concerned with uncovering individual neuronal firings but rather the
general activity below the electrode. Therefore, many electrodes are placed on the scalp so comparisons
can be made between the relative activity of different brain regions. Since an average is desired, the
electrodes often cover a large area (e.g., 1
cm
in diameter). The amplitude of the EEG is small because
the skull and scalp have a low conductivity (
σ
e
). To decrease the impedance of ion flow to the electrode,
the scalp may be
abraded
and a gel with free
Cl
−
ions is often applied.
8.3.2 Recording Preparations
In most tissue recordings there are at least three electrodes present. The
recording electrode
will detect the
potentials resulting from the electric field generated by membrane currents. In the true definition of a
potential, however, another electrode at infinity would be necessary. As this is impossible in practice, most
recordings have a second electrode, called the
ground
, that is far from any electrical activity. Common
locations for the ground would be a large slab of metal in the room or some location in the body that is
very poorly electrically connected to the recording electrode.The third electrode is called the
reference
and
is typically located in an electrically connected region of tissue but not close to the recording electrode.To
understand the role of the reference, consider that the recording electrode will detect
all
current sources
scaled by the radii. So, current sources close by will have a large impact but there will be few of them.
On the other hand, the distant current sources will individually have a small contribution but there could
be many of them. The role of the
reference
electrode is to record these distant sources so they may be
subtracted from the recording signal.
8.3.3 Filtering, Amplification and Digitization
The amplitude from a neural recording (EEG or tissue recording) may be as small as 1
μV
and contain
noise from the recording device, room, and motion artifacts.Therefore, neural signals are generally filtered
and amplified. In high-end recording systems, amplification and filtering are performed in a series of steps.
Even after filtering and amplification, the signal is still continuous. To be stored in a computer, the signal
must be
digitized
. Digitizing will transform a continuous signal (with infinitely many values) to a discrete
signal (with some finite number of values). The specifics of how to amplify, filter, and digitize a neural
signal will not be covered here but can be found in texts on biomedical instrumentation.
8.4
EXTRACELLULAR STIMULATION
If a stimulus is applied to one of the extracellular nodes in Fig. 8.2, the potential at that point (
φ
e
)
will become more negative. According to Laplace's Equation [Eq. (8.4)], this negative potential will fall
off with distance from the stimulus point. However, it is possible that even after the attenuation, the
extracellular potential outside of an active section of membrane could be driven more negative. Recall
that
V
m
=
φ
e
and that Sodium channels will open if
V
m
>V
t
m
.So,if
φ
e
outside the membrane
decreases enough, the membrane potential may reach threshold. In this way, an extracellular stimulus can
be used to activate a patch of membrane.
In a real preparation, a stimulating electrode will be surrounded by a complex tangle of axons,
dendrites, somas, and glial cells, so the exact amplitude and duration to induce an action potential will
not be known. Furthermore, a single action potential propagating down a signal axon may not be enough
to have any global impact, as it will be lost in the constant
chatter
of the neurons. Therefore, the goal of
φ
i
−
