Speech timing and working memory in profoundly deaf children after cochlear implantation

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Abstract

Thirty-seven profoundly deaf children between 8- and 9-years-old with cochlear implants and a comparison group of normal-hearing children were studied to measure speaking rates, digit spans, and speech timing during digit span recall. The deaf children displayed longer sentence durations and pauses during recall and shorter digit spans compared to the normal-hearing children. Articulation rates, measured from sentence durations, were strongly correlated with immediate memory span in both normal-hearing and deaf children, indicating that both slower subvocal rehearsal and scanning processes may be factors that contribute to the deaf children’s shorter digit spans. These findings demonstrate that subvocal verbal rehearsal speed and memory scanning processes are not only dependent on chronological age as suggested in earlier research by Cowan and colleagues (1998). Instead, in this clinical population the absence of early auditory experience and phonological processing activities before implantation appears to produce measurable effects on the working memory processes that rely on verbal rehearsal and serial scanning of phonological information in short-term memory.

Introduction

Working memory in normal-hearing children has been widely investigated for several decades, and the findings have been linked to several important developmental changes in reading and language (Henry, 1994; Hulme & Tordoff, 1989; Kail, 1988, Kail, 1997; Kail & Park, 1994; Murray & Roberts, 1968). These investigations have provided some initial clues to which memory processes are most influential in initiating developmental increases in memory span. Several researchers have suggested that increases in articulation rate may be one of several important maturational changes contributing to developmental increases in memory span because of the influence articulation rate may have on the speed of subvocal verbal rehearsal (Cowan, 1999; Ferguson, Bowey, & Tilley, 2002; Hitch, Halliday, & Littler, 1989; Hulme & Tordoff, 1989; Kail, 1988, Kail, 1997; Kail & Park, 1994). In addition, Cowan and his colleagues have proposed that developmental increases in serial scanning processes may also contribute to memory span in normal-hearing children (Cowan, 1992, Cowan, 1999; Cowan et al., 1994, Cowan et al., 1998). However, little, if any, research has examined the development and utilization of these processes in clinical populations of children that have slower rates of speech articulation and difficulties in perceiving speech, both of which may affect memory span. Examination of verbal rehearsal and scanning in a unique clinical population of children, such as profoundly deaf children who use cochlear implants could yield valuable information about the role that early sensory deprivation, degraded phonological information, and slowed speech output have on immediate memory span and provide new knowledge about the development of memory processes.

The relation between articulation rate and immediate memory span has been explained through one of the earliest and most influential models of working memory proposed by Baddeley, 1986, Baddeley, 1992. In his model, two components, the phonological store and the articulatory control process maintain phonological information in working memory through cyclically controlled subvocal repetition or verbal rehearsal. The speed and efficiency of this hypothesized covert verbal rehearsal process appears to be directly related to overt articulation rates (Landauer, 1962). Support for the relation between overt articulation, covert verbal rehearsal, and memory span has come from numerous studies examining the word length effect (Baddeley, Thomson, & Buchanan, 1975; Hulme & Tordoff, 1989), digit spans in bilinguals (Elliot, 1992; Powell & Hiatt, 1996), and articulatory suppression effects (Baddeley, Lewis, & Vallar, 1984).

The relation between articulation rate and working memory span is a reliable finding in the literature. Memory span is linearly related to measures of overt speaking rates for words (Baddeley, 1992; Baddeley et al., 1975) and nonwords (Hulme, Maughan, & Brown, 1991) in both adults and children (Hulme & Tordoff, 1989). Several recent developmental studies have shown that immediate memory span can be predicted based on the maximal rate at which children can repeat lists of words aloud (Cowan et al., 1994; Kail, 1997).

However, other research findings have questioned the relation between speaking rate, rehearsal, and memory and the role memory decay may play in adults (Lovatt, Avons, & Masterson, 2002; Nairne, 2002; Service, 1998). The standard model of working memory, proposed by Baddeley, suggests that subvocal verbal rehearsal must occur at a pace rapid enough to prevent memory decay in order for items to be preserved in memory (Baddeley, 1992). However, this memory “decay” may actually be linked to temporal changes in stimulus presentation or output interference rather than just the speed of subvocal rehearsal or may not occur at all (Crowder, 1993; Nairne, 2002; Neath & Nairne, 1995).

These considerations may also be relevant to memory processes in children, making it important to further study developmental differences in speaking rate and memory span. Previous studies have found differences between speaking rate and memory span when children of different ages are compared. Cowan et al. (1994) found differences in the speaking rates and memory spans of 4- and 8-year-old children. As expected, 8-year-old children showed the same relation between speaking rate and memory span observed in adults. That is, 8-year-olds who spoke faster displayed longer memory spans. However, the opposite relation was observed in the 4-year-old children. This finding was surprising because children at this age are assumed to be in the early stages of developing subvocal verbal rehearsal strategies (Flavell, Beach, & Chinsky, 1966; McGilly & Siegler, 1989). Such counterintuitive results suggest that the influence of speaking rate on working memory may be an important and significant developmental process to study and understand. Results such as these also suggest that the role of speaking rate and verbal rehearsal on memory span may have been overestimated in the standard model of working memory and that cue driven retrieval or recall processes may also be responsible for the reported developmental differences in memory span (Nairne, 2002).

