The Polarity Sensitivity of the Electrically Stimulated Human Auditory Nerve Measured at the Level of the Brainstem

Abstract

Recent behavioral studies have suggested that the human auditory nerve of cochlear implant (CI) users is mainly excited by the positive (anodic) polarity. Those findings were only obtained using asymmetric pseudomonophasic (PS) pulses where the effect of one phase was measured in the presence of a counteracting phase of opposite polarity, longer duration, and lower amplitude than the former phase. It was assumed that only the short high-amplitude phase was responsible for the excitation. Similarly, it has been shown that electrically evoked compound action potentials could only be obtained in response to the anodic phases of asymmetric pulses. Here, experiment 1 measured electrically evoked auditory brainstem responses to standard symmetric, PS, reversed pseudomonophasic, and reversed pseudomonophasic with inter-phase gap (6 ms) pulses presented for both polarities. Responses were time locked to the short high-amplitude phase of asymmetric pulses and were smaller, but still measurable, when that phase was cathodic than when it was anodic. This provides the first evidence that cathodic stimulation can excite the auditory system of human CI listeners and confirms that this stimulation is nevertheless less effective than for the anodic polarity. A second experiment studied the polarity sensitivity at different intensities by means of a loudness balancing task between pseudomonophasic anodic (PSA) and pseudomonophasic cathodic (PSC) stimuli. Previous studies had demonstrated greater sensitivity to anodic stimulation only for stimuli producing loud percepts. The results showed that PSC stimuli required higher amplitudes than PSA stimuli to reach the same loudness and that this held for current levels ranging from 10 to 100 % of the dynamic range.

Tinnitus Suppression by Intracochlear Electrical Stimulation in Single-Sided Deafness: A Prospective Clinical Trial - Part I

“…However, caution is required as anodic first stimulation was applied only twice per subject. A difference in tinnitus suppression between cathodic and anodic first charge-balanced electrical stimulation was initially expected as prior studies showed that anodic stimulation could be more sensitive than cathodic stimulation [Undurraga et al, 2010[Undurraga et al, , 2013Carlyon et al, 2013]. The cathodic phase is thought to stimulate the peripheral process while the anodic phase probably excites the nerve fibers more centrally [Rattay et al, 2001].…”

Reduction of stimulation coherent artifacts in electrically evoked auditory brainstem responses

“…However, alternating polarity is not an option for some investigations, for instance, at the most peripheral stages the response is polarity dependent [10]. If the desired eABR investigation requires a specific polarity, other means of artifact reduction are required.…”

“…Alternating polarity stimulation was introduced in previous publications [7][8][9][10][11] for electric artifact suppression, especially for the rejection of post pulse artifact. However, alternating polarity is not an option for some investigations, for instance, at the most peripheral stages the response is polarity dependent [10].…”

“…Fig. 3, such as CI trigger on and trigger off artifacts, CI pulse artifact (the huge artifacts during and post the pulse [7][8][9][10][11]), FNS artifact and the wave eV of the eABR. The waves eIII-eV of eABR are one of our main focuses, which are characterized by the latencies and the amplitudes of the peaks.…”

“…EABR as well as ABR are commonly obtained by averaging responses to increase signal-to-noise ratio (SNR), but because of the electric CI stimulation, some artifacts are synchronous with stimulation and cannot be removed by ensemble averaging. These phase locked stimulus artifacts are the important sources of distortion in eABR recordings [7][8][9][10][11][12], they overlap with the evoked response in both the time and frequency domains, such that conventional time windowing and frequency filtering are incapable of removing stimulus artifacts without distorting the evoked response. There are several studies on how to remove some of the CI related artifacts, such as the strong artifact during the pulse and the exponentially decaying post pulse artifact [7][8][9][10][11].…”