The role of sentence position, allomorph, and morpheme type on accurate use of s-related morphemes by children who are hard of hearing

Abstract

Purpose

Production accuracy of s-related morphemes was examined in 3-year-olds with mild-to-severe hearing loss, focusing on perceptibility, articulation, and input frequency.

Method

Morphemes with /s/, /z/, and /ɪz/ as allomorphs (plural, possessive, third person singular –s, and auxiliary and copula ‘is’) were analyzed from language samples gathered from 51 children (ages: 2;10–3;8) who are hard of hearing (HH), all of whom used amplification. Articulation was assessed via the Goldman-Fristoe Test of Articulation, 2^nd edition (GFTA-II) and monomorphemic word final /s/ and /z/ production. Hearing was measured via better ear pure tone average (BE-PTA), unaided Speech Intelligibility Index (SII), and aided sensation level of speech at 4 kHz (4kHz SL).

Results

Unlike results reported for NH children, the HH group correctly produced the /ɪz/ allomorph more than /s/ and /z/ allomorphs. Relative accuracy levels for morphemes and sentence positions paralleled that of NH children. 4kHz SL scores (but not BE-PTA or SII), GFTA-II, and word final s/z use all predicted accuracy.

Conclusions

Both better hearing and higher articulation scores are associated with improved morpheme production, and better aided audibility in the high frequencies and word final production of s/z are particularly critical for morpheme acquisition in children who are HH.

Children with mild through moderately-severe hearing losses have been shown to have difficulty with the acquisition of grammatical morphology and syntax (McGuckian & Henry, 2007; Koehlinger, Owen Van Horne & Moeller, 2013). However, the profile of grammatical use in children who are hard of hearing (HH) has not yet been carefully described. Furthermore, it is necessary to examine outcomes for this group separately from those of children with severe-profound hearing loss who utilize cochlear implants, given the differences in their auditory experiences. Because children who are HH and use hearing aids do not receive the same quality of input as normal hearing (NH) children, one might hypothesize that they would have particular difficulty with grammatical morphology. Some data tend to support this hypothesis (Koehlinger, et al., 2013; McGuckian & Henry, 2007) but other studies show that, as a group, children who are HH perform similarly to age-matched control groups (e.g., Norbury, Bishop & Briscoe, 2001).

The current study aims to describe the usage patterns observed for a set of morphemes that are likely to be difficult to hear because they are realized as /s/, /z/, or /ɪz/: third person singular –s, plural –s, copula and auxiliary ‘is’, and possessive –s. We seek to understand what factors influence use in HH children by examining characteristics of the morphemes that are known to influence accuracy, such as morpheme type, allomorph type, and sentence position and by examining child characteristics, such as audibility and articulation skills. Each of these child characteristics was examined in multiple ways to determine which would best capture the variance in morpheme use. By examining these utterance and child characteristics in relation to ostensibly similar morphemes, we may contribute to understanding the role of perceptibility and input on learning grammatical morphology.

Morpheme & Child Characteristics that Influence Acquisition in Typical Children

Allomorphic variation is one aspect of morphology that has been shown to influence acquisition. Phonetic variants of morphemes are called allomorphs. The allomorphs associated with –s related morphemes (i.e., 3^rd person singular -s, plural -s, possessive –s and forms of copula and auxiliary ‘is’) are all phonologically conditioned (Berko, 1958). When the inflection follows a voiceless sound, the morpheme is realized as an /s/ (e.g., books, stumps, the cat’s happy); following a voiced sound, the morpheme is realized as a /z/ (e.g., toys, roars, goes, the dog’s running). When the stem-final sound is an alveolar or alvelo-patalal fricative or affricate, the morpheme is produced as the full syllable /ɪz/ (e.g., witches, squishes, kisses, the princess is smiling).

Berko (1958) tested young NH children’s ability to apply these –s related inflectional markers to novel words. She observed that overall accuracy increased with age and that children more accurately formed novel plurals with the /s/ or /z/ allomorph as opposed to the syllabic /ɪz/ allomorph. This pattern of results was attributed to the fact that /s/ and /z/ allomorphs were more common in the input than /ɪz/. Input frequency has also been argued to influence the order of acquisition of different morpheme types (e.g., plural -s is 5 times more common than third person singular –s, Hsieh, Leonard, & Swanson, 1999), though other factors have also been considered (e.g., articulatory difficulty, Mealings, Cox, & Demuth, 2013).

Sentence position has also been shown to affect the use of grammatical markers. Hsieh, et al., (1999) found that inflectional markers are longer in duration when located in sentence final position than when located sentence medially and thus are easier to hear. Certain morpheme types also tend to occur in certain sentence positions due to the nature of English syntax. For instance, the plural –s inflection is not only more common overall but is also located sentence finally more often than the 3^rd person singular inflection, amplifying the input differences (Hsieh, et al., 1999). Sundara, Demuth, and Kuhl (2011) confirmed that children are sensitive to these duration differences via a grammaticality judgment task using a preferential looking paradigm. Children aged 22- and 27- months listened to sets of grammatical (He runs now, She sleeps) and ungrammatical (He run now, She sleep) sentences that differed only in the presence of the third-person singular morpheme. When located sentence finally, both groups were able to distinguish between grammatical and ungrammatical sentences. However, when the morpheme was located sentence medially, such a distinction could not be made by either group of children.

Although Hsieh et al., (1999) attributed the sentence position differences to problems with perceptibility, others attribute accuracy differences to articulatory complexity. Consonant clusters always result from the addition of the /s/ allomorph (e.g., likes) and regularly occur when /z/ (e.g., hides) is added to a stem as well if the word ends in a consonant. Consonant clusters are not formed when /z/ is appended to a vowel (e.g., plays) or when the syllabic allomorph, /ɪz/, is used (e.g., watches). Because of the distribution of word final phonemes, which are also known as codas, the syllabic allomorph, /ɪz/, is the least commonly used allomorph and the /z/ allomorph is the most common. The role of articulatory complexity has been examined by comparing production of different morphemes realized as singletons and consonant clusters in medial and final utterance position. Song, Sundara, and Demuth (2009) demonstrated that typically-developing NH children (mean age 2;2) produced the 3^rd person singular marker more accurately when it was located in a phonologically simple coda context (e.g. goes) as opposed to a complex coda context (e.g. kicks), suggesting that simpler articulatory contexts improve production. Unlike the third person singular results, word final consonant clusters did not affect production of plurals by two-year-old NH children. Plurals were affected by sentence position though, with greater accuracy observed sentence finally (Theodore, Demuth, & Shattuck-Huffnagel, 2011). Turning to the syllabic allomorph alone, greater difficulty was observed for production of plurals in sentence medial position than in sentence final position. This is arguably because young children have less time to plan and execute the morpheme and two-year olds are not yet proficient at this task (Mealings, et al., 2013). Likewise, possessives realized as clusters were likely to be reduced to a single phoneme when produced by typical two year olds and were marked through lengthening to ease the articulatory task and preserve the morphological information (Mealings & Demuth, 2014). Thus we see that morpheme type, sentence position, and allomorph may all interact to influence production accuracy in young NH children.

