Page 362 - Handbook of Biomechatronics

356 Lilach Bareket et al.

6/60 (Snellen VA), or a visual field no greater than 20degrees, in the better
eye with use of correcting lens. In the 10th revision of the World health
organization (WHO) International Statistical Classification of Diseases,
Injuries, and Causes of Death, “Low vision” function is defined as VA scores
(Snellen VA) in the better eye with the best possible correction of less
than 6/18 but at least 3/60, or a visual field of less than 20degrees. VA scores
of less than 3/60, or a visual filed of less than 10degrees are considered
“blindness.” “Visual impairment” includes both “low vision” and
“blindness.” VA is a measure of vision that compares the response of a
patient to a normative group. The term 6/60 (20/200 in feet) refers to an
individual seeing at 6m what the group saw at 60m (Bourne et al.,
2013a; Stevens et al., 2013). According to the WHO in 2010, 285 million
people were estimated to be visually impaired worldwide, of which 39 mil-
lion were blind (Bourne et al., 2013b; Mariotti, 2012). The leading causes
for blindness are cataracts (33%), uncorrected refractive error (21%), and
macular degeneration (7%). Other disorders that cause visual impairment
include retinal dystrophies, diabetic retinopathy, brain trauma, and several
infectious diseases (Mariotti, 2012; Lehman, 2012).
Visual prostheses operate by applying electrical stimulation to neurons
along the visual pathway to induce visual sensations, ultimately toward res-
toration of visual perception. Visual sensation relates to the pure physiolog-
ical process of bringing information from the environment into the body and
the brain. It includes the detection and reception of incoming light, conver-
sion of the light stimuli into neural impulses, and transmission of the infor-
mation to the visual center in the brain (occipital lobe). Visual perception
is defined as the processing and interpretation of this information by the
visual system. For example, classification of the visual stimulus into color,
movement, shape, etc., and assembly of these features into patterns. Visual
perception and is both physiological and psychological.
Anatomical targets of stimulation that are currently being explored
include the retina (da Cruz et al., 2016; Yue et al., 2015; Stingl et al.,
2015; Keser€u et al., 2012; Fujikado et al., 2016; Ayton et al., 2014), the
ON (Sakaguchi et al., 2009; Veraart et al., 1998; Fang et al., 2005), the
LGN (Panetsos et al., 2011; Pezaris and Eskandar, 2009), and the visual cor-
tex (Coulombe et al., 2007; Fernandez et al., 2005; Lowery et al., 2015;
Troyk et al., 2006)(Fig. 1).
Retinal prostheses replace the phototransduction function of the retina
in conditions where the photoreceptor cells in the retina degenerate,
targeting conditions such retinitis pigmentosa (RP) and age-related macular