Page 385 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 379

prostheses, an image compensation for gaze direction would be required.
The effect of corticothalamic activity on modifying LGN output is also little
understood (Pezaris and Eskandar, 2009).
Stimulation of the LGN to evoke visual sensation was so far examined in
nonhuman primates by analysis of eye movements in response to electrical
stimulation (Pezaris and Eskandar, 2009; Pezaris and Reid, 2007, 2009), as
well as in rats and rabbits, comparing visually and electrically generated sig-
nals recorded from V1 (Panetsos et al., 2011). Retinotopic models of
macaque and human LGN were used to simulate phosphene patterns to
investigate the electrode spacing (Pezaris and Reid, 2009). It was concluded
that a spacing of 600μm between the microelectrodes in the LGN (in three
dimensions) would theoretically allow for over 250 phosphenes per visual
hemifield in macaques, and over 800 in humans (Pezaris and Reid,
2009). To simultaneously place so many electrodes in such a small deep
structure, the investigators described a concept of a brush-like electrode
bundle splaying out from the end of the electrode sheath into the LGN.
With electrodes spaced 1mm apart in three dimensions, 250 electrodes
for each hemisphere could be placed on each side giving a total of 500
electrodes.
Pezaris and coworkers have further created simulated prosthetic vision
(SPV) for letter recognition (Bourkiza et al., 2013) and reading (Vurro
et al., 2014) with a thalamic prosthesis. In SPV studies virtual reality models
of prosthetic vision are administrated to normally sighted subjects (Chen
et al., 2009). The effects of electrode count on VA, learning rate, and
response time were examined, providing the first reports for thalamic
designs. The group recently reported a nonhuman primate model for visual
prostheses where animals are capable of performing similarly to humans on
the letter recognition task (Killian et al., 2016).

5 ENGINEERING CONSIDERATIONS FOR CORTICAL
AND RETINAL STIMULATION

In past two decades, tremendous efforts have led to substantial progress
in the ability to evoke visual sensation through prosthetic devices. In partic-
ular, patients implanted with retinal prostheses have demonstrated ability to
distinguish recognize objects, detect motion and orientation, perform sim-
ple navigation tasks, and even read large letters (Zrenner et al., 2011; da Cruz
et al., 2013; Ayton et al., 2014; Fujikado et al., 2016). Regained VA was
improved to 20/1590 or better in 8 out of 21 people as tested 5 years after