Signal Processing - March 2017 - 117
for nonoptimally placed leads. Yet, the
split ring does not provide the resolution
and shaping capabilities of the 32 contact
stimulating lead trialed intraoperatively
by Contarino et al [7]. The 32 tessellat-
ing contact resolution allows the annular
shape to slide up and down the stimulat-
ing lead in a near continuous fashion, in
contrast to jumping between nonover-
lapping dorsal-ventral segments. As
such, the sculpting possibilities are end-
less, fulfilling the real potential of stim-
ulation field shaping. Not surprisingly,
the results from the intraoperative pilot
indicated that directional stimulation
could be increased much greater before
evoking adverse events than could the
annular mimicking stimulation.
Widening the neurostimulation
parameter space
As attractive as physical shaping of
the stimulation field may appear, real
advances may alternately be available
through a better understanding of pro-
grammable stimulation parameters of
pulse frequency, width, and amplitude.
The parameter values that are used today
have been dictated by the technical
limitations of the available neurostimu-
lation devices and tuned within these
limits by clinical experience. Recent
studies have proposed various ways for
widening the parameter space with the
goal of selectively stimulating thera-
peutic target neurons at the lowest
energy possible.
Varying temporal patterns through
interleaving is unique to Medtronic's
Activa family of neurostimulators and
was first introduced in 2009. Here,
alternating pulses are emitted from dif-
ferent contacts of the same stimulating
lead, each with independently program-
mable amplitude and pulse width, but
with the same interdigitated frequency.
This was originally intended to allow
dual regions of a target substrate along
the dorsal/ventral axis of the stimulating
lead to be activated, while leaving the
region in between unperturbed. Thus,
it was thought that multiple symptoms
could be captured by multiple sites.
However, if the stimulation fields
are brought in close proximity to each
other, either by using adjacent contacts
or by increasing the amplitude and
tific and technical achievements in DBS
pulse width, then the two stimulation
systems. Notwithstanding the dramatic
fields may overlap creating a region of
improvement that DBS already brings
stimulation that will receive twice the
to the quality of life for many patients,
programmed frequency, in addition to
we are far from securing, if not defin-
nonoverlapped regions receiving the
ing, its maximum clinical outcome. For
programmed frequency. As such, two-
DBS implantation, two straightforward
tiered frequency stimulation fields can
objectives are to provide 1) pre- and
be sculpted, allowing multiple symp-
intra-operative support in localizing the
toms to be captured or alternately
DBS target area and 2) neuromarkers
adverse effects released by engaging
that depict the neurophysiological vari-
multiple temporal frequencies.
ability and, therefore, are predictive
While the St. Jude (Infinity) system
of the DBS outcome. Especially for
can drive the stimulation of different
psychiatric diseases that are typically
leads at independent frequencies, the
believed to be due to brain network
Boston Scientific (Vercise) DBS system
imbalances, DBS localization is expect-
is capable of programming indepen-
ed to benefit from approaches that link
dent frequencies on the same lead,
DBS with other, noninvasive, stimula-
for two active "areas," defined as any
tion techniques applied on the same
aggregate of contacts and/or contact seg-
functional networks [8]. For DBS
ments. Thus, the Vercise system can
programming, one possible objective is
create temporal "patterned" stimula-
to step away from the stereotyped stimu-
tion in the overlapped regions of the
lation patterns that current open-loop
generated fields. The two areas would
DBS systems provide a nd move
be driven at different
toward neuromodu-
frequencies, with the
lation that adapts
Many open questions on
initial stagger inter-
at the millisecond
the neural underpinnings
val between them
scale, where neu-
of neuromodulation are
being determined by
rons communicate.
expected to be addressed
the lagging anodal
As a surgical treat-
phase of the initial
ment
for movement
by recent scientific and
area pulse, ultimately
disorders,
DBS has
technical achievements
resulting in doublets
been historically de-
in DBs systems.
or triplets instead of
livered in an open-
continuous stimula-
loop fashion where a
tion trains. Medtronic's interleav-
preprogrammed, chronic and continu-
ing and Boston Scientific's staggered
ous stimulation pattern could not avert
independent frequencies are two ways
suboptimal clinical outcomes. Leverag-
to implement multiple frequency fields
ing the technical advances in new DBS
or patterned stimulation. However, the
devices, clinical studies show that a
clinical significance of these new tech-
closed-loop DBS (CL-DBS) system is
nical capabilities remains to be seen.
realizable. What still seems elusive is
If the utility of these simple temporal
the driving signals for such systems,
stimulation patterns can be clinically
i.e., the neural signals and their features,
demonstrated then more complicated
that are informative enough to con-
bursting capabilities could be inten-
trol the online real-time adaptation of
tionally engineered. Neurons in the
the neuromodulator. One might argue
brain lend themselves to bursting, why
that, for the current technology, the
not DBS?
best control signal is the LFP, or some
component of it. The reason is that LFP
represents the neural information inte-
Future perspectives: Toward
grated over a larger area compared to
adaptive and precise
the multiunit activity and, therefore,
neuromodulation
presumably carries more information
Many open questions on the neural
about the cardinal symptoms of the dis-
underpinnings of neuromodulation are
ease and can account for intersubject
expected to be addressed by recent scien-
IEEE Signal Processing Magazine
|
March 2017
|
117
Table of Contents for the Digital Edition of Signal Processing - March 2017
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