Page 383 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 377

elicited from all four quarters of the visual field (Veraart et al., 1998, 2003).
The indications for ON prosthesis are the same as retinal implants. The sur-
gical approaches to the ON are via a craniotomy or an intraorbital approach
(which is less invasive than craniotomy) (Brel en et al., 2006). The axons of
the ON must be intact for this site to be applicable for electrode implanta-
tion. Electrodes can either be placed as a cuff on the surface of the ON or as
penetrating electrodes. The ON is surrounded by a dural sheath within
which is contained with cerebrospinal fluid. Thus, cuff electrodes may
require higher currents and their spatial resolution would be inferior to pen-
etrating electrodes, because of the larger distance from the target ON axons.
The main advantages of this approach are the relatively easier surgical access
to the ON (compared with accessing the brain or the retina), and the pos-
sibility of targeting the entire visual field (Nishida et al., 2015; Veraart et al.,
2003). However, with more than 1 million axons passing through a 2mm
diameter nerve fiber, achieving focal stimulation is a challenge.
In the following studies by Veraat and coworkers, the influence of stim-
ulation parameters including amplitude, duration, frequency, and number of
pulses per phase on phosphene appearance (location and size) (Delbeke et al.,
2003). Despite the poor resolution of this prosthesis (four pixels), following
training the patient was able to achieve some pattern recognition, shape ori-
entation, object localization (Veraart et al., 2003; Brel en et al., 2005), as well
as perform grasping tasks (Duret et al., 2006). The second volunteer received
an eight-contact array implanted around the intraorbital section of the ON
(Brel en et al., 2006). Cortical evoked potentials and electroretinograms
(ERG) generated during electrical stimulation of the ON (in both of the
recipients) were recorded and compared with light and electrical surface
eye stimulation conducted in the normally sighted subjects (control group)
(Brel en et al., 2010).
Researchers from the Department of Ophthalmology in Osaka
University (Japan) are also developing an ON prosthesis. Their approach
is named the artificial vision by direct optic nerve electrode (AV-DONE)
(Sakaguchi et al., 2009). Here, metal wires are designed to penetrate into
the optic disc (the ON head) (Fang et al., 2006; Sakaguchi et al., 2004b,
2012). This is the point of exit for ganglion cell axons leaving the eye
and converging into the ON. Two blind patients with RP were so far
implanted with three Pt wire electrodes penetrating into the optic disc.
Round, oval, or linear phosphenes which were primarily yellow, and focally
distributed, were experienced by the patients in response to electrical
activation (Sakaguchi et al., 2009; Nishida et al., 2015).