| More than70%of the information humans use to cognize theirsurroundings comes from vision, thus blindness is a serious disability that notonly affect the quality of a patient’s daily life but also brings a great burden tosociety. As the development of science and technology, vision restoration forblind patients with a visual prosthesis has attracted worldwide interest.Chinese research team proposed a new visual prosthesis based on penetrativeoptic nerve (ON) stimulation and this new visual prosthesis need to besystematically investigated. This study basically analyzed the characteristicsof the electrically evoked potential (EEP) of the visual cortex elicited by ONstimulation and evaluated the effects of different stimulus waveforms andstimulating parameters on EEP. Our group also independently developed aMEMS-based polyimide thin-film microelectrode array for suprachoroidalelectrical stimulation. In this study, we conducted a preliminary explorationof the feasibility of the stimulating electrode array and the spatiotemporalproperties of the EEP. Details are as follows:In the study of ON stimulation, the platinum-iridium stimulatingelectrodes were inserted into the ON to stimulate the fibers. EEP wasrecorded by silver ball electrode array placed over the visual cortex. Wesystemically investigated the effects of stimulus waveforms and stimulatingparameters on the spatiotemporal properties of multi-peaked EEP. Thresholdinformation for different components in EEP and optimized stimulatingparameters were obtained. The results provided important information forunderstanding the visual signal transfer process and the optimization of thestimulating parameters. The main findings are:(1) Multi-peaked EEP couldbe evoked when electrically stimulate the ON fibers. At most five components (P0, N1, P1, P2and P3) were recorded. Different componentshad different current thresholds and temporal properties. The component withlower current threshold had shorter latency and faster temporal variation.(2)When stimulated the temporal-dorsal side of the ON, the maximal responsesite of the fast component (P1) on the visual cortex was corresponding to thesame visual field of the stimulating site in the ON. The results indicated thatthe visual topological relationship in ON is good near the eyeball and the ONhere was appropriate for implantation of a visual prosthesis.(3) Stimuluswaveforms and stimulating parameters had a great effect on cortical response.Comparing to symmetrical charge balanced stimulus pulse, stimuluswaveform with high-amplitude short-duration cathodic phase andlow-amplitude long-duration anodic phase could reduce the current threshold.The stimulus waveform which elicits larger EEP amplitude can also evokelarger spatial spread extent of EEP under the same stimulating charge density.Adding an inter-phase gap (IPG) between the two phases of the stimuluswaveform can decrease the current thresholds of EEP, which saturated at anIPG of~0.2ms; BP-C can evoke the largest response after adding an IPG. Asthe stimulating frequency increased the amplitudes of the components in EEPdecreased and the components changed distinctively. These results provideimportant experimental support for the development of the ON visualprosthesis.In the study of suprachoroidal stimulation, the MEMS thin-filmelectrode array with60stimulating sites was implanted into thesuprachoroidal space to stimulate the retina. The feasibility of electrode arraywas verified. EEP was recorded and the spatiotemporal property of the EEPwas investigated. The results indicated that suprachoroidal stimulation couldevoke EEP successfully. The current threshold to evoke EEP was48.57±6.90μA for single electrode stimulation and25.55±5.43μA for full-rowelectrode stimulation, with the corresponding charge density of25.24±3.59μC/cm2and13.28±2.82μC/cm2, respectively. The current and charge density thresholds were well below the safe limit for the electrodes and targettissue. As the current intensity increased EEP amplitudes increased, but thefocus of EEP was stable. The focus of the EEP could be altered by changingto different stimulating electrodes (the nearest distance between twostimulating electrodes is750μm). The EEP site on the visual cortex wascorresponding to the stimulated site of the retina. This study further validatedthe feasibility and effectiveness of suprachoroidal stimulation, quantifiedsome stimulating parameters and provided experiment support forsuprachoroidal prosthesis. |
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