Biomedical Engineering Reference
In-Depth Information
ing air to the animal by pre-heating it to 28-30
◦
C, close to the
core temperature maintained at
37
◦
C, to prevent mucous accu-
mulation due to the low ambient temperature (
∼
18
◦
C) within
∼
the imaging bore.
4.3. Future Directions
In recent years, careful refinement of the BOLD effect to reveal
its physiologic basis
(15-23, 29)
for future diagnostic use has
been quite encouraging
(12-14)
. However, it should be acknowl-
edged that these studies have primarily focused on neuronal func-
tions where the role of their glial counterpart has been largely
neglected
(128, 129)
. Therefore, another potential direction that
animal models, such as the ones described here, may help with is
the information about the glial activities that accompany neuronal
and/or BOLD signal changes. But this endeavor would require
development of glial-specific NMR and non-NMR methods to
complement the plethora of neuron-specific measurements that
currently exist and are routinely used in many laboratories.
Localized NMR data collection, for MRI as well as for MRS,
is based on imposing linear field gradients which are rapidly alter-
nated at high rates. The gradients are generated by currents pass-
ing through coils of various shape and orientation around the
sample. The mechanical Lorentz force acting upon the gradi-
ent coils, particularly during EPI scans for fMRI, generates waves
which are audible as loud “clicks” or “blips” in and around the
scanner. Since this is a continuous noise source during our stud-
ies, its effect can be partly negated. We are currently investigating
possibilities of applying noise-cancellation techniques for future
studies. However, studying the auditory system is a consider-
able challenge with fMRI because of susceptibility-induced signal
losses near the ear canals, in particular in the temporal auditory
cortices. Nevertheless, some methods are being developed
(126)
which may alleviate the shimming troubles in such brain locations.
The developments presented here, and more plans underway,
are readily applicable to studying effects of different tactile (i.e.,
forepaw, whisker) and non-tactile (i.e., olfactory, visual) modal-
ities simultaneously. Meredith and Stein
(130, 131)
defined two
classes of multi-sensory interactions within the receptive fields of
cortical neurons: Response enhancement and response depres-
sion. Though it appears to be a general mechanism in the cen-
tral nervous system, one region in which such multi-sensory con-
vergence and synthesis takes place
en masse
is the SC, situated
in the dorsal mid-brain, caudal to the thalamus. In the cat SC,
Meredith and Stein
(130)
showed that a conservative estimate to
the lower limit of the proportion of cells exhibiting multi-sensory
interactions in the intermediate and deep laminae of the SC is
50%, the majority of which project efferent fibers to sensorimotor
areas of the brain stem and spinal cord. These findings are consis-
tent with the hypothesized role of the SC in the control of eye,