Recently, memory recall processes in children have been examined in greater detail to determine their role in memory development (Cowan, 1992; Cowan et al., 1994, Cowan et al., 1998). More specifically, recall processes have been indexed by measures of speech-timing such as preparatory intervals preceding list recall and interword pause durations within recall. Like pre-test or non-recall based measures of speaking rate, speech-timing measures taken during actual spoken recall have provided several new insights into the relation between temporal characteristics of speech and working memory processes (Cowan, 1992; Cowan et al., 1994, Cowan et al., 1998).

In one study of speech-timing measures during immediate recall, Cowan et al. (1994) found that interword pause times may provide a reliable index of the dynamics of the memory scanning and retrieval process during development. Cowan et al. found that children’s interword pauses within spoken recall increased as list length increased. This result supports Cowan’s earlier (1992) suggestion that serial scanning may be carried out during the pauses, because longer lists require that more items be serially scanned, prolonging interword pause time. Additional evidence demonstrating that items in short-term memory are scanned during interword pauses was found in another study by Cowan et al. (1998), who reported that children with shorter interword pauses also had longer memory spans than their peers.

In addition to memory span, recall mechanisms also appear to be developmentally linked. Cowan reported that older children have shorter pause durations in immediate recall than younger children (Cowan et al., 1998). Taken together, the recent findings by Cowan et al., 1994, Cowan et al., 1998 suggest that memory span increases observed in older children might be associated with both shorter interword pauses during serial recall and faster speaking rates. According to Cowan, shorter interword pauses demonstrate that scanning mechanisms used to retrieve items from short-term memory are being executed faster and more efficiently in the older children. This factor, along with increases in articulation speed, may enhance the ability to engage in efficient memory recall as children develop. These new findings on speech timing have led Cowan and his colleagues (1998) to propose that two processing operations are used by normally developing children that affect measures of working memory—serial scanning or retrieval of items from short-term memory and subvocal verbal rehearsal of phonological information (Cowan, 1999; Cowan et al., 1998).

To our knowledge, however, there have been very few studies that have examined scanning and rehearsal processes in clinical populations of children. Early research on developmentally delayed children with mental handicaps suggested that atypical verbal rehearsal and encoding strategies were responsible for differences in digit span in this population (Ellis & Anders, 1969). Other more recent research suggests differences in central executive functioning (Conners, Carr, & Willis, 1998). Unfortunately, such conclusions concerning executive or verbal rehearsal deficits in these populations are likely to be confounded by other factors related to differences in cognition and general intelligence.

To avoid confounds related to cognition and intelligence, developmental populations that exhibit normal intelligence yet have articulatory or phonological delays for other reasons should be studied. Children with specific language impairment (SLI) are one clinical population that meets these criteria. Numerous studies have shown that children with SLI exhibit a range of deficits in working memory (e.g., Gathercole & Baddeley, 1990; Gillam & Cowan, 1995; Leonard, 1998; Sussman, 1993). These deficits are thought to be related to inefficient encoding of phonological and temporal information about speech and spoken language (Gillam & Cowan, 1995; Gillam, Cowan, & Marler, 1998) rather than discrimination and perception of speech sounds (Gathercole & Baddeley, 1990; Sussman, 1993). However, it would be interesting to examine the development and operation of working memory processes in a clinical population in which overt and covert rehearsal capabilities may be compromised and delayed due to early deficits in speech discrimination, articulation, and phonological encoding. Profoundly deaf pediatric cochlear implant users display these characteristics ideally, making them a particularly suitable clinical population in which to study verbal rehearsal and speech-timing measures in working memory in comparison to normal-hearing children.

A comparison between speaking rate, speech timing, and working memory performance in pediatric cochlear implant users and normal-hearing controls should be informative based on earlier research comparing the memory capabilities of deaf and normal-hearing children. Previous research on this clinical population has revealed, not surprisingly, large differences in phonological memory performance between deaf children and their normal-hearing age-matched peers. In one study examining phonological memory in deaf and normal-hearing children, Banks, Gray, and Fyfe (1990) found that deaf children had more difficulties recalling details previously read in written text. In phonological memory tasks that depend on encoding and retrieval of sequential information, deaf children have also been found to lag behind normal-hearing children (Waters & Doehring, 1990).

