Biomedical Engineering Reference
In-Depth Information
persistent stimulation. This time course of changes in oxygen
tension was quite consistent with the time course of intrinsic
signals (stimulus-nonspecific component) observed at 620 nm
(
Fig. 6.1B
). This result provides good evidence that the deoxy-
genation level of hemoglobin is one of the sources of intrinsic
signals.
A second source of intrinsic optical signals stems from
changes in blood volume within neural tissues. One of the isos-
bestic points of oxy- and deoxy-hemoglobin absorption is located
at 570 nm within the major absorption spectrum band. Thus,
the intrinsic signals at this wavelength should be dominated by
changes in blood volume in tissue blood vessels. Several stud-
ies provide supporting evidence for the involvement of blood
volume changes in intrinsic signals
(11, 12)
. For example, infu-
sion of extrinsic absorption dye into the blood stream increases
absorption changes elicited by neural activation at the wave-
length specific for that dye, and the time course of the signal was
nearly identical to the time course of intrinsic signals recorded
at 570 nm, where blood volume changes seem to dominate
(
Fig. 6.1C
)
(12)
.
Microstructures of neural tissue, such as intricate subcompo-
nents of neurons, multiple types of glial cells, and collection of
blood vessels of various sizes generally cause scattering of light
that penetrates the neural tissues. If neural activities are associ-
ated with changes in these microstructures, then light scattering
changes from intrinsic signals could be a component for neu-
roimaging.
Changes in tissue light scattering is indeed considered a
source of intrinsic signals since intrinsic signals are also observed
at the wavelengths outside of the major band of the hemoglobin
absorption spectrum. The light scattering component of the
intrinsic signal has unique properties that do not exist in the
absorption of oxy- and deoxy-hemoglobin. First, we can detect
light scattering changes at a wide range of wavelengths, includ-
ing infrared light, where hemoglobin absorption is minimal.
Using light scattering in the infrared offers several advantages.
Infrared light permits visualization of functional signals from tis-
sues sensitive to visible light, such as the retina, as well as detec-
tion of signals from deeper structures because of the increased
light penetration in the infrared light range compared to the
visible range. Second, we can detect light scattering changes
not only through changes in light reflection but also through
phase-sensitive detection such as optical coherence tomography
(OCT), which enables us to resolve functional structures in
greater depth. Finally, light scattering changes may have faster
time courses than the other intrinsic signals, allowing us to
resolve neural events with higher temporal resolution than sig-
nals originating from hemodynamics. Nevertheless, firm evidence
