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B.K. Baziyan, S.A. Gordeev, V. Ortmann; Comparative Evolution of Cortical Stimulation Thresholds by Intracortical and Subdural Implantation . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1513.
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Purpose: In the case of visual nerves degeneration or physical damage of both eyes, the only way to restore a visual sensation could be an implantation of multichannel electrical stimulators into the primary visual cortex of the patient. A good design practice of neural implants presumes a comprehensive investigation of stimulation thresholds and proper electrode types. The purpose of this study was to compare the stimulation thresholds of penetrating intracortical vs. subdural surface electrodes. Methods: 16 Channels intracortical stimulator comprising a 4x4 penetrating electrode matrix was implanted into cat’s striate cortex in the left hemisphere. Similar stimulator provided with sixteen surface electrodes was implanted into the right hemisphere of the same animal. The distance between each row and column was approximately 500–700 µm, and the penetrating depth was in the range of 1–3 mm. Several cats were trained to raise it’s paw in response to optical stimulus produced by a small light source with angular size of approx. 1°. Biphasic constant current pulses with different amplitudes and durations were applied in order to estimate the stimulation thresholds. The stimulation was successful, if the cat have raised it’s paw. The amplitude altered in the range of 5–150µA for penetrating electrodes and 0.1–5mA for surface electrodes. The pulse duration of both electrode types was in the range of 0.1–4 ms. The pulse frequency varied from 5 to 200 Hz. Stimulus duration was 1–3 seconds. Results: Trained cats were able to respond to electrical stimulus in the wide range of strength–duration parameters. Most reliable response was achieved by stimulation with penetrating electrodes, in which the stimulation amplitude was in the range of 20–100µA and pulse duration in–between 0.5 and 1 ms. Surface electrodes caused the reflex behaviour at much higher amplitudes in the range of 1–4 mA. Conclusions: The surface electrodes are very attractive due to simple implantation procedure and state of the art manufacturing. But the fact, that required stimulation thresholds exceed the known limits of biocompatible stimulation, makes this type of electrodes improper for long–term visual implants. The penetrating electrodes show better performance, but imply the complex implantation. Proper implantation methods and tools, especially for implantation into calcarine fissure, must be designed.
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