As for invasive cochlear nerve electric stimulation, cochlear implant is the most effective method that has become the standard of care for patients with hearing disorder. Alongside, the tympanic membrane, external auditory canal may serve as a port for improved efficiency of non-invasive stimulation. We tested the hypothesis that electric stimulation of the cochlea nerve may restore neural activities that are missing in acoustic stimulation. We have also tested a new 32-channel cochlear implant device in minipigs. The Electric Auditory Brainstem Response (EABR) was recorded to verify functionality of the cochlear nerve stimulation.
We placed an electrode in the ear canal or on the tympanic membrane in 25 human adults (10 females) and compared their stimulation efficiency by characterizing the electrically-evoked auditory sensation. Relative to ear canal stimulation, tympanic membrane stimulation was four times more likely to produce an auditory percept, required 8 times lower electric current to reach the threshold and produced 2-4 times more linear suprathreshold responses. We further measured tinnitus suppression in 14 of the 25 subjects who had chronic tinnitus. Compared with ear canal stimulation, tympanic membrane stimulation doubled both the probability (22% vs. 55%) and the amount (−15% vs. −34%) of tinnitus suppression.
After cochlear implant in 3 minipigs, we measured EABR to observe how response amplitude changed with stimulation pulse width, current level (CL), and over time. EABR amplitude was 1.56 times larger when stimulus pulse width increased from 25 µs/pulse to 50 µs/pulse. EABR threshold was significantly higher in the 25 µs/pulse group: 740 ± 242 µA and 513 ± 136 µA, respectively. The input-output growth was fitted to a sigmoidal exponential function (amplitude = max / (1 + exp(-k * (CL - CL0)))). The inclination (k) was significantly greater in the apical turn channels (112.80 ± 0.05 µA) than in the basal turn channels (57.14 ± 0.06 µA). The maximum EABR amplitude increased over two months: 0.552 ± 0.377 µV in the 1st month and 1.080 ± 0.647 µV in the 2nd month (25 µs/pulse).
Together, the previous and present results suggest that the efficiency of conventional scalpbased noninvasive electric stimulation can be improved by at least one order of magnitude via tympanic membrane stimulation. The present findings have implications as an alternative to interventions using invasive electrical stimulation such as cochlear implantation. By cochlear implantation, the EABR amplitude was significantly larger when stimulus pulse width increased, and the duration of implantation was longer over two months. The sigmoidal input-output growth was 1.97 times steeper in apical turn electrodes than in basal turn electrodes. These outcomes provide insight into understanding neural response with a 32-channel CI system.