Biomedical Engineering Reference
In-Depth Information
activity, which can be performed in animal models (
Fig. 9.5
),
may provide a way to unravel the complex relationships between
neuronal activity patterns and BOLD fMRI. Studies performed
so far in WAG/Rij rats have demonstrated that regions with
fMRI increases during SWD (e.g. barrel somatosensory cortex,
S1BF,
Fig. 9.4
) exhibit increases in neuronal firing and CBF dur-
ing SWD (
Fig. 9.5
). Meanwhile, regions with no fMRI signal
changes during SWD (e.g. primary visual cortex, V1M,
Fig. 9.4
)
show no changes in neuronal firing or CBF (
Fig. 9.5
). Fur-
ther studies will be needed, particularly in regions showing fMRI
decreases, to more fully understand these phenomena.
7. Possible
Mechanisms of
fMRI Decreases
During Seizures
From the above discussion, it follows that BOLD fMRI increases
during SWD most likely reflect focal increases in neuronal activ-
ity. However, the mechanisms of BOLD decreases during SWD
and other forms of seizure activity are not well understood. Pos-
sible mechanisms of BOLD fMRI decreases include:
(1)
A
pri-
marily vascular mechanism
, where blood supply decreases in spe-
cific brain regions during seizures. Primary causes of decreased
blood flow could include (1A)
vasospasm
or (1B)
vascular steal
.
Excessive decreases in local blood flow will decrease net oxygena-
tion, leading to a decrease in BOLD. Significant decreases in oxy-
genation (hypoxia) could also secondarily cause some reduction in
neuronal activity (
Fig. 9.2C
).
(2)
A
primarily neural mechanism
where cortical neuronal activity decreases in specific brain regions
during seizures. If neurovascular coupling is normal, reduced
neuronal activity usually leads to an excessive reduction in blood
flow (
Fig.9.2C
), again causing a net decrease in oxygenation,
and a decrease in the BOLD signal. A primary decrease in neu-
ronal activity could occur in several ways: (2A)
Decreased neuronal
activity in regions spared by seizures.
While seizures cause neu-
ronal activity to increase in regions intensely involved in epileptic
activity, network effect may cause neuronal activity to decrease
in other regions of the brain which are
not
directly involved in
the seizure. This could occur either through (2Ai)
active inhibi-
tion
, or (2Aii)
reduced excitation
in these regions. Examples of
decreased neuronal activity and CBF have been observed while
studying cortical brain regions spared by partial seizures in both
animal models
(91)
, and in humans
(5, 6)
;(2B)
Decreased neu-
ronal activity in regions involved by seizures.
A second primar-
ily neural mechanism may occur in which some cortical regions
which are
involved
in seizures could, nevertheless, show
reduced
neuronal activity. While seizure activity is generally thought to
increase neuronal activity, SWD is associated with an alternat-
ing pattern, between increases in neuronal activity during the
