Biomedical Engineering Reference
In-Depth Information
epilepsy has been demonstrated in animal models
(74-77)
and
human patients
(78,79)
. Currently, little emphasis has been placed
on BOLD signal changes outside the thalamocortical network,
but this is an area worthy of further study.
5.4. Discussion of
Human fMRI Studies
of Spike-Wave
SWD is a rhythm that arises out of normal brain circuitry and
physiology but is a distortion of normal mechanisms
(2)
. It affects
specific circuits in the brain while sparing others. Structures that
are affected by spike-and-wave discharge are the same networks
that are important for normal oscillations, and the involvement of
these structures interrupts normal brain activity. Much attention
has been paid to the possible connection between the behavioral
deficits seen in absence seizures and the cortical deactivations seen
in human BOLD studies
(69)
. This should not lead investigators
to ignore prominent BOLD changes in subcortical structures or
BOLD increases in the cortex.
fMRI increases in the thalamus have been observed repeat-
edly, but the mechanism that drives the BOLD changes is as
yet unknown. Are the BOLD increases in the thalamus due to
increased activity of excitatory or inhibitory neurons? Are neu-
rons firing more or
less
in the thalamus during SWD? Thala-
mic decreases remain unexplained (as do the mechanism that
underlies all BOLD decreases in SWD). It has also been sug-
gested that SWD may be generated by more than one mech-
anism, which might explain the varied cortical BOLD signal
changes
(25, 67)
. Finally, the relatively consistent BOLD changes
seen in the thalamus, frontoparietal cortex, and posterior cingu-
late/retrosplenial/precuneus (
Fig. 9.3
) areas may represent an
important functional network in SWD generation.
Cortical fMRI increases have not been given the same atten-
tion as cortical decreases in SWD. The possible role of these
regions in SWD generation and behavioral deficits warrants fur-
ther investigation. One author suggested that the variability in
cortical signal might have less to do with SWD generation and
more to do with the normal baseline activity of each individ-
ual
(66)
. If SWD leads to an interruption of normal activity,
then baseline activity would dictate which cortical areas showed
increases and decreases
(69)
.
5.5. Limitations of
Human Studies to
Date
Attempts to localize SWD using BOLD fMRI have produced
inconsistent results. Studies have been limited by heterogeneous
patient populations (e.g. age, medication, diagnosis, seizure activ-
ity) (reviewed in
(66)
). As has already been noted, stronger
field strengths have allowed researchers to find signal changes
that were not apparent at lower field strengths, and con-
tinued advances in fMRI research techniques may reveal a
more consistent pattern of significant fMRI signal changes.
Furthermore, while several researchers have connected their fMRI