For NH children, developing articulation skills are also implicated in morphological production. Word final /s/ and /z/ develop gradually in NH children. In their most recent textbook, Bernthal, Bankson, and Flipson (2013) compile 10 studies of age of acquisition of English consonants into a single table. The phoneme /s/ is listed as acquired as early as 3;0 (the earliest age group studied in Dodd, Holm, Hua, & Crosbie 2003, Prather, Hendrick, & Kern, 1975, and Smit, Hand, Freilinger, Bernthal, & Bird, 1990) and as late as 5;0 (boys, Smit et al., 1990). Similarly, acquisition of /z/ ranges from age 3;0 (Dodd, et al., 2003; Prather et al., 1975) to age 7;0+ (Chirlian &Sharpey, 1982; Templin, 1957). These differences are attributed to the criteria for producing an adult like /s/ or /z/ (e.g., is a dentalized /s/ accepted?), the loss of front teeth, and the complexity of the words used to elicit the target sounds. Investigation of word final clusters is much less common. Nonetheless this suggests a high degree of variability in the acquisition of /s/ and /z/, two sounds which may influence children’s ability to produce the target morphemes.

Morpheme & Child Characteristics that Influence Acquisition in Children with Hearing Loss

Grammatical Morphology Use by Children with Hearing Loss

Previous research on grammatical morphology use has included late-identified children with varying degrees of hearing loss (e.g., moderate to profound), and has found a general lag in phonological and grammatical development (Dodd, Woodhouse & McIntosh, 1992; Elfenbein, Hardin-Jones & Davis, 1994). However, these results may not generalize to children with milder degrees of hearing loss or children who are identified and have received hearing aids early in life, which is the current standard of care (JCIH, 2007).

More recent work has focused on children who are HH. For instance, McGuckian and Henry (2007) studied the use of 10 grammatical morphemes by 10 children (mean age 7;4) with moderate hearing losses and a younger MLU-matched control group (mean age 3;2) with NH. HH children produced the grammatical markers possessive –s and plural –s less often than the younger control group but they produced irregular past tense, articles and progressive -ing more often. Given the 3–4 year age gap, this does not indicate normal grammatical morpheme use, but it does suggest that the phonemes involved in realizing the morpheme influence use. Koehlinger, et al., (2013) examined finite verb morphology use by 3- and 6-year-old children with hearing losses between 20 and 79 dB HL and NH children of the same age. At both ages, the NH children were more accurate in their use of verb-related morphemes than the HH children. However, the particular allomorph employed has the potential to influence the perceptibility of these forms in the input and the use of a composite measure may have obscured differences associated with morpheme type, allomorph, and sentence position.

Articulation Abilities in Children who are HH

Children who are HH also have difficulty with articulation of speech sounds relevant for producing morphemes, including /s/ and /z/. Gordon (1987) examined consonant production in 37 mainstreamed school-aged students with moderate to profound degrees of hearing loss. She found that children with average hearing levels less than 80 dB HL made fewer consonantal errors on a sentence production task than those with greater than 80 dB HL hearing levels. However, for both groups, fricative and affricate errors were the most common types. Elfenbein, et al., (1994) measured speech production skills on the Fisher-Logemann Test of Articulation in 40 school-aged students with moderate to severe hearing losses. Results again showed that the most common error types involved were fricatives and affricates.

More recent studies support the finding that fricative production continues to be particularly challenging for children who are HH. For instance, Moeller, McCleary, Putman, Tyler-Krings, Hoover, and Stelmachowicz (2010) followed four late-identified children with a moderate to severe hearing loss. Fricative errors were more common than any other type of speech production error with the exception of one participant (S2), whose fricative errors were as common as errors with other phonemes. Three out of four children omitted postvocalic /s/ and /z/ sounds at 4 years of age and two still had difficulty at age 5. Moeller, Hoover, Putman et al., (2007) followed 12 early-identified toddlers who were HH longitudinally, and found them to be delayed compared to age-matched peers with NH in spontaneous production of fricatives in babble and early words. McGowan, Nittrouer, and Chensusky (2008) examined the spontaneous vocalizations of ten, 12-month old early-identified infants with hearing loss (> 50 dB HL) compared to ten infants with NH. Using acoustic analysis methods, they found early evidence of differences between the groups in fricative production. Collectively, these results suggest the possibility that limited or inconsistent perceptual access to fricatives affects articulation skills, though data on children with mild-moderate losses are limited.

As mentioned previously, many morphemes in English are realized as word final fricatives and clusters. Traditionally within the SLI literature, children have been screened for articulation difficulties in order to ensure that language, rather than speech, is being assessed (Goffman, & Leonard, J. 2001; Rice & Wexler, 2001). This is less common in the HH literature ~ partially due to a sense that screening children for articulation may lead to inclusion of the least severely affected children and unintentionally minimize differences between HH and NH groups. Nonetheless, difficulty with articulation may cascade into difficulties with grammatical morphology. Consider the child, S2, who presented with persistent articulation difficulties across all phonemes (Moeller, et al., 2010). She was also the last to develop verb tense marking and continued to make morpheme omissions through 84 months of age. The other three subjects presented with age-appropriate grammatical development at much earlier ages and showed faster resolution of articulation difficulties.

