Biomedical Engineering Reference
In-Depth Information
speculation regarding their role in the clinical manifestations of
absence seizures. Experiments have shown deactivations ranging
from nearly the entire cortex
(25)
to smaller subsets of the cor-
tex
(28, 63-67)
though the meaning of cortical BOLD decreases
during SWD remains unknown. Experimenters have seen bilateral
decreased BOLD signal change in the anterior and posterior inter-
hemispheric regions, lateral frontal and parietal association cor-
tices, and the posterior cingulate/retrosplenial/precuneus areas
(25,28,63-65,67,68)
. Several investigators
(63,66,69)
have noted
the similarity between the BOLD deactivation pattern generated
by SWD and those areas that characterize normal brain activ-
ity
(70)
. While much work has been done to pinpoint cortical
decreases associated with SWD, variable results have been found
and further study is necessary.
Reports of positive cortical changes in BOLD signal dur-
ing SWD are less consistent and less pronounced than negative
changes. Activations have been found in the bilateral precentral
sulci
(64)
, mesial frontal cortex, bilateral insula
(69)
, bilateral
motor cortex
(65)
, occipital cortex, and inferior parietal cortex
(66)
. Most of these studies have reported variable fMRI increases
in the lateral frontal and parietal cortical regions
(28, 68)
.The
variation and lack of consistency seen in labeling positive cortical
BOLD changes suggests additional study is necessary.
5.2. Thalamic BOLD
Changes
Like the cortex, the thalamus has long been implicated in
SWD generation and maintenance
(2, 18, 23, 71)
. Human
BOLD studies, especially those implementing continuous
EEG-fMRI, have consistently found bilateral thalamic increases
(25, 26, 28, 63, 65, 66, 68)
though some studies have also found
thalamic decreases
(26, 63)
. Some investigators have attempted
to differentiate the BOLD signal changes in different thalamic
nuclei, but such investigations are near the limit of the spatial
resolution of fMRI. Thalamic activity is, at times, seen only at
higher field strengths (3T)
(66)
and continued advances in fMRI
technology may reveal more information regarding the thalamus'
metabolic activity during SWD.
5.3. Basal Ganglia,
Cerebellum, and
Brainstem
BOLD changes outside of the cortex and thalamus have been
reported, though these changes have received much less atten-
tion, and some studies have chosen to focus solely on the corti-
cothalamic network. Increases in the cerebellum have been found
(28, 63, 69)
as have increases (probably artifactual) in the lateral
ventricles
(69)
and white matter tracts associated with the tha-
lamus
(65)
. Cerebellar activity during SWD has been previously
noted
(72,73)
, but the cerebellum's role has not yet been defined.
Decreases have also been found in the basal ganglia
(25, 28, 68)
.
The importance of the basal ganglia and the brainstem in absence
