Far-field responses to stimulation of the cochlear nucleus by microsurgically placed penetrating and surface electrodes in the cat

Object. A new generation of penetrating electrodes for auditory brainstem i mplants is on the verge of being introduced into clinical practice. This st udy was designed to compare electrically evoked auditory brainstem response s (EABRs) to stimulation of the cochlear nucleus (CN) by mi cro surgically implanted surface electrodes and insertion electrodes (INSELs) with stimula tion areas of identical size. Methods. Via a lateral suboccipital approach, arrays of surface and penetra ting microelectrodes with geometric stimulation areas measuring 4417 mum(2) (diameter 75 mum) were placed over and inserted into the CN in 10 adult ca ts. After recording the auditory brainstem response (ABR) at the mastoid pr ocess, the CN, and the level of the inferior colliculus, EABRs to stimulati on of the CN were recorded using biphasic, charge-balanced stimuli with pha se durations of 80 mu sec, 160 mu sec, and 240 mu sec at a repetition rate of 22.3 Hz. Waveform, threshold, maximum amplitude, and the dynamic range o f the responses were compared for surface and penetrating electrodes. The EABR waveforms that appeared for both types of stimulation resembled ea ch other closely. The mean impedance was slightly lower (30 +/- 3.4 k Ohm c ompared with 31.7 +/- 4.5 k Ohm, at 10 kHz), but the mean EABR threshold wa s significantly higher (51.8 muA compared with 40.5 muA, t = 3.5, p = 0.002 ) for surface electrode arrays as opposed to penetrating electrode arrays. Due to lower saturation levels of the INSEL array, dynamic ranges were almo st identical between the two types of stimulation. Sectioning of the eighth cranial nerve did not abolish EABRs. Conclusions. Microsurgical insertion of electrodes into the CN complex may be guided and monitored using techniques similar to those applied for impla ntation of surface electrodes. Lower thresholds and almost equivalent dynam ic ranges indicate that a more direct access to secondary auditory neurons is achieved using penetrating electrodes.