Biomedical Engineering Reference
In-Depth Information
functional signals). Here, red and blue patches indicate the acti-
vation for horizontal and vertical grating stimuli, respectively. In
order to clarify the reliability of maps, a pixel-by-pixel comparative
t-test was done for the horizontal against the vertical grating stim-
uli. The tests revealed that the results obtained were statistically
significant to within a 5% tolerance limit. Further, a blank minus
blank map did reveal a flat distribution without any patches.
From the fOCT map, we can make the following inferences:
1. There is a discrete distribution of activation patches across
depth, which is stimulus specific.
2. In a very superficial region (100-200
m under cortical sur-
face), there are no activation patches indicating layer 1 where
neurons are scarce.
3. In the region deeper than 100-200
μ
m, there exist several
localized patches across depth showing no regular structure.
4. The localized patches extend up to the measured depth of
approximately 1 mm. The presence of orientation columns in
primary visual cortex is well known
(39)
. This indicates that,
across depth, there might exist a continuous cylindrical organi-
zation more complex than the commonly understood colum-
nar organization.
As the maps revealed by fOCT suggest a surprisingly discrete dis-
tributed columnar organization, it becomes necessary to validate
the technique. In the present measurements, there are two poten-
tial issues that need to be considered:
1. Although we could observe a functional signal that arises as a
result of scattering change due to neural activation, it is still
not possible to specify the origin regarding the exact nature of
the scattering changes such as glial swelling or capillary dila-
tion. One way to resolve this issue may be to increase the
spatial resolution of the technique, a possibility that is tech-
nically feasible. But even with increased resolution, allowing
us to visualize the details of the structural organization, it is
still not clear whether the reflectivity changes could provide
enough contrast to resolve finer details such as neuronal cells,
glial cells and blood vessels etc. To resolve such structural dif-
ferences, one way is by specifically attaching contrast agents
such as gold particles to specific structures such as neurons. We
previously proposed a method of increasing the reflectivity in
OCT
(40)
by introducing properly sized gold particles. With
such specific labeling, we may be able to increase the reflectiv-
ity from specific structures, which would, in turn, enable us to
specify the origin of scattering change.
2. Another issue is that the site of localized scattering change
may not correspond to the site of neural activity. To address
this problem, we conducted electrophysiology recordings to
measure the neuronal activity at the sites revealed by fOCT
discussed in the following section.
μ
