Modulation of auditory percepts by transcutaneous electrical stimulation
Transcutaneous, electrical stimulation with electrodes placed on the mastoid processes represents a specific way to elicit vestibular reflexes in humans without active or passive subject movements, for which the term galvanic vestibular stimulation was coined. It has been suggested that galvanic vestibular stimulation mainly affects the vestibular periphery, but whether vestibular hair cells, vestibular afferents, or a combination of both are excited, is still a matter of debate. Galvanic vestibular stimulation has been in use since the late 18th century, but despite the long-known and well-documented effects on the vestibular system, reports of the effect of electrical stimulation on the adjacent cochlea or the ascending auditory pathway are surprisingly sparse.
The present study examines the effect of transcutaneous, electrical stimulation of the human auditory periphery employing evoked and spontaneous otoacoustic emissions and several psychoacoustic measures. In particular, level growth functions of distortion product otoacoustic emissions were recorded during electrical stimulation with alternating currents (2 Hz, 1–4 mA in 1 mA-steps). In addition, the level and frequency of spontaneous otoacoustic emissions were followed before, during, and after electrical stimulation (2 Hz, 1–4 mA). To explore the effect of electrical stimulation on the retrocochlear level (i.e. on the ascending auditory pathway beyond the cochlea), psychoacoustic experiments were carried out. Specifically, participants indicated whether electrical stimulation (4 Hz, 2 and 3 mA) induced amplitude modulations of the perception of a pure tone, and of auditory illusions after presentation of either an intense, low-frequency sound (Bounce tinnitus) or a faint band-stop noise (Zwicker tone).
These three psychoacoustic measures revealed significant perceived amplitude modulations during electrical stimulation in the majority of participants. However, no significant changes of evoked and spontaneous otoacoustic emissions could be detected during electrical stimulation relative to recordings without electrical stimulation.
The present findings show that cochlear function, as assessed with spontaneous and evoked otoacoustic emissions, is not affected by transcutaneous electrical stimulation, at the currents used in this study. Psychoacoustic measures like pure tone perception, but also auditory illusions, are affected by electrical stimulation. This indicates that activity of the retrocochlear ascending auditory pathway is modulated during transcutaneous electrical stimulation.