Audibility and Input Frequency in Children who are HH

Recent studies indicate that the limited bandwidth of hearing aids affects children’s perception of fricatives like /s/ and /z/ and that audibility must be accounted for to understand acquisition patterns. Stelmachowicz, Pittman, Hoover and Lewis (2002) examined whether children who are HH perceive regular plural endings produced as an /s/ or /z/ and irregular plurals forms that change the stem of the word by asking children to point to pictures depicting a singular referent or a plural referent. HH children were better able to identify the correct target item when it was an irregular rather than a regular plural. Performance also improved when the target words were presented in a male voice with peak energy around 4 kHz as compared to a female voice with peak energy around 6 kHz. Taken together these findings suggest that morphemes realized as /s/ and /z/ are difficult for HH children to hear. This may be attributable to the effects of restricted hearing aid bandwidth, the fundamental frequency of adult and child voices in the environment, and ambient noise or distance from the speaker. Thus, children who are HH may not reliably perceive grammatical morphemes, even when properly fit with hearing aids.

McGuckian and Henry (2007) argue that reductions in how often morphemes are perceived in the input have an influence on grammatical morpheme production by children who are HH. Perceiving a morpheme may be affected by the child’s access to certain aspects of the speech stream, to how often and where that morpheme occurs in English, and to environmental aspects, such as speaker fundamental frequency, noise, reverberation, and distance from the speaker. In children with NH, production of grammatical morphemes is highly correlated with the frequency of production in child-directed speech (Theakston & Lieven, 2008). McGuckian and Henry (2007), following Larsen-Freeman and Long (1991), argue that children acquiring English from parents who are themselves learning English as a second language (ESL) receive inconsistent input and show altered morphological acquisition. They claim that HH children also are influenced by inconsistent input, albeit for different reasons than their NH ESL counterparts. When HH children fail to perceive a morpheme, it is as if their parents had never produced the morpheme in that context, reducing the input frequency of that morpheme or allomorph. This altered input functions much like the agrammatical input that children of ESL learners receive. They support this hypothesis with their finding that the order of acquisition of morphemes is highly correlated between children who are HH and children of ESL learners.

Thus, one might predict different patterns of use of the –s allomorphs for children who are HH than what is observed in NH children due to the HH group’s limited ability to hear high frequency sounds like /s/ and /z/. Specifically, they might be more accurate with the production of the syllabic /ɪz/ allomorph because the inclusion of a vowel in the morpheme makes it more likely to be in the audible range. This allows children who are HH to receive more consistent input for /ɪz/, despite the fact that the syllabic allomorph is less common in the input in general and more likely to be in error in NH children.

Articulation skills, in particular use of word final /s/ and /z/, may also affect children who are HH (Moeller et al., 2010). There is good evidence that NH children, like adults, change their production of morphemes to accommodate production challenges, reducing consonant clusters, lengthening the final fricative, and shortening the morpheme in sentence medial position. Similar effects may be observed in the HH group. Finally, this group may be particularly influenced by changes in morpheme perceptibility due to sentence position given that shortened morphemes may be especially hard for them to hear.

Research Questions & Hypotheses

Children who are HH have difficulties with the perception (Stelmachowicz, Pittman, Hoover & Lewis, 2001, 2002; Bow, Blamey, Paatsch, & Sarrant, 2004) and production of /s/ and /z/ as speech sounds (Elfenbein, et. al., 1994; Moeller, et al., 2010) and as grammatical morphemes (Koehlinger et al., 2013;McGuckian & Henry, 2007). Production of grammatical markers is related to frequency, that is, how common the grammatical marker is in the input, and perceptibility, which is linked to its position within a sentence (Hsieh et al., 1999; Song, et al., 2009; Sundara, et al., 2011). Work with typical children has shown that more frequent allomorphs are more accurately produced (Berko, 1958) despite the fact that the more frequent allomorphs are less perceptible, calling into question the role of perceptibility. What is frequent hinges critically on what is perceptible. Thus a corpus analysis does not directly address this question. Instead the answer must be inferred from children’s performance. We are interested in determining how articulation skills, sentence position, perceptibility, and input frequency influence grammatical morpheme acquisition in children with mild-to-severe HL. Thus we ask the following questions, using productions and omissions of possessive, plural, third person singular –s, and contracted and uncontracted forms of ‘is’ from spontaneous language samples as data.

1. Does allomorph type influence grammatical accuracy in children who are HH?

   We predict that the allomorph that is most accessible in the input will be most accurately produced. Unlike NH children, who tend to produce /s/ and /z/ allomorphs most accurately (Berko, 1958), we predict that children who are HH, as a group, are more likely produce the syllabic allomorph /ɪz/, rather than the briefer allomorphs /s/ and /z/.

2. Does sentence position influence grammatical accuracy in children who are HH?

   We predict that HH children, like NH children, will show greater accuracy when the grammatical marker is located sentence finally than those in sentence medial position because the morpheme will be easier to perceive at the end of the utterance. It should be noted that, given the structure of the English language, certain morphemes may distribute differently across different sentence positions.

3. To what extent does perceptual access influence morphological acquisition?

   Children who are HH do not all have the same hearing profile and access to audible speech. Thus we predict that hearing acuity and audibility of speech will influence morphological accuracy. We especially predict that auditory access to high frequency sounds will modulate the results of the allomorph and sentence position analyses described above.

4. To what extent does articulation skill influence morphological acquisition?

   We also predict that articulation skill will influence morphological accuracy. We predict that use of word final –s and –z will be more influential than global articulation skills and that both morphological accuracy and articulation skill will be influenced by the degree of hearing loss, particularly high frequency aided audibility in the child.

Methods

Participants

Children were selected from a larger pool of 115 HH 3-year-olds who participated in the Outcomes for Children with Hearing Loss (OCHL) project. The OCHL protocol and recruitment approach is described in Holte, Walker, Oleson, Spratford, Roush, Moeller, & Tomblin (2012). All children included here had persistent, bilateral hearing loss in the mild through severe range, no other known significant language or learning disorders (see Koehlinger et al., 2013, for additional participant data, including recruitment information), and used spoken English as the primary language of communication in the home. All of the children wore personal hearing aids (48 binaural air conduction, 2 monaural air conduction, 1 bone conduction). One of the children with a monaural air conduction fitting had asymmetric hearing loss, and the poorer ear was judged to be non-aidable.

Children were selected because 1) the child had a transcribed conversational language sample with at least 40 utterances and 2) the language sample contained a minimum of 4 contexts for morphemes realized as /s/, /z/, or /ɪz/, 3) the child had participated in a screening for ability to use /s/ and /z/ word finally, and 4) the child had complete articulation testing and audiometric data (see below). Four children met all other requirements but were excluded due to limited information about the ability to use word final s/z. Following these criteria, 51 HH 3–year-olds (age range: 2;10–3;8) were retained for analysis. All but 2 children retained for analysis had data from the Comprehensive Assessment of Spoken Language core 3–4 (CASL; Carrow-Woolfolk, 1999). The syntax subtest of the CASL core is reported here since this paper focuses on grammatical development. Standard scores are reported in Table 1.