Similar results have been found more recently in deaf children using cochlear implants. In a study from our laboratory, Cleary, Pisoni, and Geers (2001) reported that deaf children using cochlear implants had significantly shorter working memory spans for both verbal and spatial patterns than normal-hearing children. Other studies have found that pediatric cochlear implant users have shorter forward and backward digit spans than normal-hearing children (Pisoni, Cleary, Geers, & Tobey, 2000; Pisoni & Cleary, 2003). However, no research has been carried out to compare the speaking rates and speech timing of deaf children with cochlear implants to their normal-hearing peers. Given the relation between speaking rate and memory span found earlier in developmental populations, this comparison may provide some new insights into why deaf children with cochlear implants display shorter immediate memory spans and why they show an enormous amount of variability on a large number of clinical outcome measures of speech and language.

The speech of deaf children has been studied for a number of years because of its importance to assessing the communicative abilities of these children (McGarr, 1981, McGarr, 1983; Osberger, Maso, & Sam, 1993; Osberger, Robbins, Todd, & Riley, 1994; Tobey & Hasenstab, 1991). In contrast, little research has examined the speech of deaf children to explore the possible influences on cognitive abilities such as memory (Pisoni et al., 2000). One of the most distinctive characteristics of deaf speech is its reduced rate of articulation. Reduced speaking rates have been found in deaf individuals prior to the availability of cochlear implants (Nickerson, 1975), as well as in cochlear implant users (Leder et al., 1987). These results suggest that overt speaking rate and subvocal verbal rehearsal speed could be responsible for the shorter immediate memory spans observed in deaf children with cochlear implants.

Speaking rate has also been linked to differences in communicative abilities such as speech intelligibility in deaf individuals (Pisoni & Geers, 2000). The intelligibility of deaf speech refers to how well short speech samples can be understood by naı̈ve, normal-hearing adult listeners. The McGarr Sentence Intelligibility Test (McGarr, 1981) was one of the first instruments developed to assess and evaluate the speech intelligibility of deaf children. Using the McGarr sentences, Pisoni and Geers (2000) found that measures of speech intelligibility in deaf children with cochlear implants were related to the speed at which the test sentences were articulated. Longer sentence durations (i.e., slower speaking rates) were associated with less intelligible speech, as measured by naı̈ve normal-hearing listeners who were asked to transcribe test sentences.

These results suggest that there are communicative advantages for pediatric cochlear implant users who are able to articulate faster. One such advantage is simply being more intelligible than their slower speaking peers. An additional advantage of the cochlear implant users who can speak faster is that they may be more capable of planning and maintaining their speech representation in working memory with less effort. Such decreased working memory demands during speech planning may result in increases in verbal fluency and articulatory precision of speech production.

In addition to the communicative advantage of having more intelligible speech, children with cochlear implants who are able to speak faster show a cognitive advantage over their slower speaking peers. Pisoni et al. (2000) found that children with cochlear implants who were able to speak faster also displayed longer memory spans, suggesting a relation between speaking rate and working memory. One factor that was found to contribute to both the articulation rate and memory spans of children using cochlear implants was the nature of the early sensory, linguistic, and communicative experiences that these children were exposed to after receiving their cochlear implants.

Communication strategies used by deaf children with cochlear implants vary across a continuum. This continuum is often divided into oral communication, in which speech is the primary method of communicating, and total communication, a method utilizing oral communication supplemented with manual signing and lip reading. By assessing where children fall on this continuum, a classification into either the oral communication or total communication group can be made. This classification method has allowed for comparisons of deaf children on a variety of communicative and cognitive measures based on the nature of the early auditory and linguistic experiences of the children (Geers, 2000; Pisoni & Geers, 2000; Pisoni et al., 2000).

In their study of working memory in deaf children with cochlear implants, Pisoni and Geers (2000) reported that oral communication users speak faster, display more intelligible speech, and have longer immediate memory spans than total communication users. This finding suggests that oral communication users’ working memory capacity is affected by linguistic and auditory experience and activities after receiving their implant and may reflect increased articulation rates (Pisoni et al., 2000). Thus, the digit span advantage displayed by oral communication children may be related to both overt articulation rate and covert verbal rehearsal abilities.

The ability of oral communication children to speak more intelligibly and more rapidly may also be a consequence of their early communicative experiences and linguistic activities after implantation. The most beneficial early experiences that the oral communication users have are undoubtedly those pertaining to oral-aural activities. Oral-aural activities are critical for speech and language development because of the role they play in helping deaf children with cochlear implants to develop efficient spoken language and phonological encoding skills. In addition to encouraging these children to produce speech, oral-aural educational environments also provide the necessary auditory feedback to deaf children using cochlear implants. Auditory feedback may be especially important for these children because it provides a direct mechanism for them to self-monitor and improve their speech articulation, speech motor control, and speech intelligibility. These differences may then affect overt articulation speed and subvocal rehearsal speed, which in turn could affect their working memory spans.

