Page 377 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 371

2.4 Intrascleral Implants
Similar to the suprachoroidal approach, in intrascleral implantation the elec-
trodes are also spatially separated from the neurons. The suprachoroidal-
transretinal (STS) prosthesis, investigated by a group from Osaka University
in Japan, is inserted into a surgically formed pocket in the sclera (Fujikado
et al., 2011). Preliminary investigations were conducted in blind and nor-
mally sighted rats using a single silver electrode and a reference electrode
in the vitreous (Kanda et al., 2003, 2004), and in rabbits using an eight-
electrode array (Nakauchi et al., 2005). Results from a semichronically clin-
ical study with a 49-electrode prosthesis implanted in two blind patients
were reported in 2011 (Fujikado et al., 2011, 2012). During the 5–7-week
study period, it was found that nine channels were electrically active, with
four to six sites successful in elicitation of phosphenes. Considerably, higher
current thresholds were required to evoke this response, as expected due to
the physical distance from target cells (Fujikado et al., 2011). A head-
mounted video camera was used to control the stimulation strength based
on visual scene, applying head movements to scan the FOV in front of them.
Patients were able to better than chance level with the device turned on in
several visual tasks including: discrimination of shape thickness and object
localization. In object localization task patient performance improved with
training (Fujikado et al., 2011). Recently, the team reported a 1-year clinical
trial with a similar device implanted in three patients with RP (Fujikado
et al., 2016). All of the recipients were able to describe the appearance of
phosphenes in 24–28 out of the 49 electrodes. Two out of three patients
were able to perform significantly better with the device turned on as com-
pared with the device turned off in localization, discrimination, and mobility
tasks (Fujikado et al., 2016).
The recent progress with retinal devices reveals their potential in res-
toration of visual perception in individuals blind due to retinal degenera-
tion. There are many advantages to placing electrode grids on or near to
the retina including a large area of the visual field which can be covered,
avoidance of intracranial surgery, as well as the possibility to place an opti-
cal sensor in the eye to avoid the need for tracking of eye position to com-
pensate for gaze shifts. However, retinal implants target a relatively small
proportion of overall blindness conditions with the rare inherited disease
RP being the main indication. Cortical prostheses can potentially target
multiple other cases of blindness where retinal or ON prostheses cannot
be applied.