Bilateral Cochlear Implant and Bimodal Hearing with Cochlear Implant and
Hearing Aid 

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 and 4 bilateral supplied subjects was carried out
to assess speech understanding and localization ability. The bimodal
subject group was provided with the same type of hearing aid. 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. All subjects of the
bilateral cochlear implant subject group displayed localization ability
with varying precision. One bilateral supplied subject showed a
localization accuracy of 10 degrees. An explanation for these remarkable
results is difficult as the bilateral cochlear implants are controlled by
two independent speech processor devices running without a common clock,
thus interaural time difference cues seem to be distorted. Interaural
level differences seem to carry enough directional coding information
although different compression schemes might distort the interaural
envelope differences to some extent.