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E. Zrenner, H. Benav, A. Bruckmann, U. Greppmaier, A. Kusnyerik, A. Stett, K. Stingl, R. Wilke; Electronic Implants Provide Continuous Stable Percepts in Blind Volunteers Only if the Image Receiver is Directly Linked to Eye Movement. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4319.
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To compare subretinal electrode-generated percepts elicited by stationary stimulation with those elicited by light-sensitive subretinal arrays during patient initiated, natural eye movements.
The subretinal implant consists of two arrays: a) 4 x 4 electrode field controlled externally for direct electrical stimulation, b) a light sensitive "chip" (3 x 3 x 0.1mm) with 1500 photodiodes, amplifiers and electrodes (50 x 50µm, 70µm apart). While both arrays stimulate fixed retinal locations, light-mediated stimulation from the photodiode array follows the patient's unrestricted eye movements.
1. Percepts from continuous, direct, electrical stimulation with pulse duration (PD) of 1-4 ms, presented at a 0.3 Hz repetition rate, fade after approx.15 sec; at 2 Hz after approx. 2 sec; and at 10 Hz after approx. 0.5 sec. With the light sensitive subretinal chip, however, objects like grating patterns or letters can be perceived continuously (repetition frequency = 7 Hz, PD = 1 ms). Objects scanned in this natural way activate a range of adjacent pixels on the chip, i.e. eye movements and microsaccades continuously shift the "electrical image" on the retina, thus preventing mechanisms of image fading (e.g.Troxler effect).2. The ability to discriminate two consecutive electrical stimuli depends nonlinearly on both, the retinal distance between stimulation and repetition frequency (2 maxima at ~280 and >560 µm; 70 Hz and <5 Hz, respectively).3. In contrast with the classical Troxler effect, the retinal "electric Troxler effect" is not dependent on perception: pulses with amplitudes below perception threshold are capable of suppressing subsequent suprathreshold stimuli.
Continuous stimulation of fixed electrodes can provide only quickly fading percepts (similar to the Troxler effect). In contrast, percepts of images constantly moving across the subretinal light-sensitive chip -- due to saccadic and fixational eye movement -- remain visible without measurable temporal limitation. We conclude that real-time information of eye movements (an efference copy) is required by the brain to generate stable percepts. This is only available if the light sensitive part of the implant moves exactly with the eye, as in the subretinal microphotodiode implant.
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