Ординатура / Офтальмология / Английские материалы / Artificial Sight Basic Research, Biomedical Engineering, and Clinical Advances_Humayun, Weiland, Chader_2007

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of nerve-cell processes, the “neuropil” appears normal. The neurons themselves (some indicated as “N”) also appear normal. In the section, some of these neurons are within 70 m of the tip site. At the time that these stimulating microelectrodes were implanted in the cochlear nucleus, a recording electrode was implanted near the cat’s inferior colliculus, a structure in the brainstem to which the neurons of the cochlear nucleus project. This electrode is too large to record the electrical

(A)

(B)

(C)

Figure 16.9. (a) A histologic section through the site of the tip of one of 4 microelectrodes that had been implanted in a cat’s cochlear nucleus for 2588 days. (b) An averaged evoked response recorded in the cat’s inferior colliculus while stimulating in the cochlear nucleus with the microelectrode whose tips site is shown in A. The first and second components of the response are indicated. (c) Plots of the amplitude of the early component of the AERs evoked from the microelectrode whose tip site is shown in (a).

activity of individual neurons but it does record a manifestation of the partly synchronized electrical activity of the population of nerve cells that is activated by the microstimulation in the cochlear nucleus. Figure 16.9b shows an example of this averaged evoked response (AER), which was generated by averaging the responses to 2048 successive presentation of the stimulus in the cochlear nucleus. Figure 16.9c shows plots of the amplitude of the early component of the AERs that were evoked from the microelectrode whose tip site is shown in Figure 16.9a. The graph’s abscissa is the amplitude of the 150 s stimulus current pulses. Throughout the 7 years, the threshold of the neuronal response was quite constant (approximately 5 to 8 A) and the growth of the response with increasing stimulus amplitude also was very constant. This stability of the neuronal response is consistent with the good condition of the neural elements surrounding the tip site (Figure 16.9a).

Conclusions

Certainly, many hurdles remain to in order develop a visual prosthesis based on ICMS, and to determine its place amongst the alternative approaches to a visual prosthesis. At this time, there is not even solid evidence that the topographic projection of the retina onto the primary visual cortex can be exploited so as to convey to the patient the percept of a complex shape. However, these issues now are being investigated in animal studies (3, 26). In other animal studies, we have shown that with an appropriate choice of stimulus parameters, it should be possible to safely activate the small populations of neurons in the visual cortex that mediate either simple visual percepts such as phosphenes or some higherorder percepts, including the perception of motion by an object in the visual field. The blunt-tipped activated iridium microelectrodes used in our studies have been shown to evoke neuronal responses that are stable over many years in the brain, and the small arrays of these microelectrodes are compatible with an approach of “tiling” the surgically accessible portions of the visual cortex with numerous closely spaced arrays. Certainly, the intracortical microelectrodes and the associated implantable electronic components will continue to evolve, as will technique for safely and efficiently implanting large numbers of microelectrode into the cortex.

Acknowledgments. We thank Clarence Graham, Jesus Chavez, David Minik, Nijole Kulevitute, and Alfred Tirado for able technical assistance. Mr. Leo Bullara was responsible for much of the design of the electrode arrays developed at HMRI. We thank Edna Smith and the animal care staff for excellent care of the animals and Cheryl Long provided secretarial assistance.

The animal studies conducted at HMRI were approved by the Animal Care & Use Committee of HMRI, and were performed under the guidelines set forth in the Guide to Care and Use of Laboratory Animals (1996 edition). This work was

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supported in part by research grant RO1-NS40690-01A1 and by contracts NO1- NS-8-2388, NO1-NS-5-2324, and NO1-NS-5-2324 from the National Institutes of Health.

References

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26.McCreery DB, Yuen TG, Agnew WF, and Bullara LA (1992) Stimulation with chronically implanted microelectrodes in the cochlear nucleus of the cat: histologic and physiologic effects. Hear Res 62: 42–56.

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to fragile retinal tissues. So we have developed indirect stimulating method of artificial retina that an electrode is inserted into the scleral pocket. And we already reported that our original stimulating method ‘suprachoroidal-transretinal stimulation (STS)’ was able to elicit electrical evoked potential (EEP) in acute experiment in rabbits [1]. Then chronic implanting method was developed and EEP responses were recorded without attenuation for 3 weeks [2]. As is widely known, although biphasic charge balanced pulse is used, the neural tissue will be damaged if excessive amount of electrical current is injected [3, 4]. In our STS method, electrode does not contact directly with retina, so it is principally safer to inject current compared with other method. In this study, we investigated the safety range of current by STS using a high-current continuous stimulation. And we studied the tissue change induced by STS with a current beyond the safety limit.

