Electrically evoked auditory brainstem response: growth of response with current level.

The electrically evoked brainstem response (EABR) was measured in cochlear implant users who had received either the Ineraid multichannel implant or the Nucleus multichannel implant. Although both implants use a multi-electrode array, they are different in a number of ways. In the Ineraid system the electrodes can be accessed directly through a percutaneous plug and stimulation is generally on four different intracochlear electrodes relative to a common ground outside the cochlea. In the Nucleus implant stimulation is accomplished via an internal coil and stimulation is bipolar between pairs along the 22 electrode array. The ABR waveforms were similar for both groups of subjects, consisting of a series of 3 or 4 positive peaks at the highest levels of stimulation. Using the normal stimulation mode (bipolar for Nucleus and monopolar for Ineraid), users of both devices demonstrated an increase in response amplitude and a decrease in response latency with increases in current level. The threshold of response tended to be higher and growth of the response with level tended to be more gradual for Nucleus users than for Ineraid users. However, with bipolar stimulation for both implant types, when the stimulating electrodes were closely spaced the threshold of response was higher and the growth of amplitude with level was more gradual than the case where the electrodes were separated further. When bipolar stimulation and similar electrode spacing was used, the response growth and threshold were similar for both implant types. Results from neither device showed a strong correlation with performance on word recognition tests.