appropriate criteria for electrode dimensions and charge density characteristics discussed elsewhere should be met the electrodes also facilitate nrt measurements to optimise placement during implantation initial implantation away from the ampulla of the lateral canal produced small responses at high current thresholds that were not in the canal plane these were associated with limited dynamic range current limits and recruitment of facial nerves were a problem revision of the electrode placement closer to the ampulla produced thresholds an order of magnitude lower and no cross talk to the facial nerve precise electrode placement near the ampullae of the semicircular canals is critical for robust activation of the vestibular system as determined by eye movements in response to electrical stimulation and the ocular reflex the ocular reflex is a function of vestibular stimulation electrode placement that is too shallow or too deep results in weak or absent vestibular responses fortunately a reliable intraoperative tool for assisting in optimal electrode placement has been developed animal trials have demonstrated that like the cochlear nerve the vestibular afferents produce an electrically evoked compound action potential ecap that can be recorded from the implant using standard clinical neural response telemetry software when these responses can be recorded robust electrically evoked eye movements are obtained when the responses cannot be recorded no or minimal eye movements are obtained from stimulation correctly positioned electrodes produce large higher velocity nystagmic eye movements.