Material and Methods

Animals: Ten Dutch-belted rabbits were used. Twenty eyes were used for stimulation experiment (contents are referred to Table 17.1) and six eyes of them were for histological study. The rabbits were anesthetized with ketamine (50 mg/kg) and xylazine (20 mg/kg) cocktail injection. In need of additional anesthesia, half of first bolus was injected. The procedures used on the animals conformed to the Institutional Guidelines of Osaka University and the ARVO Resolution on the Use of Animals in Ophthalmic Research.

Electrodes: Stimulating electrode is shown in Figures 17.1a, b. A single Pt wire electrode (diameter: 100 m 200 m, polyurethane coating) which was embedded in silicone plate (2 × 5 × 0 3 mm, manufactured by Unique Medical, Osaka, Japan) was used. And the electrode wire was bent perpendicularly at the tip, with 50 m protruding from the plate surface. The surface area (including

(diameter: 100 m, polyurethane coating) was used. The tip 3 mm was bent perpendicularly, and the top 2 mm was uncoated.

Table 17.1. Tissue change by STS, These data derived from 20 eyes with macroscopic observation, 11 eyes were used for 100 m electrode stimulation, and 9 eyes were used for 200 m electrode stimulation. A fraction, for example, +4/4 means every time damaged of 4 times stimulation. Six eyes of them (with 1 mA, 1.5 mA, 2 mA of both electrode) were observed with light microscope. (ND; not done)

(A) A Photo of stimulating electrodes

(B) A schema of stimulating electrodes

Figure 17.1. (a) A photo of stimulating electrodes. Left: 100 m electrode. Right: 200 m electrode. Silicone plate size is 2 × 5 × 0 3 mm Bar = 1 mm. Dark-colored wire part is coated with polyurethane. (b) A schema of stimulating electrode. An electrode wire is protruding 0.05 mm from silicone plate. The surface area of electrode is 2 36 × 10−4 cm2 for 100 m electrode, and 6 28 × 10−4 cm2 for 200 m electrode (including top and lateral area).

Surgery: Sclerotomy was performed at the area just beneath the visual streak of rabbits as thin as possible until choroid was observed. And the stimulating electrode (100 m or 200 m) was attached on the fenestrated sclera with 8-0 Vicryl and 5-0 Dacron suture. Return electrode was placed in the vitreous cavity at the ora serrata. One electrode was used on one eye for one-time stimulation, and for secondary stimulation, eye and electrode was changed.

Stimulation: The schema of STS is shown in Figure 17.2. The retina over the electrode was stimulated by biphasic pulse (anodic first, duration: 0.5 msec, frequency: 20 Hz) with a current ranged from 1 to 3 mA continuously for an hour. The stimulus shape was made with signal processor (SEN-7203, Nihon-Kohden, Shinjyuku, Japan) and generated by the isolator (WPI-A365, WPI, Sarasota, USA). The voltage between stimulating and return electrode during stimulation was measured with oscilloscope (TPS-2014, Tektronics, Beaverton, USA).

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A schema of STS

Figure 17.2. A schema of STS. A transretinal electric current is injected from the stimulating electrode to the vitreous electrode. A biphasic charge-balanced current is generated by isolator (WPI-A365), and the voltage between active and return electrode is monitored by oscilloscope (TPS-2014). A square surrounded place is dissected for histological study.

After stimulation, funduscopy was performed to study whether there was a tissue change or not.

Histology: When both eyes stimulation and funduscopy were over, the rabbit eyes were enucleated and marked the scleral dent where the protruded electrode was attached. The eyes were fixated with 2.5% glutaraldehyde (0.1M phosphate buffer, Ph: 7.4), embedded in paraffin and sectioned with 3 m thickness around the marked scleral site with an interval of 20 m and staining with hematoxylin-eosin.

Results

The results of tissue change observed with funduscopy are shown in Table 17.1. For 100 m electrode, no tissue change was observed with a current of 1 mA

For 200 m electrode, no tissue change was observed with a current of 1 mA n = 2 and 1 5 mA n = 2 ; however, retinal change was observed with a current of 2 mA n = 4 and 3 mA n = 1 . With the eyes examined with a current of 1 mA, 1.5 mA, and 2 mA of theses 3 parameters for both electrodes, one preparation was made for observation with light microscope. Histological examination showed the residual scleral thickness was 50–100 m in all preparations n = 6 .

Representative examples of no histological change with low currents are shown in Figures 17.3a, c.