Page 388 - Handbook of Biomechatronics

382 Lilach Bareket et al.

suprachoroidal and subretinal configuration (Argus II), respectively (Ahuja
et al., 2013; Ayton et al., 2013). This means higher stimulation currents are
required to activate the neurons (relative to other stimulation sites). Another
issue is that more current is spreading to neighboring cells. Focal activation
of the RGCs is highly desired to achieve spatially distinct phosphenes
perception, and therefore current spreading means lower resolution.
The visual cortex is a relatively large area to place electrodes compared
with the retina, and thus a large number of electrodes can be employed with-
out requiring further reduction in electrode dimensions. The electrodes are
placed to interface the surface of the cortex or penetrate into the tissue cor-
tical arrays. The penetration of the electrode into the tissue contributes to an
increase in the precision of stimulation and to a decrease in the power con-
sumption of the device. There is a dramatic reduction in the threshold to
stimulation of phosphenes as the electrodes penetrate deeper into the tissue
(Bak et al., 1990). The ideal depth of electrode penetration for human intra-
cortical stimulation is approximately 2–2.25mm where stimulation thresh-
olds are within the 1.9–77μA range (Bak et al., 1990; Schmidt et al., 1996).
Therefore, due to the lower stimulation currents, the risk for epilepsy is
lower with ICMS compared with surface stimulation (Bezard et al., 1999;
Goddard, 1967).
A significant proportion of the visual cortex is submerged in the calcarine
fissure. If this part of the cortex is not included in the stimulation, an
hourglass-shaped visual field results. Head scanning which places the camera
at different angles will help the individual “fill in the gaps” and create a more
complete picture of the visual scene. This has been studied using simulation
in normally sighted individuals wearing a heads up mounted display in gog-
gles (Lowery et al., 2015). Placing electrodes within the calcarine cortex may
also require varying length electrodes and different carrier designs. Alterna-
tively, additional electrodes can be positioned in secondary visual cortex to
cover some of the missing visual field (if the electrodes do not penetrate this
hidden calcarine cortex) (Lewis et al., 2015). Placement of electrodes on the
medial surface of the hemisphere potentially covers the peripheral vision and
central vision, but the orientation of these electrodes would be orthogonal to
a transmitting coil on the scalp and wireless data transfer would be restricted.
Insertion of penetrating cortical arrays cannot be achieved with pressure
from the surgeon’s finger and requires a pneumatic-actuated inserter (Utah
array insertion tool) (Normann et al., 1999). The Utah array insertion tool
operates through a pneumatically controlled piston which pushes the elec-
trodes through the pia, a relatively tough fibrous layer covering the brain