Stiles, D.J., Bentler, R.A., & McGregor, K.K. (2012). The speech intelligibility index and the pure-tone average as predictors of lexical ability in children fit with hearing aids. Journal of Speech Language and Hearing Research, 55, 764-778.
This editorial discusses the clinical implications of an independent research study and does not represent the opinions of the original authors.
Despite advances in early hearing loss identification, hearing aid technology, and fitting and verification tools, children with hearing loss consistently demonstrate limited lexical abilities compared to children with normal hearing. These limitations have been illustrated by poorer performance on tests of vocabulary (Davis et al., 1986), word learning (Gilbertson & Kamhi, 1995; Stelmachowicz et al., 2004), phonological discrimination, and non-word repetition (Briscoe et al., 2001; Delage & Tuller, 2007; Norbury, et al., 2001).
There are a number of variables that may predict hearing-impaired children’s performance on speech and language tasks, including the age at which they were first fitted with hearing aids and the degree of hearing loss. Moeller (2000) found that children who received earlier aural rehabilitation intervention demonstrated significantly larger receptive vocabularies than those who received later intervention. Degree of hearing loss, which is typically defined in studies by the pure-tone average (PTA) or the average of pure-tone hearing thresholds at 500Hz, 1000Hz, and 2000Hz (Fletcher, 1929), has been significantly correlated with speech recognition (Davis et al., 1986; Gilbertson & Kamhi, 1995), receptive vocabulary (Fitzpatrick et al., 2007; Wake et al., 2005), expressive grammar, and word recognition (Delage & Tuller, 2007) in some studies comparing hearing-impaired children to those with normal hearing.
In contrast, other studies have reported that pure-tone average (PTA) did not predict language ability in hearing-impaired children. Davis et al. (1986) tested hearing-impaired subjects between five and18 years of age and found no significant relationship between PTA and vocabulary, verbal ability, reasoning, and reading. However, all subjects scored below average on these measures, regardless of their degree of hearing loss. Similarly, Moeller (2000) found that age of intervention affected vocabulary and verbal reasoning, but PTA did not. Gilbertson and Kamhi (1995) studied novel word learning in hearing-impaired children ranging in age from seven to 10 years and found that neither PTA nor unaided speech recognition threshold was correlated to receptive vocabulary level or word learning.
At a glance, it seems likely that degree of hearing loss should affect language development and ability, because hearing loss affects audibility, and speech must be audible in order to be processed and learned. However, the typical PTA of thresholds at 500Hz, 1000Hz, and 2000Hz does not take into account high-frequency speech information beyond 2000Hz. Some studies using averages of high-frequency pure-tone thresholds (HFPTA) have found a significant relationship between degree of loss and speech recognition (Amos & Humes, 2007; Glista et al., 2009).
Because most hearing-impaired children now benefit from early identification and intervention, their pure-tone hearing threshold averages (PTA or HFTPA) might not be the best predictors of speech and language abilities in every-day situations. Rather, a measure that combines degree of hearing loss as well as hearing aid characteristics might be a better predictor of speech and language ability in hearing-impaired children. The Speech Intelligibility Index (SII; ANSI,2007), a measure of audibility that computes the importance of different frequency regions based on the phonemic content of a given speech test, has proven to be predictive of performance on speech perception tasks for adults and children (Dubno et al., 1989; Pavlovic et al., 1986; Stelmachowicz et al., 2000). Hearing aid gain characteristics can be incorporated into the SII algorithm to yield an aided SII, which has been reported to predict performance on word repetition (Magnusson et al., 2001) and nonsense syllable repetition ability in adults (Souza & Turner, 1999). Because an aided SII includes the individual’s hearing loss and hearing aid characteristics into the calculations, it better represents how audibility affects an individual’s daily functioning.
The purpose of the current study was to evaluate the aided SII as a predictor of performance on measures of word recognition, phonological working memory, receptive vocabulary, and word learning. Because development in these areas establishes a base for later achievements in language learning and reading (Tomasello, 2000; Stanovich, 1986), it is important to determine how audibility affects lexical development in hearing-impaired children. Though the SII is usually calculated based on the particular speech test to be studied, the current investigation used aided SII values based on average speech spectra. The authors explained that vocabulary acquisition is a cumulative process, and they intended to use the aided SII as a measure of cumulative, rather than test-specific, audibility.
