Optimizing the Clinical Fit of Auditory Brain Stem Implants Christopher J. Long, Ian Nimmo-Smith, David M. Baguley, Martin O’Driscoll, Richard Ramsden, Steven R. Otto, Patrick R. Axon, and Robert P. Carlyon Objective: To develop and implement a new audio- logical fitting procedure for auditory brain stem implants (ABIs), based on an efficient algorithm, and to compare it with two procedures presently used in clinical practice. Design: First, the different procedures were com- pared by using computer models and simulations with normal-hearing subjects (N  4). This allows for an analysis of the accuracy of the procedures in a way that is not possible when testing ABI users. The root-mean-square error between the order es- timated by the procedure and the true order was calculated. In addition, ABI users (N  2) were tested with the new procedure to see if it could be successfully applied in clinic. The degree of vari- ability of their results across runs and sessions was analyzed. Results: The tests of the normal-hearing subjects showed that our proposed procedure required sig- nificantly fewer trials (22 on average) than proce- dures presently used in clinic (with 76 and 234 trials on average for the two other procedures tested) to produce the same degree of accuracy. Computer modeling also demonstrated this advantage. Addi- tional testing showed this advantage was main- tained under a variety of conditions relevant to the clinic. The two patients tested were able to use this procedure with success, even though they were poor at discriminating the pitch of electrodes. The patients showed results consistent with having about 4 to 5 discriminable groups of electrodes with the 12 to 14 electrodes tested. Conclusions: The proposed procedure requires fewer trials to produce a clinically useful result and is well tolerated in the clinic. An additional advan- tage is that it allows testing to be broken down into several “blocks,” each containing a small number of trials. If the variability between blocks is small, information can be combined across blocks to in- crease the accuracy of the result. If the variability is large, perhaps between blocks on different days, this may reflect a significant change in the percepts generated by the implant, and signal to the clinician that a significant alteration in the fitting is re- quired. We recommend its use in ABI user fitting and in cochlear implant fitting when pitch ranking is problematic. (Ear & Hearing 2005;26;251–262) For many patients with profound sensory hearing loss of cochlear origin, cochlear implants can restore useful hearing by electrically stimulating the resid- ual neural structures, probably the spiral ganglion cells. For a cochlear implant to function, it requires the presence of the auditory nerve to connect the cochlea to the brainstem. Neurofibromatosis type II (NFII) is a condition characterized by bilateral tu- mors of the auditory-vestibular nerves (vestibular schwannomas). Patients with NFII nearly always have a severe-to-profound hearing loss due either to the destruction of the auditory nerves by the tumors themselves or by the surgery to remove the tumors. For such people, some hearing ability may be re- stored by an auditory brain stem implant (ABI). ABIs do not provide the same level of benefit as multi-channel cochlear implants. One reason for this may be that the ABI, which is typically placed over the surface of the dorsal cochlear nucleus, does not usually excite neural elements in a tonotopically ordered way, perhaps because many of the cells and their dendrites run parallel to the surface (Moore, 1987). This contrasts with the situation for cochlear implants, in which it is known a priori that elec- trodes positioned in the basal end of the cochlea will produce higher pitches than those located in the apical end. Accordingly, clinicians have to rely on ABI patients’ reports of the relative pitches pro- duced by each electrode to map higher-frequency bands of sound onto electrodes that evoke progres- sively higher pitches. This approach has been shown to provide significant benefits to speech perception: For example, Otto (Reference note 1) compared speech perception in users of the Nucleus ABI (Co- chlear Limited) with the electrodes assigned to fre- quency channels in a pitch-ranked order with that when the assignment was random. The pitch rank- ing produced substantial improvements in tests of speech perception; from 20 to 55% correct for vowel MRC Cognition and Brain Sciences Unit (C.J.L., I.N.-S., R.P.C.), Cambridge, United Kingdom; Department of Oto-Neurological Surgery (D.M.B., P.R.A.), Addenbrooke’s Hospital, Cambridge, United Kingdom; Manchester Royal Infirmary (M.O., R.R.), Manchester, United Kingdom; and Auditory Implants and Per- ception (S.R.O.), House Ear Institute, Los Angeles, California, 0196/0202/05/2603-0251/0 • Ear & Hearing • Copyright © 2005 by Lippincott Williams & Wilkins • Printed in the U.S.A. 251