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Early Assessment of Speech Development in Cochlear Implanted Children

¹Simona Serban, ²Viorel Zainea

¹Institute of Phonoaudiology and Functional Surgery “Prof. Dr. D. Hociota”, Bucharest, ROMANIA

²University of Medicine and Pharmacy “Carol Davila”, Bucharest, ROMANIA

 

Abstract

Background: Studying cochlear implant with young children brings in fundamental methodological problems. Objective analysis of speech and sound production is a valuable tool in assessing young children during the first year after cochlear implantation. The first six months after device switching on are essential for the child’s accommodation with the cochlear implant.

The authors present the case of a girl with bilateral profound hearing loss who received cochlear implant at the age of 3. An objective analysis of her speech production was made during the 6 months post-implant activation.

The methodology included quantitative and qualitative analysis of sounds and speech. The speech production spectrograms indicate consistent spectral modifications. The multidimensional voice analysis shows that Jitter presents the best improvement.

Key words: cochlear implant, objective analysis, speech production, spectrograms, multidimensional voice analysis, Jitter

 

Correspondence:

Viorel Zainea

Institute of Phonoaudiology and Functional Surgery “Prof. Dr. D. Hociota”

Bucharest, ROMANIA

 

Background

A cochlear implant is a surgically implanted electronic device designed to provide hearing to patients with severe to profound sensor neural hearing loss. The device replaces the work of hair cells in the inner ear, when the hair cells do not work but the hearing nerve does. The cochlear implant consists of internal and external components. The external component is a speech processor that detects mechanical sound energy and converts it into a distinct code of electric stimuli. The internal receiver-stimulator accepts, decodes and transmits signals to an array of electrodes implanted within the scala tympani which stimulates the auditory nerve. The electrical impulses then travel along the auditory pathways to the brain. The cochlear implant remains the solution for hearing rehabilitation in children suffering from pre-lingual deafness, who prove they do not benefit from conventional hearing aids.

Early auditory deprivation has severe consequences on the individual by inducing:

- anatomic changes (degeneration of the auditory nerve, reducing the size of neurons in cochlear nuclei, superior olivary complex, loss of synaptic connections in the cortex, cortical reorganization)

- functional changes

- behavioural changes

Degenerative effects of deafness become manifest after the age of 3 and cerebral plasticity is affected after 6 years. Consequences of the sensorial deprivation can be avoided by early appropriate intervention, as close as possible to the moment of deafness appearance.

Restoring sensorial input by classic hearing aid rehabilitation or cochlear implant allows activations and reinforcements of neuron connections and colonisation of the cerebral territory.

Statistics show that 43 % of the implanted children at age 2 came to have normal speech at the age of 8-9 years old, compared to a percentage of only 16 % of the children who were implanted before the age of 4 (1). For children who undergo cochlear implantation before 4 years of age, it becomes evident around 2 to 2 1/2 years after the operation whether a child develops language near normal or not (2).

In the first year of life, the normally hearing child goes through several speech development phases, from continuous phonation (weeks 0-6) to non-repetitive articulatory movements (after 1 year); table 1 (6).

table 2.1 1

Early implantation reduces speech development delay. Comparing early implanted children with their normally hearing pairs, Boolard shows that in implanted children:

  1.  vocabulary acquisition is twice as fast
  2. speech learning is also fast
  3. sentences are shorter
  4. the communication mode of implanted child becomes essentially auditory after 18 months of experience with implant

Cochard (7) distinguishes three evolution phases in post-implant speech development:

  1. rapid and homogenous increase of speech acquisition after implantation, followed by
  2. slow progress of the pre-linguistic phase and
  3. heterogeneous evolution with varied difficulties in speech development

The most difficult implant accommodation period is the first 6 months after implantation. Speech training during this period is extremely important for further performances of the child with cochlear implant.

Case History

We present the case of a girl with congenital bilateral sensor neural profound hearing loss. The deafness aetiology is unknown. The diagnosis of hearing loss was made at age of 2. The trial period of conventional amplification with powerful nonlinear hearing aids was without benefit. She underwent cochlear implantation at age of 3. The child did not babble before implantation and she used conventional signs to communicate. The device was activated 4 weeks after surgery and ACE strategy with 900 pps / electrode stimulation rate was selected for mapping. The auditory verbal training was administrated two times a week.