Sixteen hearing-impaired children with hearing aids (CHA) and 24 children with normal hearing (CNH) between six and nine years of age participated in the study. All of the hearing-impaired children had bilateral hearing loss and had used amplification for at least one year. All participants used spoken language as their primary form of communication. Real-ear measurements were used to calculate the aided SII at user settings. Because the goal was to evaluate the children’s actual audibility as opposed to optimal audibility, their current user settings were used in the experiment whether or not they met DSL prescriptive targets (Scollie et al., 2005).
Subjects participated in tasks designed to assess four lexical domains. Word recognition was measured by the Lexical Neighborhood Test and Multisyllabic Lexical Neighborhood Test (LNT and MLNT; Kirk & Pisoni, 2000). These tests each contain “easy” and “hard” lists, based on how frequently the words occur in English and how many lexical neighbors they have. Children with normal lexical development are expected to show a gradient in performance with the best scores on the easy MLNT and poorest scores on the hard LNT. Non-word repetition was measured by a task prepared specifically for this study, using non-words selected based on adult ratings of “wordlikeness”. In the word recognition and non-word repetition tasks, children were simply asked to repeat the words that they heard. Responses were scored according to the number of phonemes correct for both tasks. Additionally, the LNT and MLNT tests were scored based on number of words correct. Receptive vocabulary was measured by the Peabody Picture Vocabulary Test (PPVT-III; Dunn & Dunn, 1997) in which the children were asked to view four images and select the one that corresponds to the presented word. Raw scores are determined as the number of items correctly identified and norms are applied based on the subject’s age. Novel word learning was assessed using the same stimuli from the non-word repetition task, after the children were given sentence context and visual imagery to teach them the “meaning” of the novel words. Their ability to learn the novel words was evaluated in two ways: a production task in which they were asked to say the word when prompted by a corresponding picture and an identification task in which they were presented with an array of four items and were asked to select the item that corresponded to the word that was presented.
On the word recognition tests, the children with hearing aids (CHA) demonstrated poorer performance than the children with normal hearing (CNH) for measures of word and phoneme accuracy, though both groups demonstrated the expected gradient, with performance improving in parallel fashion from the hard LNT test through the easy MLNT test. There was a correlation between aided SII and word recognition scores, but PTA and aided SII were equally good at predicting performance.
On the non-word repetition task, which requires auditory perception, phonological analysis, and phonological storage (Gathercole, 2006), CHA again demonstrated significantly poorer performance than CNH, and CNH performance was near ceiling levels. PTA and aided SII scores were correlated with non-word repetition scores. Beyond the effect of PTA, it was determined that aided SII accounted for 20% of the variance on the non-word repetition task, which was statistically significant.
The receptive vocabulary test yielded similar results; CHA performed significantly worse than CNH and both PTA and aided SII accounted for a significant proportion of the variance.
The only variable that predicted performance on the word learning tasks was age, which only yielded a significant effect on the word production task. On the word identification task, both the CHA and CNH groups scored only slightly better than chance and there were no significant effects of group or age.
As was expected in this study, children with hearing aids (CHA) consistently showed poorer performance than children with normal hearing (CNH), with the exception of the novel word learning task. The pattern of results suggests that aided audibility, as measured by the aided SII, was better at predicting performance than degree of hearing loss as measured by PTA. Greater aided SII scores were consistently associated with more accurate word recognition, more accurate non-word repetition, and larger receptive vocabulary.
Although PTA or HFPTA may represent the degree of unaided hearing loss, because the aided SII score accounts for the contribution of the individual’s hearing aids, it is likely a better representation of speech audibility and auditory perception in everyday situations. The authors point out that depending on the audiometric configuration and hearing aid characteristics, two individuals with the same PTA could have different aided SIIs, and therefore different auditory experiences.
The results of this study underscore the importance of audibility for lexical development, which in turn has significant implications for further development of language, reading, and academic skills. Therefore, the early provision of audibility via appropriate and verifiable amplification appears to be an important step in the development of speech and language. The SII, which is already incorporated into some real-ear systems or is available in a standalone software package, is a verification tool that should be considered a standard part of the fitting protocol for pediatric hearing aid patients.
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