Table 1.

Information about participants.

Variable	n	M	SD
Age	51	37.90	2.92
Hearing Measures
BE-PTA (dB HL)	51	49.89	11.80
BE unaided SII (%)	51	24.18	18.43
4k SL (dB SPL)	51	18.96	10.19
Speech/Language Measures
GFTA-II Raw	51	36.67	15.83
GFTA-II SS	51	89.64	16.70
CASL-Syntax	49	86.73	14.09
MLUw	51	2.38	0.72

Note: SS= Standard Score; GFTA= Goldman Fristoe Test of Articulation; BE-PTA= Better Ear Pure Tone Average; CASL= Comprehensive Assessment of Spoken Language

Data collection

Hearing measures

Audiological testing was completed on all children. Results are shown in Table 1. Conditioned play audiometry was used to obtain air conduction pure tone thresholds at 500, 1000, 2000 and 4000 Hz. These 4 values were averaged and the lower value was retained for the better ear pure tone average (BE-PTA). BE-PTA ranged from 31.25–82.5 dB HL (M= 49.89 dB HL) ¹.

A second value, the speech intelligibility index score (SII) (ANSI S3.5, 1997; Bentler, Cole, & Wu, 2011; French & Steinberg, 1947; Kryter, 1962) was also calculated for each ear and the higher score was retained for use in analyses. Broadly speaking, SII measures how much access the child has to the speech spectrum given the degree and configuration of their hearing loss, either with or without hearing aids (see Holte et al., 2013 and Koehlinger et al., 2013 for further information on how SII was calculated). Unaided SII for 65 dB input ranged from 0 to 74 in our data set, with possible values being from 0% (no access to speech) to 100% (complete access).

A third measure, aided sensation level (SL) at 4 kHz, was derived by computing the difference between the behavioral threshold at 4 kHz in dB SPL and the output of the hearing aid in dB SPL for the 1/3 octave band centered at 4 kHz. This is a measure of how audible the average speech spectrum is at 4 kHz with hearing aids, which may give a more specific estimate of high-frequency speech audibility than SII or BE-PTA (McCreery & Stelmachowicz, 2011). Scores for this measure ranged from −5 to 36 dB SPL.

Articulation Measures

All children retained for analysis had data from The Goldman Fristoe Test of Articulation 2^nd edition (GFTA-II; Goldman & Fristoe, 2000). Raw and standard scores from both the GFTA-II are reported in Table 1. The GFTA-II served as a global measure of articulation ability with the raw score (number of errors) used in the regression analyses below. It is worth remarking on the composition of the GFTA-II here given later measures that look exclusively at word final s/z use. The GFTA-II samples 39 consonants and clusters in English in various word positions for a total of 80 possible points. Although s/z appear in several words they are each only scored once in word final position (2/80 points). Thus using the GFTA-II as a continuous articulation measure was appropriate, but did not address the question of whether production of word final s/z, in particular, influenced morpheme accuracy.

Information about the use of word final s/z was gathered via an experimenter-created single word naming task for 44 participants. As a part of this task, children were tested on articulation via six mono-morphemic words with word final consonants [s] and [z] (bus, nose, cheese, horse, hose, buzz). Children were given picture cards and asked to name/identify the object on the card. Responses were scored as correct if children gave an approximation of the target phoneme (distortions were accepted as correct).

Eleven children did not participate in this task due to fatigue, examiner error, etc. In these cases, the participants’ language samples, described below, were analyzed for words that provided obligatory contexts for [s] and [z] as mono-morphemic final consonants (e.g., these, this, squeeze). We computed percent correct use of monomorphemic word final [s] and [z] for 4 to 7 opportunities. As mentioned previously, four of these 11 children who otherwise met criteria for participation did not have enough obligatory contexts (< 4) to separate articulation abilities from morphology skills and were excluded. Although we initially intended to use this as a continuous measure, the data were not normally distributed and therefore were treated categorically. Based on the criteria in the SLI literature, which typically uses a 80% cutoff (e.g., Leonard, Eyer, Bedore & Grela, 1997; Rice, Wexler & Herschberger, 1998), children were grouped into categories of poor (<80%) and good (>80%) use of word final [s] and [z] in later analyses. Table 2 shows the number of children in each group and the hearing information associated with these groups of children. Neither of these measures of word final –s and –z production assessed use in clusters directly.

Table 2.

Hearing data grouped by children with high and low articulation skills for word final s/z.

	>80% on artic		< 80% on artic
n	36		15
Hearing Measures	M	SD	M	SD	p-value
4kSL (dB SPL)	20.08	8.79	16.27	12.93	0.22
BE -unaided SII (%)	27.46	18.59	16.33	16.01	0.048
BE-PTA (dB HL)	48.38	11.4	53.52	12.33	0.16

Language sample elicitation

All children participated in 15-minute conversational language samples (following Hadley, 1998) in which children played with play-doh and talked with a parent and an examiner. Of the 51 children who participated in the full 3-year-old battery of testing, 26 HH children also had language samples from a 5-minute art gallery task following the protocol described in Adamson, Bakeman & Deckner (2004). Because number of obligatory contexts, t (49) = 1.55, p = .12, and average accuracy, t (49) = 1.16, p = .25, did not differ between cases where children had conversational samples only and conversational samples plus art gallery samples, these samples were added together when available in order to maximize the size of the language sample to be analyzed from each participant. Data collection methods and transcription conventions are reported in more detail in Holte, et al., (2013) and Koehlinger, et al., (2013). Generally speaking, samples were transcribed following SALT conventions (Miller & Iglesias, 2010), with some adjustments to enhance the ability to search for relevant grammatical morphology (e.g., copula and auxiliary were coded with unique codes, which is not standard SALT protocol). Inter-transcriber reliability was computed for 10% of the language samples for each of the following criteria: utterance boundaries (M= 92%, range, 89–95%), words produced (M= 95%, range, = 92–96%), coding for bound morphology (M= 88%, range, 85–90%).

