Page 390 - Handbook of Biomechatronics

384 Lilach Bareket et al.

electrode/tissue interface (Merrill et al., 2005), leading to formation of gas
bubbles and pH shifts that damage the tissue (McCreery et al., 1994, 2010).
To prevent these processes, the net charge used in stimulation should be
neutralized in some way, such as shorting poststimulus, series capacitors
in line with each electrode, and carful circuit design (Bartlett et al., 1977;
Brummer et al., 1983). The electrodes can be intermittently stimulated in
various patterns of anode/cathode geometry. This may enhance the focus
of the stimulation (Berenstein et al., 2008). Some electrodes may fail with
adjacent electrodes maintaining function. Reconfiguration of stimulation
parameters and patterns may maintain device efficacy overtime.
In the case of ICMS, the goal is to activate small areas of the visual cortex
around each individual electrode to produce single visual percepts, thus
reducing crosstalk. According to a recent study in rats, the optimal stimulus
is an extended pulse train of low amplitude and low frequency (Watson
et al., 2016). In the retina, challenges of stimulation strategies include not
only the containment of neural activation as in the former case, but also
others derived from the neural architecture of the retina. For electric field
containment, different return configurations are commonly used, as this
technique allows for modifying the shape of the electric field, and therefore
the extent of the neural excitation. In a monopolar configuration, the return
electrode is placed far from the active electrode thus producing wide neural
activation with low activation thresholds (Matteucci et al., 2013). In a bipo-
lar strategy, the return electrode is placed in the vicinity of the active. This
configuration produces more contained activation, but when biphasic pulses
are used, it may also activate the area around the return electrode during
charge recovery (Dokos et al., 2005). In a multipolar disposition
(Matteucci et al., 2013; Spencer et al., 2016), current returns through a
group of electrodes (see Fig. 6). Containment of the electric field is also
being targeted by using field overlapping techniques, a strategy that com-
bines different electric fields, for example, to reduce activation thresholds
while containing neural activation (Matteucci et al., 2013, 2016). Concom-
itant stimulation has to be managed carefully as it can also produce neural
inhibition (Barriga-Rivera et al., 2017).
Another important challenge in retinal electrostimulation is to become
able to elicit neural responses that encode appropriately visual stimuli. With
more than 32 functional RCGs (Baden et al., 2016), researchers are using
high-frequency stimulation to selectively target different information
streams (Twyford et al., 2014). Although these approaches are promising