Biomedical Engineering Reference
In-Depth Information
the stimuli. The resolution could be estimated to be 4.5/2000, a crude ambulatory vision (Humayun
et al., 1996). The electrical stimulation threshold has been found to be dependent on the location of
the electrode; the macular region required higher threshold currents than the peripheral retina to
elicit visual perceptions. Also, patients with less advanced RP or AMD required lower threshold
currents than those with more advanced disease stages. These findings are important because lower
thresholds would allow for smaller electrodes and greater resolution. Lower threshold values in
healthier retinae were later confirmed by other experiments (Suzuki et al., 1999; Chen et al., 1999;
Weiland et al., 1999; Rizzo et al., 2000; Majji et al., 1999). Perhaps the most important result of
these studies was associated with form recognition. Patients were able to identify crude forms such
as letters or a box shape during the short period of electrical stimulation testing. When the electrical
stimulation ended, there was no persistence of the image. Another group later confirmed this result
of form recognition by electrical stimulation of a healthy retina of a sighted volunteer (Rizzo et al.,
2000). Other important psychophysical perceptions of this study included flicker fusion (at a
frequency of 40 to 50 Hz) and different color perceptions (Humayun et al., 1999a,b).
17.1.3
Optic Nerve Prostheses
Attempts have been made to stimulate the optic nerve to generate optical impulse. But the high
density of the axons within the optic nerve (1.2
10
6
within a 2-mm diameter cylindrical structure)
could make it difficult to achieve focal stimulation and detailed perception. In addition, any surgical
approach to the optic nerve requires dissection of the dura and can have harmful side-effects.
Similar to the retina prosthesis approach, optic nerve stimulation requires intact RGCs and is
limited to outer retinal pathologies. A volunteer with retinitis pigmentosa and no residual vision
was chronically implanted with an optic nerve electrode connected to an implanted neurostimulator
and antenna. An external controller with telemetry was used for electrical activation of the nerve
which resulted in phosphene perception. Low perception thresholds allowed for large current
intensity range within safety limits. In a closed-loop paradigm, the volunteer was using a video
camera to explore a projection screen. The volunteer underwent performance evaluation during the
course of a training program with 45 simple patterns. After learning, the volunteer reached a
recognition score of 63% with a processing time of 60 sec. Mean performance in orientation
discrimination reached 100% with a processing time of 8 sec (Veraart et al., 2003).
17.1.4
Sensory Substitution Devices
An alternative to direct stimulation of the visual system has been tried and is being developed to aid
visually handicapped people. This method attempts to convert visual information into vibro-tactil
or auditory signals (i.e., sensory substitution devices) (Margalit et al., 2004; Brabyn, 1982; Rita
et al., 1998).
17.2
CURRENT CONCEPTS IN RETINAL PROSTHETIC DEVICES
Worldwide, there are currently several groups involved in the process of development of retinal
prostheses. Their respective work is classified according to the location of the device in relation to
the retina: on the retinal surface (
epiretinal
), or behind the retina (
subretinal
). There is also another
group in Japan developing a new approach, suprachoroidal-transretinal stimulation (STS), where
the array is to be placed in the suprachoroidal space (Kanda et al., 2004).
Epiretinal implantation has the advantage of keeping the majority of electronics outside the eye,
minimizing the risk of failure, and optimizing the ease of replacement or upgrading the electronics.
Additionally, the majority of the intraocular electronics could be placed in the vitreous cavity, a
naturally existing space, which is fluid filled postsurgery. This greatly helps in dissipating the heat
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