Biomedical Engineering Reference
In-Depth Information
for the contribution of the light scattering changes has not been
demonstrated until recently. Here, we will focus on the light
scattering component of intrinsic signals. In particular, we will
show evidence for the involvement of light scattering changes in
intrinsic signals, applications of OISI to functional imaging of the
retina using the light scattering component, and depth-resolved
functional imaging with OCT.
2. OISI for
Functional
Imaging from
Retina (Functional
Retinography)
With the increasing number of people suffering from vision-
threatening retinal diseases, such as age-related macular degen-
eration, there is an urgent need for the development of objective
methods to measure retinal function, by which functional disor-
der can be detected before symptomatic or structural changes
occur. The distribution of retinal responsiveness could not be
adequately mapped by conventional techniques, such as elec-
troretinogram (ERG). Therefore, we have developed a record-
ing system for measuring flash-evoked intrinsic signals from the
macaque retina
(13, 14)
.
We have applied OISI to macaque retina, and successfully
demonstrated the topography of cone- and rod-induced neu-
ral function by measuring the light reflectance changes follow-
ing flash stimulus. Because of the differences in anatomical struc-
tures, the properties of intrinsic signals in retina are quite differ-
ent from those in the cerebral cortex. Here, we will focus on the
light reflectance changes in two discrete regions in the posterior
retina: fovea and posterior retina apart from the fovea (perifoveal
regions).
The ocular fundus of Rhesus monkey under anesthesia was
monitored via a modified fundus camera equipped with a CCD
camera (
Fig. 6.2
). The intrinsic signals evoked by white dif-
fuse flash stimuli were calculated by dividing the averaged images
obtained after the flash by those obtained before the flash
(13,14)
(see details in Appendix 1). With an observation light wave-
length of 630 nm, the light reflectance from the fovea increased
(retinal image became brighter), whereas, the light reflectance
from the perifoveal regions decreased (retinal image became
darker) following a flash stimulus (
Fig. 6.3A
). The increase
in light reflectance following a flash in the foveal region was
attributed to the bleaching of photopigments by visible light
(15-18)
. On the other hand, the decrease in light reflectance in
the perifoveal region was attributed to the hemoglobin-related
reflectance changes or tissue light scattering changes, which can
be observed in the cerebral cortex
(13)
. The pseudocolor map
(
Fig. 6.3A
) shows that the signals in the foveal and perifoveal
