Objective: Cochlear microphonic responses (CMs) were measured in a rabbit m
odel in the intact ear (CM1) and in ears with a partially implantable piezo
electric middle ear implant (P-MEI) (CM2) to investigate the characteristic
s of speech transmission of the P-MEI in vivo.
Design: The spectra of pure tones, voices, and elicited CM1 and CM2 obtaine
d from the round window before and after the implantation of a P-MEI device
were calculated by using fast Fourier transform. Frequency response functi
ons of CM1 and CM2 were used to demonstrate the functional similarity betwe
en the implanted P-MEI and the normal ossicular chain. The coherence functi
ons between the voices and CM1 and between CM1 and CM2 were evaluated to ch
aracterize speech transmission of the P-MEI in vivo. Ten rabbit ears were u
sed in this study. Pure tones, six Chinese vowels, and six Chinese characte
rs were the acoustic stimuli. The CMs elicited by a list of bisyllabic word
s were tape-recorded and then recognized by subjects with normal hearing.
Results: Using pure tones at the same intensity of 90 dB SPL, frequency res
ponse functions of the CMs between the two states (the intact ear with norm
al hearing and ears with a P-MEI device at the medium volume) were calculat
ed showing great resemblance in shape. Compared with that at 1 kHZ, gain fa
ctors were 10 and 20 dB, respectively, at higher frequencies. The correlati
on and spectral analyses of the vocalizations, CM1 and CM2, demonstrated th
at the harmonics of CM1 were approximately identical as those of the voices
between 0.5 and 5.0 kHz with coherence functions of about 0.7 similar to 1
at the formants' frequencies, whereas the harmonics of CM2 between 0.5 and
2.5 kHz were enhanced with the coherences near to unity at the formants' f
requencies, and others <0.5 kHz and >2.5 kHz were attenuated. The recogniti
on score of the CMs elicited by a list of bisyllabic words was >90% using s
ubjects with normal hearing.
Conclusions: Data from this study suggest that cochlear microphonic potenti
als can be used as an important tool to evaluate objectively whether the im
plantation of a P-MEI device is successful and whether the quality in speec
h transmission of the P-MEI is satisfactory. Thus, the method would be of s
ignificance to clinically ascertain the performance of the device in vivo.