Biomedical Engineering Reference
In-Depth Information
26, 28, 29)
). Results from these studies form the basis for specu-
lation regarding the functional role of RSC. Bressler et al.
(30)
have suggested that such correlated signal fluctuations may be a
phenomenon representing the functional connection of cortical
areas analogous to the phenomenon of “effective connectiv-
ity” defined by Friston et al.
(31)
. Thus, a family of cognitive-
origin hypotheses (in contrast to biophysical-origin hypotheses)
has emerged. Gusnard and Raichle
(29)
, for instance, have sug-
gested that such coherence indicates the presence of a “default
mode of brain function” in which a default network continuously
monitors external (e.g., visual stimuli) and internal (e.g., body-
functions, emotions) stimuli. Other cognitive-origin hypothe-
ses suggest that low-frequency fluctuation is related to ongoing
problem-solving and planning
(28)
. Biswal, Hyde and colleagues
(26)
and Xiong, Fox and colleagues
(27)
observed that analyses
of resting-state physiological fluctuations reveal many more func-
tional connections than those revealed by task-induced activation
analysis. They hypothesized that task-induced activation maps
underestimate the size and number of functionally connected
areas and that functional networks are more fully revealed by RSC
analysis.
Here, we discuss various animal studies that affect the
CBF oscillations and hence the low frequency fluctuations in
BOLD signal. The spatiotemporal characteristics of the low
frequency fluctuations in brain oxygenation was mapped non-
invasively using BOLD weighted fMRI imaging using phys-
iological perturbations known to alter CBF oscillations. The
resting state connectivity maps obtained from normocapnia and
changes in mean arterial pressure (MAP) and differences between
them are discussed. Low-frequency BOLD signal fluctuations
were studied both in the time-domain and frequency-domain.
These animal results were also consistent with earlier stud-
ies done in humans using fMRI. Functional maps obtained
using the power spectrum of the frequencies and cross correla-
tion analysis indicate a significant reduction in the resting state
low-frequency BOLD physiological fluctuations in the cortical,
sub-cortical and deeper brain structures during normal physi-
ological states. Exsanguination led to an enhancement in the
amplitude of the low frequency BOLD fluctuations and spa-
tially expanded to most of the cortical, sub-cortical and deeper
brain structures. The behavior of the resting state BOLD sig-
nal fluctuations similar to CBF fluctuations, during exsanguina-
tion, indicates a myogenic dependence. However, a high intra-
hemispheric symmetry in the BOLD fluctuations with similar
phase characteristics in other regions in this study except the hip-
pocampus suggests that the fMRI-BOLD signal fluctuations in
anesthetized rats may carry information from underlying neural
activity.