The language samples allowed us to compute MLU in words (MLUw) for all subjects. MLUw, as opposed to MLU in morphemes, was used so as to avoid penalizing the children who presented with grammatical morpheme production difficulties. Information about MLUw is reported in Table 1. The average language sample contained 128.31 utterances (SD = 57.77, range = 40 – 360). Recall that children with fewer than 4 obligatory contexts or 40 utterances in the sample were excluded; some children who are HH may present with greater language learning challenges than might be assumed from the information here. Sixteen children had between 4 and 9 obligatory contexts; 27 had between 10 and 30 obligatory contexts; and 8 had more than 30 contexts. Table 3 shows the average number of obligatory contexts per child per morpheme type broken out by sentence position and allomorph type. The distribution of contexts is uneven across the various cells, generally reflecting natural distribution of these contexts in English. For example, plurals are more common utterance finally and copulas are more common medially. As we will return to later, our choice of statistical method was designed to deal with the fact that different children contributed different numbers of responses.

Table 3.

Information about the distribution of obligatory contexts for each morpheme across sentence position and allomorph contexts.

		Medial				Final				All Positions
Morpheme		-s	-z	-iz	All	-s	-z	-iz	All	All Allomorphs
Plural	M (SD)	1.56 (0.73)	2 (1.62)		2.31 (1.74)	2.25 (2.17)	4 (3.15)	1 (0)	4.85 (4.19)	5.9 (5.15)
	range	0–3	0–8		0–8	0–9	0–14	0–1	0–22	0–29
	N	9	23		26	20	44	2	46	48
Possessive	M (SD)	1 (0)	1.92 (1.56)		1.79 (1.48)		4.25 (5.85)		4.25 (5.85)	2.62 (4.38)
	range	0–1	0–5		0–5		0–13		0–13	0–18
	N	2	12		14		4		4	16
Copula	M (SD)	8.02 (6.91)	2.74 (1.83)	3.94 (4.88)	12.56 (10.69)			1.29 (0.76)	1.29 (0.76)	13.26 (11.58)
	range	0–33	0–7	0–25	0–45			0–3	0–3	0–51
	N	49	34	36	50			7	7	50
Auxiliary	M (SD)	2.08 (1.61)	2.59 (2.45)	1.17 (0.41)	2.79 (2.82)					2.79 (2.82)
	range	0–5	0–11	0–2	0–12					0–12
	N	13	17	6	28					28
3rd Singular	M (SD)	1.47 (0.99)	1.56 (0.86)	1 (0)	2.08 (1.63)	1 (NA)	1 (0)		1 (0)	2.11 (1.65)
	range	0–4	0–4	0–1	0–7	0–1	0–1		0–1	0–7
	N	15	18	2	25	1	5		5	27
All Morphemes	M (SD)	9.35 (7.80)	5.44 (4.83)	4.19 (5.05)	16.53 (13.24)	2.3 (2.18)	4.38 (3.51)	1.22 (0.67)	5.52 (4.95)	22.02 (17.25)
	range	0–34	0–21	0–26	1–56	0–9	0–14	0–3	0–23	4–74
	N	49	43	36	51	20	45	9	46	51

Note: The mean (M) and standard deviation (SD) of opportunities was calculated using only those (N) children who had at least 1 obligatory context available. The range includes all children. The distribution of values reflects the natural distribution of contexts in English (e.g., auxiliary is rarely heard utterance finally, even in adult speech).

Morphology Coding

All utterances that provided an obligatory context for morphemes realized as /s/, /z/, or /ɪz/ were extracted from the language samples using SALT searches and coded. Each morpheme (n=1,176) was coded for accuracy, morpheme type, sentence position, and allomorph. Accuracy was coded as correct or omitted. Although we planned to allow considerable variance to account for immature articulation skills, the majority of the productions coded as correct were reported by the transcribers to be adult-like approximation of the target (e.g. watch-ɪz, girl-z) and good reliability on transcription was achieved. Commission errors (e.g., They is happy) were discarded (n=27). Table 4 illustrates the coding scheme used with sample utterances marked for morpheme type, accuracy, allomorph type and sentence position.

Table 4.

Sample utterances coded for morpheme type, accuracy, sentence position and allomorph type.

Morpheme	Sample Utterance	Accuracy	Sentence Position	Allomorph Type
Plural –s	Two snake.	Omitted	Final	/s/
	Make some eggs.	Correct	Final	/z/
Possessive	Elmo’s daddy.	Correct	Medial	/z/
	Bert nose.	Omitted	Medial	/s/
3rd singular	He jumps.	Correct	Final	/s/
	It go fast.	Omitted	Medial	/z/
Copula is	This my bowl.	Omitted	Medial	/ɪz/
	Ernie’s all gone.	Correct	Medial	/z/
	That’s mine.	Correct	Medial	/s/
Auxiliary is	He coloring.	Omitted	Medial	/z/
	He is jumping.	Correct	Medial	/ɪz/
	This going to be the big one.	Omitted	Medial	/ɪz/

Morpheme type was coded as possessive, third person singular, plural, copula or auxiliary. If the morpheme type was not determinable from the utterance (e.g., My cook my ones.) the item was discarded (n=14).

Allomorph was coded as /s/, /z/, /ɪz/ and indeterminate. The allomorph was determined by the final phoneme of the target word stem in the transcription and not through acoustic analysis. For example, if a marker was preceded by a voiceless sound (e.g., book), then the allomorph was coded as /s/, word final voiced sounds were coded as /z/, and word final alvelo-patalal fricatives or affricates were coded as /ɪz/. If the allomorph was not determinable from the utterance as it was transcribed (e.g., J’s gonna cut your thing.) the item was discarded (n=12). These discards were primarily due to the fact that names were replaced with initials at the time of transcription in order to protect participant privacy. Contracted copula and auxiliary forms of ‘is’ were coded for allomorphs as described above. If the omission was an obligatory context that allowed contraction, it was treated as such. If a full form of ‘is’ was required due to the phonetic context (e.g., the princess happy), it was coded as the syllabic allomorph.

Sentence position was coded as initial, medial, and final. All sentence initial morphemes were overtly produced, uncontracted, forms of ‘is’ (e.g., is he happy?) and were discarded (n=26). Only 1 instance of elision contributed to the utterance final uses of ‘is’. Thus, medial and final forms of all morphemes were analyzed. 1,049 items (89.2% of the data) were retained for analysis. Due to the nature of English, sentence position and morpheme type are not independent; note, for instance, that there are no cases of auxiliary in utterance final position in our data. Statistical methods that account for uneven numbers of items per variable were selected with this in mind.

