Download 13th International Conference on Cochlear Implants and Other

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Transcript
P2-10 Sound coding
P2-10-1
The prerequisites for language acquisition: how congenitally deaf children process vowel length after
cochlear implantation - an EEG study
1,2
2
Vavatzanidis N.K. , Hahne A. , Mürbe D.
2
1
Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 2University Hospital Dresden, Saxonian Cochlear Implant
Center, Dresden, Germany
Background: Congenitally deaf and severely hearing-impaired children can get access to hearing when
receiving a cochlear implant - a neuroprosthesis that directly stimulates the auditory nerve. If implanted at a
young age (< 4 years), chances are good for acquiring normal oral speech, despite the prolonged absence of
any auditory stimulation and the non-natural input.
Understanding what infants actually hear with the implant in the critical age of language acquisition would help to
both understand the auditory system and its plasticity after an input-deprived period as well as how language
acquisition evolves if it starts considerably later than normal due to the absence of sensory input. This is even
more the case when considering studies with normal hearing children where a lack of sensitivity to basic auditory
cues such as vowel length or syllable stress co-occurs with later language impairment (Friedrich et al., 2004,
2009). In order to assess which features children process when presented with their first auditory input, we used
electroencephalography. We were especially interested in auditory features essential for further language
development and how the processing of these features develops in the first months of implant use. We thus
began investigating vowel length as one of the most basic but linguistically relevant cues. In German lengthening
of a vowel can be both semantically relevant as well as a marker of syllable stress, which is relevant for speech
segmentation and thus for language acquisition.
Methods: 14 congenitally deaf children (age at implantation: 0;9-3;7 years, mean: 1;7 years - time of implant
use: 0-10 months) were tested repeatedly electrophysiologically: 1) before the implantation as a baseline
condition, 2) directly after first fitting and after 3) two, 4) four, 5) six and 6) eight months of implant use. Syllables
with either short or long vowel duration were presented in a classical oddball paradigm to elicit the mismatch
negativity (MMN) (adopted from Friederici et al., 2002). A control group matched gender and age of implanted
children measured after 4 months of implant use.
Results: 2 months after the first hearing experience with the implant, the deviating long stimulus elicited a typical
negative deflection in the difference wave (effect of stimulus length: p < 0.001). No mismatch response was
detectable for the implanted children pre-operatively and at the time of first fitting. After four months of implant
use, the ERPs were strikingly similar to those of the control group.
Conclusion: Whereas directly after first activation of the implant there is no sign of discrimination between long
and short syllables, a robust response can be seen after 2 months of hearing experience. Already after four
months the discriminative response of implanted children resembles that of age peers. Thus one of the
fundaments for further language acquisition is laid after a relatively short period of time.
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