Biomedical Engineering Reference
In-Depth Information
redox state may, at least, in part be linked to rhythmic variations
in CMR
O2
.
It should be emphasized that the origin of the slow cere-
bral fluctuations of CBV and CYTox remains to be determined.
It is unlikely to be entirely vascular (“vasomotion”), in view
of the complex frequency/time and interhemispheric architec-
ture of these fluctuations in cats and rabbits
(16, 17)
.Thereare
a variety of neuronal, glial, and vascular phenomena that may
offer their contributions to what finally appears as a measurable
“fluctuation”
(22)
. For instance, glutamate-induced intracellu-
lar calcium waves within the glial syncytium may represent an
energy-dependent indirect reflection of activity within focal neu-
ronal fields. Such factors would need to be carefully dissected
by future efforts. An interesting example of such multifactorial
components of the slow fluctuations emerges from the study
of CBV and CYTox during the transition from slow-wave sleep
to REM sleep in the cat, as will be discussed in the following
sections.
We first demonstrated a significant temporal correlation of
SLFs, both within and across hemispheres, in primary sensori-
motor cortex during rest
(9, 10)
. Nearly 74% of the time series
from these voxels correlated significantly (after filtering the fun-
damental and harmonics of respiration and heart rates) while only
a few voxel time courses (
3%) correlated with those in regions
outside of motor cortex. Subsequently, Hampson et al.
(23)
demonstrated the presence of RSC in sensory cortices, specifi-
cally auditory and visual cortex. In their studies, signal from visual
cortex voxels during rest (first scan) was used as a reference and
correlated with every other voxel in the brain. Significant num-
ber of voxels from the visual cortex passed a threshold of 0.35,
while only a few voxels from outside the visual cortex passed the
threshold. They have demonstrated similar results in the auditory
cortex
(24)
.
Lowe et al.
(25)
extended Biswal, Hyde and colleague's
(26)
results by showing such correlations over larger regions
of sensorimotor cortex (i.e., across multiple slices). Xiong, Fox
and colleagues
(27)
established relationships between motor and
association cortex. Similar to earlier results, they observed RSC
between sensorimotor cortex areas (primary, premotor, secondary
somatosensory). Further, however, they observed RSC relation-
ships between these motor areas and association areas, specifi-
cally anterior and posterior cingulate cortex, regions known to
be involved in attention. Greicius et al.
(28)
observed RSC in
anterior and posterior cingulate areas. Subsequent observation of
activation during a visual attention task indicated similar cingulate
activity.
These studies have established the foundation for “resting-
state functional connectivity studies” using fMRI (e.g.,
(23, 25,
<