Statistical Methods

Our goal was to determine what child factors (hearing, articulation skills) and utterance level factors (morpheme of interest, allomorph, sentence position) are best predictors of accurate production of grammatical morphemes. A generalized linear mixed model (GLMM) with a logit link was used to relate the child factors to ability to produce grammatical morphemes. This approach weights the reliability of a factor based on how many data points are available, something that is critical given the unevenness of the data across children and morpheme types. A factor with more data has smaller confidence intervals and is more likely to be significant; a factor with less data has larger confidence intervals and is less likely to be significant. Thus type II errors are most likely given the distribution of our data. A random subject effect was used to account for the repeated measurements per child because the number of obligatory contexts was not consistent across children. A total of 1049 opportunities from 51 children were analyzed. Specifically, we wanted to determine the degree to which sentence position, morpheme, and allomorph affect probability of correct production. In addition, hearing abilities and articulation, as measured by word final use of s/z and raw GFTA-II scores, were expected to influence correct usage. Interactions were also investigated and kept in the model if significant. In the statistical analysis, word final use of s/z is treated as a dichotomous variable where a score greater than 0.8 was considered “good word final articulation” and a score of less than 0.8 was considered “poor word final articulation” because this measure was not normally distributed and could not be treated as a continuous variable in the model. This variable was dichotomized, rather than categorized another way, because that best reflected participants’ actual performance. GFTA-II, which met assumptions for normality, was entered as a continuous variable. Analyses were carried out in PROC GLIMMIX of SAS v9.3.

Results

As predicted, both utterance level properties and child characteristics influenced accuracy and these variables also interacted with each other, suggesting that both types of information are important for understanding language development in children who are HH. Results from the final model are given in Table 5. Using boxplots to help visualize the distribution of correct use, Figure 1 displays accuracy information by morpheme type and Figure 2 displays the same information for allomorph and utterance position. In both plots, children with only one instance (thus placing their accuracy either at 0% or 100%) were excluded to avoid skewing the visual presentation of the data, though these data were included in the regression model. At the level of the individual utterance, allomorph affected overall accuracy (p = 0.018), but sentence position was nonsignificant as a main effect (p=0.192).

Table 5.

Effects included in the final regression model.

Effect	Degrees of freedom	Chi-square value	p-value
Allomorph	2	8.04	0.018
Sentence Position	1	1.7	0.192
Morpheme	4	12.51	0.014
Word Final s/z skills	1	1.77	0.183
4k SL Hearing	1	16.1	<0.0001
GFTA-II Raw Score	1	12.7	0.0004
Allomorph*4kSL	2	12.23	0.002
Allomorph *Word Final s/z	2	6.86	0.03
Allomorph *GFTA	2	7.02	0.03
Sentence Position * Word Final s/z	1	9.81	0.002

Denominator Df = 1032

Figure 1.

Boxplot of accuracy of each morpheme type.

Note: Children who produced each morpheme only once were dropped from the figure to avoid skewing the visual representation of the data. The number of participants included for each morpheme is noted above the labels.

Figure 2.

Boxplot of accuracy of each allomorph in sentence medial and sentence final position.

Note: Children who produced each allomorph in a particular sentence position only once were dropped from the figure to avoid skewing the visual representation of the data. The number of participants included for each morpheme is noted in the legend.

Turning to children’s abilities, we saw that children’s articulation skills influenced accuracy through global articulation skills (GFTA-II raw scores, p=0.0004). Even though the main effects for sentence position and word final s/z skills (p=0.183) were not significant, they did significantly interact with other variables and thus were retained in the model. Of all three hearing measures, only the 4kHz SL measure (p<0.0001) was influential. Neither BE-PTA nor unaided SII were significant when entered alone or in concert with other variables and thus were excluded from the model.

Children’s hearing and articulation abilities led to different outcomes depending on the allomorph being produced, with allomorph type interacting with 4kHz SL (p=0.002), word final s/z skills (p=0.03), and GFTA-II raw scores (p=0.03). Sentence position also interacted with word final s/z skills (p = 0.002). These interactions modulate the main effects reported above and thus we discuss each in turn, beginning with allomorph type.

Although we had no specific predictions about morpheme type, it was included in the model to account for known variance associated with developmental trends. Morpheme type was significant (p=.014). Morpheme accuracy mirrored the profile reported for typically-developing children closely: plural and possessive were most accurate and auxiliary is and third person singular –s being the least accurate (see Table 6 for odds ratios and p-values). It was the only utterance level variable that did not interact with either another utterance level variable or with a child characteristic. Morpheme type was not evenly distributed across allomorph or sentence position, due to the natural distribution of these morphemes in English, which may have made it more difficult to detect these interactions. Elicited production methods would complement this approach to further explore this question.

Table 6.

Odds Ratios (above the diagonal) and p-values (below the diagonal) for morpheme by morpheme comparisons of accuracy

	Plural	Possessive	Copula	Auxiliary	3rd Singular
Plural	--	1.03	1.68	2.76	4.11
Possessive	.96	--	1.64	2.69	4.00
Copula	.13	.34	--	1.64	2.44
Auxiliary	.02	.08	.11	--	1.49
3rd Singular	.001	.02	.01	.35	--

Allomorph Type

Although there was a main effect of allomorph type, with /ɪz/ being more accurate on average than /s/ or /z/ forms (/z/ vs. /ɪz/ OR =0.33, p = .004; /s/ vs. /ɪz/ OR =0.30, p<.001; /s/ vs. /z/ OR = .90, p=.70; see Figure 2), allomorph type also interacted with all the child level variables (p<.05). Children’s overall articulation skills and word final s/z skills both predicted their overall accuracy with the allomorphs. As can be seen in Figure 3, regardless of GFTA-II raw score, children were highly accurate with /ɪz/. Global articulation skills affected /s/ and /z/ accuracy, with declines in morpheme use as the number of errors on the GFTA-II increased. Word final s/z skills also appeared to predict children’s accuracy with the different allomorphs. Children with “good” word final s/z skills did not differ in their ability to produce the various allomorphs (p>0.10). Children with “poor” articulation skills were more likely to produce /ɪz/ than /s/ (OR = 8.3, p= <0.001) or /z/ (OR = 5.3, p =0.02).

Figure 3.

Association between GFTA-II raw scores (number of errors) and accuracy level for each allomorph.

