Abstract
While noninvasive brain stimulation is convenient and cost effective, its utility is limited by the substantial distance between scalp electrodes and their intended neural targets in the head. The tympanic membrane, or eardrum, is a thin flap of skin deep in an orifice of the head that may serve as a port for improved efficiency of noninvasive stimulation. Here we chose the cochlea as a target because it resides in the densest bone of the skull and is adjacent to many deep-brain-stimulation structures. We also tested the hypothesis that noninvasive electric stimulation of the cochlea may restore neural activities that are missing in acoustic 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 eight times lower electric current to reach the threshold and produced two-to-four 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. These findings extended previous work comparing evoked perception and tinnitus suppression between electrodes placed in the ear canal and on the scalp. Together, the previous and present results suggest that the efficiency of conventional scalp-based noninvasive electric stimulation can be improved by at least one order of magnitude via tympanic membrane stimulation. This increased efficiency is most likely due to the shortened distance between the electrode placed on the tympanic membrane and the targeted cochlea. The present findings have implications for the management of tinnitus by offering a potential alternative to interventions using invasive electrical stimulation such as cochlear implantation, or other non-invasive transcranial electrical stimulation methods.