The nerve-electrode interface of the cochlear implant: Current spread

One of the fundamental facets of the cochlear implant that must be understo od to predict accurately the effect of an electrical stimulus on the audito ry nerve is the nerve-electrode interface. One aspect of this interface is the degree to which current delivered by an electrode spreads to neurons di stant from it. This paper reports a direct mapping of this current spread u sing recordings from single units from the cat auditory nerve. Large variations were seen in the degree to which the differ ent units are selective in responding to electrodes at different positions within the sca la tympani, Three types of units could be identified based on the selective ness of their response to the different electrodes in a linear array, The f irst type of unit exhibited a gradual increase in threshold as the stimulat ing site was moved from more apical to more basal locations within the scal a tympani, The second type of unit exhibited a sharp local minimum, with ra pid increases in threshold in excess of 6 dB/mm in the vicinity of the mini mum. At electrode sites distant from the local minima the rate of change of the threshold approached that of the first type of units. The final type o f unit also demonstrated a gradual change in threshold with changing electr ode position, however, two local minima, one apical and one basal, could be identified. These three types are hypothesized to correspond to units which originate a pical to the electrode array, along the electrode array and basal to the el ectrode array.