Biomedical Engineering Reference
In-Depth Information
low-pass filtered the data at 2 Hz, the effective sample period is
500 msec and thus the number of photons/sample period is 10
7
.
The shot noise in this measurement should then be approximately
3
10
−
4
of the resting intensity. Consistent with this prediction,
the noise in the measurements shown in
Fig. 3.9C
is less than
2
×
10
−
3
of the resting intensity. In preliminary measurements,
we found that the shot noise and noise from respiration and heart
beat were similar in magnitude.
The spatial resolution shown in
Fig. 3.9
, D-E is on the order
of 20
×
m, far better than might have been anticipated from the
measurements illustrated in
Fig. 3.12
. However, both factors
that could contribute to blurring are minimized in the measure-
ments shown in
Fig. 3.9
. First, scattering will be lower because
the Calcium Green-1 dextran is only in the outer two layers of the
olfactory bulb. Second, out-of-focus signals will be small because
the glomerular layer is only 100
μ
mthick.
Because many (up to 50) maps of this sort can be
obtained from each preparation, considerable information has
been obtained about the input from the nose to the bulb. How-
ever, it remains a challenge to determine the map of the out-
put of the bulb carried by the mitral/tufted cells. If both the
input and the output were known, we would have a strong clue
about the function of this first way station in processing olfactory
information.
μ
3. Methodological
Considerations
The three examples given above involved fractional intensity
changes and signal-to-noise ratios that are not large. To measure
these signals, the noise in the measurements had to be a substan-
tially smaller fraction of the resting intensity. In the sections that
follow, some of the considerations necessary to achieve such a low
noise are outlined.
3.1. Signal Type
Sometimes it is possible to decide in advance which kind of opti-
cal signal will give the best signal-to-noise ratio, but in other
situations, an experimental comparison is necessary. The choice
of signal type often depends on the optical characteristics of the
preparation. Extrinsic birefringence signals are relatively large in
preparations that, like axons, have a cylindrical shape and radial
optic axis
(59)
. However, in preparations with spherical symme-
try (e.g., cell soma), the birefringence signals in adjacent quad-
rants will cancel
(60)
. Thick preparations (e.g. mammalian cortex)
also dictate the choice of signal. In this circumstance, transmitted
light measurements are not easy (a subcortical implantation of a
light guide would be necessary), and the small size of the absorp-
tion signals that are detected in reflected light
(59, 61)
meant that
fluorescence or reflectance would be optimal
(62)
. Fluorescence
