Biomedical Engineering Reference
In-Depth Information
4. Discussion
4.1. Summary of fMRI
and Neurophysiologic
Results
We demonstrated the ability to deliver four independent sen-
sory stimuli to anesthetized rats lying inside an MRI scanner and
replicated identical conditions during neurophysiologic measure-
ments outside the scanner. Noticeably, however, evoked BOLD
responses with olfactory, visual, and whisker stimuli exhibited
slight variability during the stimulation period (e.g., the BOLD
response in the S1
BF
declines), whereas forepaw stimuli demon-
strated relatively stable BOLD signal change. This is most likely
a reflection of the nature of forepaw stimuli in which strong and
highly synchronous barrages of identical efferent inputs invade
the contralateral S1
FL
, causing massive activity of cortical neu-
rons, and which, in turn, may partially explain the more stable
BOLD response during forepaw stimulation. It is unlikely that the
forepaw stimulus is out of the physiological range, because the
systemic physiology was unaffected by the stimulus as observed
similarly with the other stimuli. Since we could tightly control
the systemic physiology (i.e., pH
≈
7.35; pCO
2
≈
37 mmHg;
pO
2
>
100 mmHg; blood pressure
≈
95-110 mmHg; core tem-
37
◦
C) for
perature
-chloralose anesthetized rats which were
given visual, whisker, and forepaw stimuli, we did not exclude
a high number of animals from our studies. However, for the
urethane-anesthetized rats receiving olfactory stimuli (i.e., no tra-
cheotomy), temperature was the only parameter that could be
efficiently controlled, and which unfortunately did not prove to
be sufficient as we had to exclude nearly half of the experiments
due to poor maintenance of systemic physiology. These issues are
discussed further below (
see
Section 4.2
).
The variable BOLD responses with different sensory inputs -
which ranged from diffuse odorant maps (in the olfactory bulb)
to highly localized somatosensory and visual foci (in the brain) -
presumably reflected neurophysiologic variations across the sen-
sory modalities as well as their neuroanatomic underpinnings.
The localized neurophysiologic responses, measured here with
the electrical (i.e., LFP or MUA) and/or the coupled CBF signal
and which were in general agreement with the fMRI results, can
be used to provide complementary insights to the multi-modal
basis of the BOLD signal change
(12-14)
. Generally, the MUA
responses from the brain (i.e., DLG, V1, S1
BF
,andS1
FL
)were
quite comparable, with 5-7 Hz incremental change in ensemble
firing rate for the respective stimuli. However, the CBF changes
in the S1
BF
was nearly half that observed in the S1
FL
. But the time
lags to reach the CBF peak were nearly identical with whisker and
forepaw stimuli, whereas the CBF responses reached peak 1-2 s
earlier than the BOLD responses.
≈
α
