Psychophysics of a prototype peri-modiolar cochlear implant electrode array

Psychophysical measurements were performed in three hearing-impaired adult subjects: implanted with a CI22 cochlear prosthesis (Cochlear Ltd.) fitted with a developmental peri-modiolar electrode array. The array was manufactu red with a curvature approximating that of the inner wall of the scala tymp ani but, after straightening and insertion, lay on average about half way b etween the inner and outer walls of the scala. All subjects were tested wit h bipolar stimulation; two were also tested with monopolar, employing the m ust basal electrode as the return. Maximum comfortable level and threshold reduced with decreasing distance of electrode from the modiolus, whereas dy namic range increased. The linearity of the loudness growth function did no t vary significantly with electrode position but the function was more non- linear for lower maximum comfortable levels. Current level discrimination, normalized with respect to dynamic range, improved with decreasing distance of electrode from the modiolus in two subjects. Pitch varied regularly wit h insertion depth of the stimulated electrode for bipolar stimulation in tw o subjects and also for monopolar stimulation in one subject. Electrode dis crimination was enhanced by closeness to the modiolus. Whereas the forward masking patterns for bipolar stimulation of electrodes close to the modiolu s had a sharp double-peaked structure, those for monopolar stimulation were flatter and had a single peak. (C) 2001 Elsevier Science B.V. All rights r eserved.