In vivo Studies of the Optic Nerve Using a Chronically Implantable Cuff Electrode

Abstract

Purpose:: A novel optic nerve cuff electrode has been developed in order to record or stimulate the optic nerve during in vivo studies. The experiments aim to progress the development of an optic nerve visual prosthesis.

Methods:: This study complies with the declaration of Helsinki and the protocol has been approved by the ethical committee of the School of Medicine at St-Luc University Hospital. A nerve cuff electrode with spikes which penetrate down to the level of the optic nerve was manufactured from biocompatible materials. Four New Zealand white rabbits (4.2-5.5kg) were implanted with the new electrode (ranging from 1 day to 2.5months). Field potentials generated by the optic nerve were obtained during light or electrical stimulation of the eye. The potentials were compared with electroretinograms (ERG) and cortical visual evoked potentials (VEP) recorded simultaneously to the optic nerve response. Flashes were generated by a white LED at varying intensities (1-3 cd.s/m2) and durations (0.1ms to 1s). More complex stimulations such as checkerboard patterns or a multifocal stimulus were generated on a standard CRT monitor. The cuff electrode was also used to electrically stimulate the nerve. Threshold measurements were made by increasing the amplitude of stimulation until a VEP could be elicited. Finally, a novel inner retinal potential was recorded using an ERG electrode whilst retrogradely stimulating the eye from the optic nerve.

Results:: Reproducible recordings with a high signal to noise ratio were obtained using the electrode. The potentials from the nerve had a bandwidth of 50-200Hz and are similar in shape to the oscillatory potentials of the ERG. The signal was abolished by cutting or anaesthetizing the nerve. The waveshape changes when increasing the luminosity or duration of the flash stimulus. At very long flashes it was possible to separate the on and off responses of the nerve. The multifocal recording shows how each electrode contact is able to record from a subpopulation of fibers which correspond to an area of the visual streak in the multifocal ERG. The nerve could be stimulated by the electrode and elicited a VEP at low thresholds. The ERG recorded when electrically stimulating the nerve demonstrates a separation of classes of retinal ganglion cells with different conduction velocities.

Conclusions:: The new electrode allows chronic, in vivo, multicontact stimulation or recording from the optic nerve. The electrode may provide a better alternative to the one already being used for the optic nerve visual prosthesis.

Keywords: electrophysiology: non-clinical • optic nerve • electroretinography: non-clinical