2.1 -1 

Figure 1: speech production at 2 months after implant activation

2.1-2 

Figure 2: speech production at 4 months after implant activation

2.1-3 

Figure 3: speech production at 6 months after implant activation

2.1-4 

Figure 4: first sentence recording

Material and Method

The child’s voice production was recorded with DAT equipment at 2, 4 and 6 months post implant activation.

Records were analyzed with respect to quantity and quality.

The quantitative analysis refers to speech elements and the rate of its changes during the first 6 months after the device activation. The qualitative analysis highlights changes of the formant structure and voice acoustic parameters. Qualitative analysis was performed with a computerised voice and speech analyser (CSL Kay, 4300 B) and Multi-Speech software version 2.01 (Kay Elemetrics Corp).

Spectrografic analysis used wideband spectrograms covering a frequency range from 0 to 17916 Hz. The filter bandwidth used for the spectrograms was 647 Hz. The spectrographic analysis was made on vowels /a/, /i/, /o/, /u/. The voice assessment was made on a sample of vowel /a/.

Results

The speech production inventory shows a gradual and fast acquisition of speech elements:

  1. 2 months after the activation: vowels are present (figure 1)
  2. 4 months after the activation, the following are added (figure 2):

consonants (fricatives, plosives, dentals)

repetitive articulatory movements

non-repetitive articulatory movements

  c. 6 months after the activation the first words are articulated (figure 3) and

  d. the first sentence is pronounced (figure 4);

The objective analysis of vocal emission and voice parameters, based on “a” sustained phonation indicates :

  1. changes of spectral characteristics (figure 5): the first formant and the fundamental frequency have a decreasing tendency, F1 going down from 1748 Hz to 1271 Hz, and Fo from 635 Hz to 317 Hz; the mean values in children are 1249 Hz for the first formant (F1) and 281 Hz for the fundamental frequency (F0)
  2. changes of voice parameters: multidimensional voice parameters diagrams (MDVP) show a reduction of Jitter and v Am values (figure 6)

2.1-5 

Figure 5: spectrogram changes of vowel

2.1-6

Figure 6: MDVP changes over the first 6 months


Discussion

After 4 months of exposure to sound the implanted child starts babbling. At 6 months after implant activation, she articulates words and sentences. The language inventory counts 20 words at 6 months of experience with cochlear implant. The child’s speech intelligibility for unfamiliar persons is 50%. The qualitative analysis of speech production indicates consistent spectral modifications. Even if there is an improvement of certain acoustic parameters (especially Jitter), the multidimensional voice analysis shows that the audio-phonator feedback is not complete.

Conclusions

The study of young children raises fundamental methodological problems when evaluating the efficiency of cochlear implants. The qualitative analysis of voice emission is a valuable method in evaluating the progress of cochlear implanted children during the first year following surgery. It provides useful information for the guidance of auditory verbal training and cochlear implant programming sessions.

 

Glossary of Terms

Formants = regions of prominent energy in the acoustic spectrum of a speech sound

Fundamental frequency = lowest component frequency in the acoustic spectrum of a speech sound

Formant 1 (F1) = first frequency region above fundamental frequency of prominent energy in the acoustic spectrum of speech sound

Spectrogram = short-term spectrum of a speech signal, displayed as a function of time

CSL = computerized speech laboratory = computerized voice analyzer manufactured by Kay Elemetrics Company

MDVP= multi dimensional voice program = specialized software for complex analyses of voice

Jitter = acoustic parameter that refers to the cycle to cycle variations in the period of the vocal fold waveform; it is also known as frequency variation

V Am = peak amplitude variation = shows the mean variation in the amplitude of the vocal fold vibration calculated on a sample of 11 periods

 

REFERENCES

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4.Oller, K. -. Metaphonology and infant vocalizations. In Lindblom, B. &Zetterstrom, R. eds. Precursors of early speech. New York: Stockton, 1986, 21–36

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