Genetic predisposition and sensory experience in language development: Evidence from cochlear-implanted children

Abstract

Recent neurobiological studies have advanced the hypothesis that language development is not continuously plastic but is governed by biological constraints that may be modified by experience within a particular time window. This hypothesis is tested based on spontaneous speech data from deaf cochlear-implanted (CI) children with access to linguistic stimuli at different developmental times. Language samples of nine children who received a CI between 5 and 19 months are analysed for linguistic measures representing different stages of language development. These include canonical babbling ratios, vocabulary diversity, and functional elements such as determiners. The results show that language development is positively related to the age at which children get first access to linguistic input and that later access to language is associated with a slower-than-normal language-learning rate. As such, the positive effect of early experience on the functional organisation of the brain in language processes is confirmed by behavioural performance.

Keywords:

• Sensitive periods
• Development of grammar
• Hearing impaired
• Cochlear implantation
• Biological constraints
• Environmental factors
• Sensory input deprivation
• Language production
• Experience
• Deaf

Acknowledgements

This work was supported by grants from the Netherlands Organisation for Scientific Research (NWO VIDI grant # 276-75-004 to the first author) and from the Fund for Scientific Research Flanders (to all authors).

Notes

¹An additional alternative method for measuring vocabularity diversity was used to exclude any remaining effects of sample size. This alternative method is based on a mathematical model of the curvilinear relationship between the size of a language sample and the range of vocabulary it contains. It has been implemented in a software program, vocd (Mc Kee, Malvera, & Richards, 2000) by which the value of a single parameter D is obtained. This value representing the best fit of the theoretical curve to the empirical curve is derived through a series of TTRs by randomly sampling words and has been shown to be a highly reliable measure of vocabulary diversity.

²The outcomes of the alternative lexical diversity measure D were clearly in line with those based on TTR. At 24 months for instance, CI children have a median D value of 9.847 (range 3.05–60.71), whereas hearing children have a median of 35.48 (range 20.94–50.22). The difference between both populations is statistically significant (MWU p=.004).