Effects of electrical current configuration on potential fields in the electrically stimulated cochlea: field models and measurements.

Potential distributions measured within the scala tympani of the anesthetized guinea pig support the assertion that focusing is possible when currents are appropriately delivered to the electrodes in the scala tympani. Results obtained with a lumped-element model agree with measurements made in the inner ears of monkeys during monopolar and bipolar stimulation. The predictions are closer for potential distributions apical to the stimulating electrode than they are for basal distributions. In one monkey, in which electrodes were implanted in the middle ear as well as in the inner ear, we obtained measurements of the impedance from inside the scala tympani to points within the middle ear. These impedances are smaller that those initially used in the model, in which the round window membrane was assumed to have a relatively high impedance. A model of the common ground configuration was developed using finite electrode impedances. Finite impedances broaden the potential distributions in this model. Potential distributions from the lumped element model are compared with those obtained with an analytical model, to suggest ways in which focused and unfocused stimuli can affect the excitation of neurons in the implanted ear.