Electrical field imaging as a means to predict the loudness of monopolar and tripolar stimuli in cochlear implant patients

• Published in: Hearing research Vol. 270; no. 1; pp. 28 - 38
• Main Authors: Berenstein, Carlo K., Vanpoucke, Filiep J., Mulder, Jef J.S., Mens, Lucas H.M.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.12.2010; Elsevier
• Subjects: Acoustic Stimulation; Aged; Auditory Threshold; Biological and medical sciences; Cochlea; Cochlea - physiopathology; Cochlear Implantation - instrumentation; Cochlear Implants; Correction of Hearing Impairment - psychology; Ear and associated structures. Auditory pathways and centers. Hearing. Vocal organ. Phonation. Sound production. Echolocation; Ear, auditive nerve, cochleovestibular tract, facial nerve: diseases, semeiology; Electric Stimulation; Electrodes; Fundamental and applied biological sciences. Psychology; Hearing; Humans; imaging; Loudness Perception; Medical sciences; Middle Aged; Non tumoral diseases; Otorhinolaryngology. Stomatology; Persons With Hearing Impairments - psychology; Persons With Hearing Impairments - rehabilitation; Psychoacoustics; Psychophysics; Signal Processing, Computer-Assisted; Vertebrates: nervous system and sense organs; Z far mono; Z near; EFIm; ζ peak mono / tri; EFI; Z eff tri; Z eff mono; BEDCS; PACTS; RCF; ANOVA; Z far; MCL; RMS; Z eff; Z near mono; ζ current tri / mono; Human; Electrode configuration; Loudness; Basilar membrane; Implant; Growth; Electrophysiology; Complement; Inner ear; Organ of hearing; Cochlea; Field potential; Scala tympani
• ISSN: 0378-5955; 1878-5891
• DOI: 10.1016/j.heares.2010.10.001

Tripolar and other electrode configurations that use simultaneous stimulation inside the cochlea have been tested to reduce channel interactions compared to the monopolar stimulation conventionally used in cochlear implant systems. However, these “focused” configurations require increased current levels to achieve sufficient loudness. In this study, we investigate whether highly accurate recordings of the intracochlear electrical field set up by monopolar and tripolar configurations correlate to their effect on loudness. We related the intra-scalar potential distribution to behavioral loudness, by introducing a free parameter ( α) which parameterizes the degree to which the potential field peak set up inside the scala tympani is still present at the location of the targeted neural tissue. Loudness balancing was performed on four levels between behavioral threshold and the most comfortable loudness level in a group of 10 experienced Advanced Bionics cochlear implant users. The effect of the amount of focusing on loudness was well explained by α per subject location along the basilar membrane. We found that α was unaffected by presentation level. Moreover, the ratios between the monopolar and tripolar currents, balanced for equal loudness, were approximately the same for all presentation levels. This suggests a linear loudness growth with increasing current level and that the equal peak hypothesis may predict the loudness of threshold as well as at supra-threshold levels. These results suggest that advanced electrical field imaging, complemented with limited psychophysical testing, more specifically at only one presentation level, enables estimation of the loudness growth of complex electrode configurations. ► The electrical dynamic range is independent from the amount of current focusing, ► A peri-modiolar electrode array increases the effect of focusing and efficiency, ▶ The number of recruited nerve fibers increases linearly with increasing current levels, uniform neural survival assumed, ► Threshold and supra-threshold loudness growth corresponds with the peak of the intracochlear electrical field