Biomedical Engineering Reference
In-Depth Information
inner and middle layers are nourished by retinal vessels and cap-
illaries. We believe that the light scattering changes following
activation of the cone photoreceptors are probably the source
of the fast intrinsic signals observed at the fovea because the
foveal avascular region is not subject to changes in hemoglobin
concentration or blood volume following neural activation
(14)
.
The rapid darkening observed at the perifoveal regions may also
be derived from light scattering changes because, under infrared
light, the change in the optical signal due to deoxygenated
hemoglobin concentration is thought to be much smaller than
that from tissue light scattering
(27)
. The light scattering changes
following a flash are thought to be derived from microstruc-
tural changes in the outer segment disks, membrane hyperpo-
larization, cell swelling, and changes in the composition of the
inter-photoreceptor matrix. Recent functional OCT studies using
blood-free slice preparations
(28,29)
or in vivo retina
(30)
showed
that the reflectance in the photoreceptor layer is strongly modu-
lated by neural activation followed by microscopic morphological
changes.
As for the sources of the slow signals in the perifoveal regions,
our recent studies have suggested that the light scattering changes
due to blood flow changes in capillaries are the main contribu-
tors to changes in reflectance
(42)
. Direct measurement of blood
flow with laser Doppler flowmetry has shown that a flash stimulus
evokes a slow increase in blood flow only in the perifoveal regions
and its time course exactly matched that of the slow component
of the intrinsic signals we described above. A fast increase in blood
flow, however, has not been observed either in foveal or perifoveal
regions.
We have shown that the light scattering change, which is inde-
pendent of blood oxygenation level, also correlates well with neu-
ronal activity and can be used for mapping neural function. We
believe that this fast scattering signal is of great value for mapping
neuronal activity because it may have better spatial and tempo-
ral resolutions than the blood-flow- or blood oxygenation related
signals.
3. Optical
Coherence
Tomography (OCT)
for Functional
Imaging Resolved
in Depth
(Functional OCT)
In conventional OISI with CCD cameras, the measured reflected
light is actually the integrated signals over depths determined
by the collection optics. Hence potential variations in functional
organization across depth may go undetected. Optical coher-
ence tomography (OCT) is an optical imaging technique that has
the potential to show reflectivity at specific depths because the
method is a sensitive measure of refractive index variations across