Children’s access to high frequency sounds (4kHz SL) also interacted with allomorph (p=0.002). Apparently contradicting our hypothesis, children with low 4kHz SL scores were poor at producing all three morphemes, with /ɪz/ being particularly poor. However, as shown in Figure 4, as 4kHz SL increased, the odds of producing /ɪz/ accurately increased more rapidly than the odds of producing /s/ or /z/. This may primarily be a measurement problem – 4kHz SL, although reflective of high frequency hearing, may be too low in frequency to pick up on variation in /s/ and /z/ use, resulting in a flat (non-predictive) line for these allomorphs. Use of other measures (e.g., 6kHz SL) may result in better predictions for these sounds.

Figure 4.

Association between 4kHz SL hearing measure and accuracy level for each allomorph.

Sentence Position

Sentence position and word final s/z skills also interacted (p=0.002). Children with “good” word final s/z skills are better at morphemes in sentence final position than in sentence medial position (OR = 3.7, p=0.0004). Children with poor word final s/z production skills were equally poor at producing morphemes in both positions (p=0.66). Two-way interactions between utterance position and allomorph type and three-way interactions involving child characteristics and utterance characteristics were not detected. This could be because they do not occur or it could be due to limited power. Larger samples with more evenly distributed exemplars may be required to rule out this possibility since these interactions have been observed in NH populations using elicited production (e.g., Mealings, et al., 2013).

Hearing and Articulation Skills

To further explore the relationship between hearing and children’s accuracy, we also considered which hearing measures predicted children’s articulation skills. The global articulation measure, GFTA-II raw scores, were predicted well by BE-PTA (R²=.12) and unaided SII (R²=.11). 4kHz SL was not a significant predictor (R²=.04). Figure 3 shows the relationship between GFTA-II raw scores and allomorph accuracy; Figure 5 shows the relationship between GFTA-II raw scores, word final s/z production and morpheme accuracy. It was not possible to run statistical analyses for children’s word final s/z abilities because the distribution of the data violated assumptions about normality but Figure 5 plots this information after converting both the GFTA-II and the articulation probes to percentages to assist the reader in interpreting these measures together.

Figure 5.

Relationship between percentage correct use of phonemes scored on the GFTA-II (filled circles), word final s/z use on the articulation probe (open diamonds) and overall accuracy for morphemes.

Note: The trend line reflects GFTA-II data only.

Discussion

We evaluated the performance of 3-year-old children who are HH on their production of a variety of morphemes realized as /s/, /z/ and /ɪz/. In many ways HH children are similar to their NH peers; though the overall accuracy rate is lower (Koehlinger et al., 2013), the relative accuracy levels for the 5 morphemes reported here generally mirrored those reported elsewhere for NH children (Brown, 1973).

The results for allomorph and sentence position hinged on children’s articulation skills. Although overall accuracy was generally low, children who are HH with good word-final articulation (s/z use >80%) tend to show patterns that approximate those seen in NH children. For these children, all three allomorphs were produced equally well. For children with poor word final articulation (s/z use < 80%) the allomorph /ɪz/ was produced correctly 5–8 times more often than the /s/ and /z/ allomorphs. This is a reversal of the pattern seen in NH children – recall that NH children produce the /s/ and /z/ allomorphs more accurately than /ɪz/ (Berko, 1958). Results for sentence position also were affected by children’s ability to use s/z word finally. Like NH children, children who were HH with good word final s/z skills produced morphemes more accurately sentence finally than sentence medially (for NH children, see Song et al., 2009). Children with poor word final s/z skills were equally poor at producing the morphemes in both positions, suggesting than any utterance final advantage was not being realized.

Articulation and Hearing

We hypothesized that degree of hearing loss and articulation skills would affect morphological accuracy. Thus, it was unexpected that neither BE-PTA nor unaided SII directly entered in to the model. Both have been shown in the past to affect MLU and verb morphology use in children who are HH, suggesting an influence on grammatical development (Koehlinger et al., 2013). These verb morpheme composites used in previous studies also include syllabic forms like am, and are and other morphemes like past tense –ed that are not realized using fricatives. In contrast, in this study, 4kHz SL is a significant predictor of s-related morphology, which includes both noun and verb markers. 4kHz SL measures audibility in a high frequency region of the spectrum, an area known to be influential for perception and production of fricatives in morphological contexts (Stelmachowicz, et al., 2001), a point we will return to when we consider the role of allomorphs on accuracy.

Global articulation skills are also strong predictors of grammatical morphology use and it may be that the degree of hearing loss indirectly influences grammatical development. BE-PTA and unaided SII predicted GFTA-II raw scores, but not morphological accuracy. The converse was true for 4kHz SL scores – these scores predicted morphological accuracy, but not GFTA-II raw scores, our global measure of articulation. That said, BE-PTA only accounted for approximately 12 percent of the variance in the global measure of articulation. On average, every additional 10 dB of hearing led to a decrease of 4.4 raw score points (errors) on the GFTA-II. It seems surprising that BE-PTA accounted for so little of the variance in global articulation skills given the strong association between degree of hearing loss and articulation reported in previous studies (Stelmachowicz, Pittman, Hoover, Lewis, & Moeller, 2004; Tomblin, Oleson, Ambrose, Walker, & Moeller, 2014). However, it is worth bearing in mind the age of the children included in this study. Recall that even 3-year-old children with NH are highly variable in their articulation abilities (Smit, et al., 1990). The unexplained variance in overall articulation abilities may be due to maturational factors present in young children more generally and not unique to the population of children who are HH. BE-PTA may be a stronger predictor of both morphological accuracy and articulation skill for older children whose motor skills are more stable.

Since GFTA-II raw scores are not heavily weighted toward the use of /s/ and /z/, but instead sample a variety of phonemes in English across initial, medial, and final word positions, this measure is not as tightly related to s/z grammatical morpheme use as the word final s/z measure was. It would have been more transparent to relate hearing measures directly to the articulation of word final s/z but these measures were highly skewed and a parametric relationship could not be fit. Taken together, these results suggest that the general hearing profile only has an indirect influence on s/z morpheme acquisition by way of articulation ability. Aided access to high frequency sounds, on the other hand, appears to be directly important for grammatical development of s/z morphemes. Further research including measures of children’s access to higher frequency sounds than measured here (measuring aided audibility at 6kHz or 8kHz or using a high-frequency average for aided audibility) would enhance our understanding of the role of hearing in morphological development.

