Biomedical Engineering Reference
In-Depth Information
1996; Normann et al., 1999; Jones and Normann, 1997; Maynard et al., 1997; Nordhausen et al.,
1996). Experiments done with intracortical electrodes confirmed that the stimulus current threshold
was 10 to 100 times lower than that for stimulation using surface electrodes (Dobelle et al., 1976).
Subjects were also able to perceive phosphenes at a predictable and reproducible location in the
visual space (Schmidt et al., 1996). Separate patterned perceptions could be evoked by electrical
stimulation via electrodes spaced as close as 500 mm apart. The preferable stimulation location was
probably the fourth layer of the visual cortex (Dobelle et al., 1976).
Undoubtedly, the lower current threshold of the intracortical microstimulation, the predictable
forms of generated phosphenes, the capability of increasing the number of electrodes, power
requirement improvement, and the current reduction per microelectrode are the main advantages
of the intracortical microstimulation approach (Dobelle et al., 1976; Schmidt et al., 1996). Also, the
skull protects both the electronics and the electrode array. The cortical prosthesis will bypass all
diseased neurons distal to the primary visual cortex, and hence, has the potential to restore vision to
the largest number of blind patients. However, there are problems in this approach. Spatial
organization is more complex at the cortical level and two adjacent cortical loci do not necessarily
map out to two adjacent areas in visual space, so that patterned electrical stimulation may not
produce the desired patterned perception. In addition, the convoluted cortical surface makes it
difficult for implantation, and surgical complications can have serious and devastating complica-
tions, including death.
17.1.2
Concept of Retinal Prostheses
A patent for a subretinal microphotodiode was issued to Graham Tassicker in the 1950s. This is the
first written record of a retinal prosthesis. The device was never realized as described in the patent.
During the early 1970s, it was found that blind humans can also perceive electrically elicited
phosphenes in response to ocular stimulation, with a contact lens on the cornea as the stimulating
electrode (Potts et al., 1968; Potts and Inoue, 1969, 1970). When obtainable, these electrically
elicited responses indicated the presence of at least some functioning inner retinal cells. Clearly,
stimulation through a single channel on the cornea could not produce form vision, yet it did
demonstrate that electrical stimulation at the level of the eye could evoke phosphenes in blind
subjects.
The idea of stimulating the remaining inner retina with an electrode array on the retinal surface
came about due to the fact that a number of blinding retinal diseases are predominantly outer retinal
or photoreceptor degeneration (Stone et al., 1992; Santos et al., 1997; Humayun et al., 1999a,b).
Two of the more common outer retinal degeneration diseases are retinitis pigmentosa (RP) and age-
related macular degeneration (AMD). Incidence of RP is 1 in 4000 live births; there are approxi-
mately 1.5 million people affected worldwide and it is the leading cause of inherited blindness
(Berson et al., 1993). AMD is the main cause of visual loss amongst older adults (
>
65 years old) in
Western countries. Annually, there are approximately 700,000 new patients in the U.S. who lose
vision due to AMD; 10% of these become legally blind each year (Curcio et al., 1996).
Analysis of eyes with outer retinal degeneration suggests that cells are present, but the retinal
circuitry is disrupted. Morphometric analysis of the RP retina has revealed that many more inner
nuclear layer cells (bipolar cells and others — 78.4%) are retained compared to outer nuclear layer
(photoreceptors — 4.9%), and ganglion cell layer — 29.7% (Potts and Inoue, 1970; Stone et al.,
1992). Similar results were obtained from AMD patients (Kim et al., 2002). Thus, it appeared
feasible to stimulate remaining retinal neurons. However, more recent studies of animal and human
retina with outer retina disease have shown that the retinal circuitry and structure undergoes
significant changes after photoreceptor loss.
Stimulation in blind humans with temporarily implanted electrodes demonstrated the principle
of electrical stimulation with epiretinal electrodes. Focal electrical stimulation elicited phosphenes
in all patients; four out of five patients were able to describe spatial and temporal aspects of
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