Biomedical Engineering Reference
In-Depth Information
is applied to separate the MUA (10
2
-10
4
Hz) from the LFP
10
2
Hz) signals. SUA and MUA signals are typically represen-
tative of signals from neuron(s) in the microelectrode's vicinity
(spanning 10-100
(
<
m). LFP signals can integrate potentials over
much larger distances (
μ
m to mm) to represent the aggregate
cellular activity in that region.
Both electroencephalography (EEG;
(67)
) and magnetoen-
cephalography (MEG;
(68)
) allow mapping with much better
brain coverage because the measurement devices nearly surround
the entire head. EEG measures summed activity of post-synaptic
currents, whereas MEG measures tiny magnetic fields (in fT
range) produced by synchronized ionic currents flowing in den-
drites. Since EEG and MEG signals originate from slightly dif-
ferent cortical locations and are acquired with different temporal
resolutions (ms vs.
μ
s), the types of information obtained vary.
Because both EEG and MEG suffer from the inverse problem
(i.e., difficulty localizing origin of signal), the spatial resolution
suffers (mm to cm). EEG signals are susceptible to body move-
ments and MEG signals from the brain sometimes compete with
higher magnitude environmental noise. However, non-invasive
use of EEG and MEG in humans is invaluable for basic science
and clinical research.
μ
4.2. Optical Imaging
Optical imaging techniques are based on the discovery
(69)
that
optical properties of cells, and therefore tissue, change during
activity. Sensitivity of all optical techniques is limited by the mag-
nitude of the changed optical properties of the system. At present,
four dynamic optical techniques dominate: optical intrinsic signal
(OIS), voltage sensitive dye (VSD), near infra red spectroscopy
(NIRS), and laser-Doppler flowmetry (LDF). These methods can
be used to measure a variety of dynamic events in cells (e.g., ion
flux) and vessels (e.g., blood oxygenation). OIS and VSD are pri-
marily applied in animals because of the need for a window (e.g.,
4
4mm
2
) on the skull and typically have high spatial resolution
(10-50
×
m). The data are limited to superficial regions of the
brain because of limited light penetration in tissue (mm range).
NIRS is a non-invasive technique and suited for use in humans.
The spatial resolution of NIRS and LDF is limited by the dis-
tance between the emitting and detecting probes. Most of these
techniques allow two dimensional images of brain activity with
relatively high temporal resolution (10-100 ms).
OIS is perhaps the most widely used optical technique in
neuroscience
(70)
, in part, because it does not require exoge-
nous optical probes. However, the origin of the task-dependent
changes in the intrinsic signal is complex. Changes in blood
oxygenation (increase or decrease in deoxyhemoglobin), blood
volume (vessel dilation, capillary recruitment, etc.), and light-
scattering that accompany cell swelling with activation, all can
μ
