Studies on the intelligibility of hearing-impaired children

In the domain of HI children’s speech, there is a long tradition of investigating these children’s speech intelligibility. In the 1970s and 1980s, several studies examined the speech of deaf or partially-hearing children and adolescents through transcription tasks and included both the

judgements of experienced and inexperienced listeners. In terms of scores, there seemed to be a consensus that the speech of deaf individuals is around 20% intelligible to inexperienced listeners (Markides, 1970;

Osberger et al., 1993; Smith, 1975; Svirsky et al., 2000a). Interestingly, all studies reported a higher intelligibility score for the experienced listeners (Ferguson, 1981; Markides, 1970; McGarr, 1983; Monsen, 1978 - but see Mencke et al. 1983).

In the 1990s, the positive effect of cochlear implantation on speech perception was established and studies started to address the speech intelligibility of children with CI. Initially, researchers were especially interested in how children with CI performed as opposed to those with a traditional acoustic HA. For example, the study of Miyamoto et al. (1996) found that CI users reached speech intelligibility scores of over 40% after 3.5 years of device experience. These scores exceeded those of HA users with a hearing threshold between 101 and 110 dB HL, but were lower than the scores obtained by HA users with a lower hearing threshold. Similarly, Osberger et al. (1994) reported a mean score of 48% for CI users that used oral communication as their primary communication mode. Thus, in the early studies on the speech intelligibility of children with CI, the children did not yet reach comparable levels of speech intelligibility to children with traditional HA. However, these studies were characterised by large ranges in chronological age and hearing thresholds of the tested subjects.

Moreover, the lack of additional (hearing related) information within and between the studies complicate the interpretation and comparisons between those early studies. Gradually, factors such as the age of

1999; Miyamoto et al., 1999). More specifically, it was found that HA children with hearing levels of 90-100 dB HL and the group of CI children that was implanted earliest, i.e. between the ages of 2 and 4, both obtained intelligibility scores of 60-90% (Löhle et al., 1999). Compared to the study of Miyamoto et al. (1996), children with CI thus seemed to have improved to a point that they were on a par with HA users. This trend continued and children with CI increasingly surpassed children with HA (Baudonck et al., 2010b; Gillis, 2017; Lejeune & Demanez, 2006). For example, in the study of Van Lierde et al. (2005), listeners rated the intelligibility of children with CI to be only slightly impaired, whereas that of the children with HA was judged to be moderately impaired. Thus, in the studies that compared the speech intelligibility of CI and HA children, CI users initially performed poorly, yet changes such as earlier implantation led to the fact that the CI children oftentimes outperformed peers with HA, leading to “better outcomes than have historically been expected for children with severe and profound hearing impairments” (Flipsen & Colvard, 2006: 103).

Current state of the art: comparisons CI-HA-NH

Because of the promising results of cochlear implantation,

“language acquisition and development on a par with that of children with normal hearing is no longer considered unrealistic” (Chin et al., 2003: 441).

Therefore, rather than comparing with children with HA, the speech of children with CI was increasingly often compared with that of NH children.

One of the first studies comparing the two groups consisted of a transcription task of imitated short sentences (Chin & Tsai, 2001). In this longitudinal study of 3-to-6-year old children with CI, the mean

intelligibility score was significantly lower than that of NH children at each year interval. However, the intelligibility increased with each year and some children reached scores of over 80% (Chin & Tsai, 2001). Therefore, children with CI are considered as “gap closers” when their development of speech intelligibility is compared longitudinally to that of NH children (Nicholas & Geers, 2007; Yoshinaga-Itano et al., 2010).

However, the group of children with CI is characterised by a large amount of variability. This variability can be ascribed to the heterogeneity of CI users and thus, more specifically, to characteristics related to the individual child, their hearing or their environment (Boons et al., 2013).

Amongst others, studies on intelligibility have taken into account the mode of communication, e.g., oral or total communication (Osberger et al., 1994;

Tobey et al., 2004; Vieu et al., 1998), the age of implantation (AlSanosi &

Hassan, 2014; Habib et al., 2010; Tye-Murray et al., 1995) and the length of device use (Montag et al., 2014; Peng et al., 2004; Uziel et al., 2007).

Focus: factors age of implantation and length of device use

The question whether age of implantation or length of device use is most crucial in children with CI has been an ongoing discussion for quite some time. Concerning the age of implantation, there is convincing evidence of a so-called sensitive period in which the plasticity of the auditory system is at its highest (Kral & Sharma, 2012). Therefore, implantation in this time frame, i.e. preferably before the age of two, is recommended and leads to the best speech and language outcomes (Nicholas & Geers, 2007; Svirsky et al., 2007). Similarly, in terms of the

(2010) found that the speech intelligibility of children implanted between 8 and 24 months was significantly higher than that of children implanted between 24 and 36 months of age.

Concerning the length of device use, early studies questioned whether intelligibility of congenitally deaf children develops at all in the first two years of device use (Osberger et al., 1994; Tye-Murray et al., 1995).

Therefore, studies started to investigate the long-term effects of cochlear implantation on children’s speech intelligibility. In most of these studies, speech intelligibility was assessed through five-point rating scales such as the Speech Intelligibility Rating task (SIR) (Cox & McDaniel, 1989). The first (longitudinal) studies showed that, after three years of device use, the children’s speech was only intelligible to experienced listeners who had access to lipreading cues, i.e. SIR score 3 (Allen et al., 1998; Calmels et al., 2004; Inscoe, 1999). In later studies, the assessment after three years of device experience showed a median score of 4 (Bakhshaee et al., 2007; De Raeve, 2010). Studies investigating an even longer length of device use showed that some children reached ceiling scores. After approximately six years of device use, 85% of the CI children – mostly implanted before the age of two – reached level 4 or 5 on the SIR, meaning that their speech is intelligible to experienced as well as inexperienced listeners (Toe &

Paatsch, 2013).

Still, in studies on the effect of length of device use, the factor age of implantation shows its importance: even with prolonged device experience, earlier implanted children perform significantly better than children implanted later in childhood. More specifically, children implanted around the age of five years and assessed with a sentence

imitation task after seven years of device use reached an average intelligibility score of 68% (Peng et al., 2004). Contrarily, early implanted children with about the same length of device use and type of task reached near ceiling intelligibility scores (Chin et al., 2012; Chin & Kuhns, 2014;

Montag et al., 2014).

Thus, a combination of early implantation and several years of device use seems to lead – at least in imitation tasks – to nearly perfect intelligibility scores. For spontaneous speech, it is still unclear whether this combination of factors affect children’s speech intelligibility and hence, it is one of the aims of this dissertation to investigate the intelligibility of this type of speech. More precisely, the intelligibility of early implanted children with CI and children with NH will be compared. The children with CI were implanted before the age of two and had approximately six years of device experience. More information on the specific children is provided in chapters 2 and 3.