Page 379 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 373

of seven letters per minute and with an accuracy of up to 90% (Donaldson,
1973). Dobelle also implanted subdural electrodes that were connected via a
plug in the skull, and via an electrical cable to a computer processing images
from a video camera. Dobelle’s patients were able to perceive phosphenes,
identify shapes, navigate the environment, and in one patient, to read
Braille. The currents required to generate the phosphenes with this device
were in the low-mA range (Dobelle and Mladejovsky, 1974; Dobelle, 2000;
Dobelle et al., 1974).
Penetrating cortical microelectrodes [intracortical microstimulation
(ICMS)] produce more localized phosphenes at currents in the μA
range. This is a >100-fold reduction in current output that significantly
lowers the risk for seizures (Lewis et al., 2016a; Davis et al., 2012).
A preliminary human study examining ICMS of visual cortex was pub-
lished in 1990, the results of which added significant impetus to the effort
to develop a cortical visual prosthesis (Bak et al., 1990). The three sighted
volunteers were able to perceive phosphenes from ICMS at currents up to
100 times lower than those required by surface stimulation. Moreover,
the phosphenes were discriminable when stimulated by electrodes
700μmapart (Bak et al., 1990). Further work identifying thresholds of
total charge delivered and charge density, beyond which neuronal dam-
age could be expected to occur (McCreery et al., 2010), supported the
progression to a more systematic evaluation of ICMS of visual cortex
in a blind volunteer (Schmidtetal.,1996). A key finding from this study
was that the chronically blind subject, who was unable to perceive phos-
phenes from surface stimulation, perceived phosphenes from ICMS in a
similar manner to sighted volunteers in the previous report (Schmidt
et al., 1996).
Currently, there are five groups around the world that are developing
brain implants for bionic vision. Intracortical electrodes are investigated
by the “CORTIVIS” group at the Universidad Miguel Hernandez
(Spain) (Fernandez et al., 2005, 2007), Sawan and coworkers at the Poly-
technique Montreal (Canada) (Coulombe et al., 2007; Mohammadi et al.,
2012), Troyk and coworkers at the Illinois Institute of Technology
(United States) (Troyk et al., 2006; Srivastava and Troyk, 2006a,b), and
the Monash Vision Group (MVG) at Monash University (Australia) with
the commercial entity Gennaris Neural Systems Inc. (Lowery et al.,
2015). Subdural electrodes were recently implanted in humans by Second
Sight Medical Products Inc. (United States). The subdural array (Orion)
was placed on the medial occipital cortex of a blind subject and phosphenes