K 169Evoked cortical activity following subretinal electrical stimulation with implantable devices
H. N. Schwahn, F. E. Gekeler, H. G. Sachs, K. Kobuch, V.-P. Gabel, E. Zrenner
Introduction: We established methods that provide an objective measure for the transfer of visual information to the visual cortex mediated by subretinal microphotodiode-arrays in animal models (i.e. micropig and rabbit). These implantable devices are intended to replace/restore photoreceptor function in degenerated retina.
Methods: Light-stimulation: (1) Multifocal visual stimuli of various size and intensity were applied to anaesthetised micropigs or rabbits. Electrical stimulation: (2) The retina was electrically stimulated by monopolar voltage pulses ranging from -3 V to +3 V (200 to 400 µsec) referenced to a corneal electrode using acutely implanted multielectrode polyimide foils.
(3) Silicon microphotodiode-arrays (retina-chip), that produced electrical pulses upon "stimulation" with infrared light patterns were implanted chronically. Cortical recording: (4) Evoked field potentials in correlation with either light- or electrical stimulation were recorded above the visual cortex with chronically implanted epidural electrode arrays. In addition, the topography of the resulting fieldpotentials was assessed by means of a modified multifocal technique. (5) After acute stimulation (2 to 5 hours) eyes were histologically examined.
Results: (1) In both rabbit and micropig electrically evoked cortical potentials could be recorded immediately after implantation of the subretinal devices. (2) Increasing stimulation amplitude produced increasing response amplitudes up to 15 mV. (3) It was possible to characterise the topography of the cortical activation following multifocal light- and electrical stimulation in micropigs.
Discussion: Reproducible recordings of stimulus-correlated field potentials in the visual cortex provide an objective measure to evaluate to the feasibility of a subretinal visual prosthesis. They allow (1) to characterise the cortical representation of a subretinal electrical stimulation and (2) to routinely monitor long-term functional stability of implanted microphotodiode-devices (biocompatibility, toxicity).
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