Highlights
- •This review focuses on interhemispheric inhibition and its role in the healthy and stroke lesioned brain.
- •This
review discusses the importance of considering measurement method and
phase of movement when comparing studies associating interhemispheric
inhibition with functional recovery and also emphases the need to target
specific neural circuitry to improve stroke recovery.
- •Current
and novel neurostimulation techniques are discussed with respect to
how; they might be most effectively applied to target interhemispheric
inhibition and; enhance stroke rehabilitation.
Abstract
Background/Objectives
Interhemispheric
inhibition in the brain plays a dynamic role in the production of
voluntary unimanual actions. In stroke, the interhemispheric imbalance
model predicts the presence of asymmetry in interhemispheric inhibition,
with excessive inhibition from the contralesional hemisphere limiting
maximal recovery. Stimulation methods to reduce this asymmetry in the
brain may be promising as a stroke therapy, however determining how to
best measure and modulate interhemispheric inhibition and who is likely
to benefit, remain important questions.
Methods
This
review addresses current understanding of interhemispheric inhibition
in the healthy and stroke lesioned brain. We present a review of studies
that have measured interhemispheric inhibition using different
paradigms in the clinic, as well as results from recent animal studies
investigating stimulation methods to target abnormal inhibition after
stroke.
Main findings/Discussion
The
degree to which asymmetric interhemispheric inhibition impacts on stroke
recovery is controversial, and we consider sources of variation between
studies which may contribute to this debate. We suggest that
interhemispheric inhibition is not static following stroke in terms of
the movement phase in which it is aberrantly engaged. Instead it may be
dynamically increased onto perilesional areas during early movement,
thus impairing motor initiation. Hence, its effect on stroke recovery
may differ between studies depending on the technique and movement phase
of eliciting the measurement. Finally, we propose how modulating
excitability in the brain through more specific targeting of neural
elements underlying interhemispheric inhibition via stimulation type,
location and intensity may raise the ceiling of recovery following
stroke and enhance functional return.
Abbreviations:
TMS (
Transcranial magnetic stimulation),
rTMS (
repetitive transcranial magnetic stimulation),
GABA (
γ-amino butyric acid),
tDCS (
transcranial direct current stimulation),
TBS (
theta-burst stimulation),
iTBS (
intermittent theta-burst stimulation),
cTBS (
continuous theta-burst stimulation),
S-IHI (
Short latency interhemispheric inhibition),
L-IHI (
Long latency interhemispheric inhibition),
iSP (
ipsilateral silent period),
fMRI (
functional magnetic resonance imaging)
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