Download 13th International Conference on Cochlear Implants and Other

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Transcript
S14-11
Application of a test measuring frequency modulation difference limen as a tool to assess processing of
temporal fine structure information in cochlear implant patients
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Majchrzak A. , Lorens A. , Zgoda M. , Putkiewicz J. , Skarzynski H.
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World Hearing Center, Institute of Physiology and Pathology of Hearing, Kajetany/Warsaw, Poland
Among limitations of present-date systems one of the most important is a deficit in representation of fine
structure information. Studies on signal decomposition and the importance of its components (envelope and fine
structure) indicate temporal fine structure (TFS) to be significant for music and speech reception in difficult
listening conditions.
Therefore, representing fine structure information by cochlear implant systems in a way that it can be perceived
and utilized by patients may allow further improvement of outcomes. Temporal fine structure can be represented
by especially designed processing strategies, which have been an active area of research in recent years. In
patients after PDT temporal fine structure information is contained in preserved low-frequency hearing. However
so far there is only a limited number of studies on how and to what extent cochlear implant users can take
advantage of this information. Thus, the extent to which TFS information is available to implanted patients is in
present unknown. Evaluation of access to fine time structure information by an individual patient can provide
optimal choice of a stimulation method (EAS with or without the frequency overlap, EC or ES), as well as
detailed parameters of electrical stimulation (including in particular the speech processing strategy).
A method that has been chosen for this purpose is a frequency modulation difference limen (FMDL)
measurement, as there is a body of evidence for a temporally based mechanism in frequency modulation
detection for low-frequency carriers (deteriorating with increasing frequency and breaking down at about 4-5
kHz), particularly at low-modulation rates (< 20 Hz). FMDL test was created using forced choice adaptive
procedure with adjustable parameters and graphical interface.
Measurements using the test were conducted with basic signal frequency parameters: carrier frequency 250 and
1000 Hz, modulation rate 2 Hz, signal level selected individually by the patient as most comfortable. A threealternative forced choice 2-down 1-up adaptive procedure was used with feedback given to the patient. Results
of normal hearing were obtained as reference values. Preliminary measurements in cochlear implant patients
(with EAS, EC and ES) were conducted showing FMDL test can be a useful tool to evaluate access to TFS
information. Further research using the test may give important notes on possibilities of utilizing fine structure
information by cochlear implant patients.
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