Activation zones in cat visual cortex evoked by electrical retina stimulation
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Background. A retina implant for restoring simple basic visual perception in patients who are blind due to photoreceptor loss requires optimisation of stimulation parameters for obtaining high spatio-temporal resolution. We developed effective low-power epi-retinal stimulation and intracortical recording in semichronically prepared cats.
Methods. Individually driveable fibre electrodes were inserted through a small scleral incision and positioned at the area centralis. Polyimide–platinum film electrodes were inserted via a corneal incision and fixed by instillation of perfluorocarbon liquid on the internal limiting membrane. For electrical stimulation we used short charge-balanced current impulses of 100–400 µs duration and amplitudes ranging from 1 to 100 µA. During stimulation we recorded multiple single-cell and population activities from areas 17 and 18. Recordings were stored digitally. Stimulus–response relations including response strength, cortical activation zones, information transmission, and electrical receptive fields were analysed off-line.
Results. We found low-threshold activations with fibre electrodes and polyimide–platinum film electrodes in close mechanical contact to the retina. Retinal stimulation with bipolar charge-balanced impulses resulted in cortical activation zones corresponding to 1–5° visual angle at paracentral locations dependent on the eccentricity of the retinal stimulation point. Retino-cortical transinformation analysis revealed 20–30 bits/s per electrode, corresponding to 10–15 four-level pictures/s. Electrical receptive fields had sizes of 1–3° visual angle.
Conclusions. Coarse visuomotor coordination and navigation seems possible with retina implants.
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