Biomedical Engineering Reference
In-Depth Information
the contralateral S1
BF
for consistent and reproducible studies,
both in the same subject and across a group of subjects. While this
device provides easy control of whisking frequency, we are investi-
gating more refined amplitude variations by controlling flow rates
of compressed air.
While ambulatory functions fulfilled by the rodent's forelimbs
and hind limbs may seem mundane compared to the perceptions
of smell and whisking, somatosensory sensing by the limbs rep-
resents an important component of the multi-faceted environ-
mental cues that a rodent must experience for survival. Upon
stimulation of either limb, afferent signals arrive through the
medial-lemniscal system at the contralateral ventral posterior lat-
eral nucleus of the thalamus, prior to connecting with the limb-
specific contralateral primary somatosensory cortex. Electrical
stimulation of nerve fibers innervating the touch-sensitive limbs
is the most widely used method of sensory stimulation of the
rat, both in bench neurophysiologic
(91-94)
and fMRI
(95-101)
studies.
Forepaw stimulation in the rat is a very popular model in
many fMRI laboratories
(95-98)
and can be easily applied iden-
tically inside and outside the magnet
(98-101)
. The pulse ampli-
tude is typically in the range of a few mA and the pulse dura-
tion is usually a fraction of a ms. The electrodes are typically
small enough to be placed subcutaneously in the forepaw's skin.
It is easily applied and produces very well localized and strong
electrical and hemodynamic cortical responses. During electrical
forepaw stimulation, robust activations in the contralateral S1
FL
were reproducibly observed in different sessions as well as across
many subjects under
4.1.4. Tactile: Forepaw
Stimulation
α
-chloralose anesthesia (
Figs. 10.7 and
10.8)
. The peak S1
FL
response was near 1-3 Hz forepaw stim-
ulation frequency
(63-65)
which is quite different from the S1
BF
response during whisker stimulation. This model has been widely
used for biophysical/physiologic characterization of the BOLD
image contrast
(102-108)
, perhaps in part, because the averaged
BOLD response localized in the contralateral S1
FL
is much larger
than observed with other sensory modalities.
4.2. Challenges of In
Vivo NMR and Bench
Experiments
We have described some experiences with anesthetized rat prepa-
rations in our laboratory. But the awake experiment still has desir-
able features. However, the few fMRI studies that have reported
on awake but restrained rats
(109-111)
do not demonstrate the
same level of reproducibility in BOLD activations as anesthetized
rats, perhaps in part, because the spontaneous BOLD oscilla-
tions can be as large as the induced BOLD responses
(111)
and
furthermore, even the BOLD responses can be quite variable
(110)
. We, and many others, hypothesize that “the anesthetic
state may be thought of as a tool for limiting the repertoire