Input Frequency & Morpheme Availability

Morpheme Type

It is suggested that how often NH children hear a grammatical marker influences how early it is acquired (Berko, 1958; Hsieh, et al., 1999). Our findings for children who are HH that plurals, possessives, and copulas were most accurate and 3^rd person singular was least accurate mirrors the order of acquisition, documented for NH children (Brown, 1973). Thus we observe similar, but delayed, patterns of acquisition regardless of the level of grammatical development that we examine (Koehlinger, et al., 2013). Certainly, longitudinal studies would be more reliable for determining order of acquisition but accuracy rate tends to align well with age of mastery (Brown, 1973; de Villiers & de Villiers, 1973). As can be seen in Figure 1, when there are enough tokens produced by the individual child to judge, most children are not at either ceiling or floor; rather many children are in the middle range once children who only have one opportunity to produce each form once are excluded. The one exception is possessive for which too few children had sufficient contexts to judge. The current results are somewhat surprising because McGuckian and Henry (2007) reported that HH children showed an alternate ranking of morpheme accuracy due to inconsistent input that was attributable to differences in perceptibility. The order of morphemes that we report does generally align with their work with the exception of possessives. Our studies also differ considerably—they focused on a wider range of morphemes and older children than we do here. Relatively speaking, we have very few instances of possessives to analyze (see Table 3) and our results might change if we introduced more opportunities for production. Elicited production would enhance these findings by ensuring sufficient productions to analyze with confidence. Further study will be required to determine whether the order of acquisition differences that they reported for –ing and –ed generalize to a larger group of children.

Sentence Position

Like the results from morpheme type, the results on sentence position generally matched previous findings (Song, et al., 2009), though children’s articulation abilities were particularly influential. Recall that Song and colleagues (2009) have argued that utterance final placement both reduces motor planning demands by reducing the co- articulation requirements and enhances perceptibility. They convincingly showed that consonant clusters also lead to reduced accuracy. In this study, children with good word final s/z skills were 3.7 times more accurate in sentence final position than in sentence medial position. Children with poor word final s/z skills were equally poor at both sentence positions. Even when placed in facilitative situations, children with poor articulation skills were not able to demonstrate knowledge of grammatical morphemes realized as –s or –z. Our articulation screening measure did not evenly sample singletons and clusters so we cannot ensure that difficulty producing word final –s clusters was not a problem, but the interaction between poor articulation skills and utterance position is notable nonetheless.

Allomorphs

Critically for questions about the role of input, we hypothesized that children who are HH would have higher levels of accuracy for inflectional markers realized as /ɪz/ as opposed to /s/ and /z/ because they would have more consistent access to the presence of the morpheme in the input when it is produced as a full syllable. Looking across all children, our hypothesis seems to be supported since the /ɪz/ allomorph is clearly more accurate than the /s/ or /z/ forms (p<.0001).

When we examine the results according to groups of children, two factors seem to be particularly influential with regard to allomorph use: word final s/z skills and access to high frequency sounds. These two factors may be confounded. Approximately two-thirds of the children produced word final s/z more than 80% of time, in line with criteria applied in standard studies of SLI. The remaining 15 children produced word final s/z between 0–80% of the time. As can be seen in Table 2, the two groups do not differ with regard to their overall hearing profile.

The performance of the good articulation group on each allomorph was similar to their overall performance; all three morphemes were between 64–68% correct. This would indicate that these children are applying rules for inflection even-handedly across the different production contexts. It is still notable that, for this group, the /ɪz/ allomorph is trending toward being more accurate than /z/ (OR: 1.3) and that overall accuracy levels remain rather low.

For the poor articulation group, a wide range of abilities at producing word final s/z is present. Nonetheless, within this group the use of /ɪz/ (M=.88) was clearly more accurate than the use of /s/ (OR: 8.6) or /z/ (OR: 5.3) allomorphs. At first this appears to be readily attributable to the difficulty that these children have with the final consonant sound by itself. As has been found in other studies, articulation skills are interfering with our ability to assess the language abilities of these children. Presumably remediation for articulation of word final s/z would lead to improvements in morphological accuracy.

However, the interaction between 4kHz SL and allomorph suggests that perception may also be a contributing factor. The regression model predicted that children with poor access to high frequency sounds would perform particularly poorly with /ɪz/ and at higher (but still non-mastery) levels for /s/ and /z/. As access to sounds in the 4 kHz range improved, use of /ɪz/ improved more rapidly than use of the other allomorphs. One explanation for these findings is related to measurement - we may need to extend the 4kHz measure to 6kHz or 8 kHz in order to better capture the variability in the acquisition of s-related morphemes. Nonetheless, this is clearly a promising start since this measure provides insights that are entirely lacking from the more standard pure-tone average and SII measures, which both contain limited weighting for frequencies above 2 kHz. Alternatively, although /s/ and /z/ are less audible to children with low 4kHz SL scores, these two allomorphs may still be sufficiently common in the input that other factors aid acquisition. Because they are more common, they may be heard more often in an absolute sense, allowing children to identify and extract cues from other aspects of the speech stream (McMurray & Jongman, 2011). These cues, in combination with more robust input heard in quiet environments, may allow all children to acquire these forms. The /ɪz/ form, on the other hand, may be sufficiently rare such that only when a child has enough hearing in the right ranges can they detect the pattern and deploy the morpheme. This would be consistent with predictions of the surface hypothesis (Leonard, 1989), which argues that morpheme acquisition is influenced by the audibility of the morpheme in combination with its frequency and transparency.

Clinical Implications

Our findings put the emphasis on the need to jointly assess morpheme use and articulation skills in children with HL. Future studies should explore morpheme production through elicited production in order to better assess the role of morpheme type and consonant cluster use by these children. Assessment of the syllabic allomorph may provide a means of examining morpheme use in children who are HH with especially poor articulation skills. This allomorph may also lend itself to serving as initial therapy targets because it is easier to perceive and produce. Even if a child cannot produce the fricative, the clinician can detect use of the morpheme through the vowel. Targeting articulation directly may also be beneficial. Moeller et al., (2010) and Bow et al., (2004) showed that when articulation skills improved so did morphological production and/or perception. Presentation of targets in utterance final position and manipulation of the phonological properties of the target word may also enhance accuracy during the early stages of therapy. Future work should examine whether these recommendations alter the efficacy of therapy for language-related targets. Investigations of whether improving audibility in the high frequencies leads to improved language skills are also warranted.