Sound localization with combined use of a Cochlea-Implant and a hearing aid
in different ears 

Uwe Baumann (1), Bernhard U. Seeber (2)
(1) Klinikum Großhadern/ENT/Audiology, Univ. of Munich, Marchioninistr. 15,
    Munich, BY 81377 Germany,  
(2) AG Technische Akustik, MMK, Munich Univ. of Technology, Arcisstr. 21,
    Munich, BY D-80290 Germany

After successful cochlear implantation on one ear, some patients continue
to use their hearing aid at the opposite side. They report an improved
understanding of speech especially in noise as well as a better perception
of music when hearing aid and cochlear implant are worn in combination. A
survey with 11 bimodal subjects was carried out to assess speech
understanding and localization ability. Ten subjects were users of the
MED-EL Combi 40 plus implant, one subject with a COCHLEAR CI24M. The
subjects were provided with the same type of hearing aid (PHONAK PZ A4).
One of the two program options of the hearing aid made use of a directional
microphone. A week after the initial fitting a fine tuning of the hearing
aid program was performed and speech tests were conducted subsequently. The
localization tests were carried out in an anechoic room in complete
darkness. To minimize non-binaural hints a roving level paradigm was used.
Subjects pointed to the direction of sound incidence by use of a trackball
with a computer-controlled laser-pointer. The high precision of the method
permits the discussion of the displayed localization results in terms of
the accuracy of localization.  

Results: The additional usage of a hearing aid improved speech
understanding in 9/11 subjects of the bimodal supported group. Two subjects
with substantial residual hearing of the bimodal group also showed improved
localization ability, five subjects were able to differentiate the side of
sound origin. The use of the directional microphone of the hearing aid
improved speech understanding in some subjects. No influence on
localization performance was observed with the directional microphone
program. Interaural level differences seem to carry enough directional
coding information although different compression schemes might distort the
interaural envelope differences to some extent.