Deaf children with cochlear implants who use either communication mode are likely to rely on covert verbal rehearsal strategies in many language processing tasks because such mechanisms have been measured in deaf children without cochlear implants (Bebko, 1984; Liben & Drury, 1977). In addition, it has been shown that when carrying out memory tasks, deaf children, like their normal-hearing peers, display word length effects which are assumed to reflect speed of articulation (Campbell & Wright, 1990). More importantly, in a recent study examining verbal and spatial working memory in a sample of deaf children using cochlear implants, Cleary et al. (2001) found evidence of verbal rehearsal and encoding in the cochlear implant users. In some cases, the verbal rehearsal strategies of the children with cochlear implants were as efficient as the strategies used by normal-hearing children. Based on these earlier findings, it is reasonable to expect that the cochlear implant users in the present study are capable of some kind of covert verbal rehearsal as well. Previous findings also suggest that covert verbal rehearsal in cochlear implant children may be related to speaking rate (Pisoni & Geers, 2000). If this hypothesis is correct, we would expect that both the cochlear implant users and the normal-hearing children in the present study who speak at faster rates should display longer immediate memory spans.

The present study was designed to investigate and expand on the earlier results showing a relation between speaking rate and memory span in deaf children with cochlear implants. In addition, we were interested in examining speech-timing measures during memory span recall in this clinical population. Measures of speaking rate and speech timing during recall were examined in a group of deaf children who use cochlear implants and in an age-matched control group of normal-hearing, typically developing children. Measures of articulation rate and subvocal rehearsal speed were obtained by examining sentence durations from a non-speeded sentence repetition task. The strength of the relation between articulation rate and working memory in each group of children was compared to determine how rehearsal processes might differ between the two populations. To assess speech timing during spoken recall, response latencies, articulation durations of the test items, and interword pauses in digit span lists were measured in both the cochlear implant and normal-hearing groups of children.

The importance of these speech-timing measures to understanding the processes used in immediate memory is based on Cowan’s recent proposal that articulation rate in recall reflects subvocal verbal rehearsal speed and that pause durations in recall reflect the time spent scanning and retrieving items from short-term memory (Cowan, 1999). Speech-timing measures obtained during the deaf children’s digit span recall were examined to determine if the differences in scanning information in short-term memory would be comparable to the findings observed previously in normal-hearing children and the current normal-hearing control group. The relation between speech timing and memory span was also investigated to determine how it influences the digit span differences between cochlear implant and normal-hearing children and between total communication and oral communication users. These comparisons are critical in order to uncover the reasons for the shorter memory spans exhibited by profoundly deaf children with cochlear implants. We hypothesized that the observed differences in immediate memory span are related to a reduced efficiency of verbal rehearsal and/or scanning processes and ultimately derive from the early period of sensory and linguistic deprivation that these children experienced prior to receiving their cochlear implants.

We predicted that both measures of speech timing, subvocal rehearsal speed and rate of serial scanning, would be atypical in the deaf children with cochlear implants, particularly the total communication users because of their reduced exposure to spoken language. These differences were expected to be observable through decreased articulation rates in the sentence repetition task and longer interword pauses during the recall portion of the digit span task. We assume that such results would be related to the nature of the deaf children’s unique developmental history and the early absence of linguistic experience and activities which attenuate or prevent the efficient verbal encoding, rehearsal, and retrieval of phonological information from working memory that normal-hearing children routinely experience in the typical language learning environment.

Section snippets

Participants

Thirty-seven deaf 8- to 9-year-old children (M=8.70, SD=0.51) who use cochlear implants were recruited for this study. Twenty-five of the children were male, and 12 were female. The deaf children were tested at Central Institute for the Deaf (CID) in St. Louis, Missouri as part of a larger ongoing study (Geers, 2000). Most of the deaf children had a congenital profound hearing loss. Five of the children lost their hearing after birth, between the ages of 9 and 18 months (M=14.00, SD=4.58). The

WISC-III digit span scores

Differences in digit span reported previously in deaf children with cochlear implants and normal-hearing children were replicated in the present study. As expected cochlear implant users displayed shorter digit spans than their age-matched normal-hearing peers. Additionally, total communication users showed shorter forward digit spans than oral communication users. Fig. 2 illustrates these differences. These results suggest that the deaf children with cochlear implants, particularly children

Discussion

The results of this study replicated previous findings showing that profoundly deaf children with cochlear implants have shorter digit spans than their normal-hearing peers. As expected, deaf children with cochlear implants also displayed longer sentence durations than their normal-hearing peers. In addition, within the group of deaf children with cochlear implants, total communication users displayed slower speaking rates and shorter forward digit spans than the oral communication users. These

Acknowledgements

This research was supported by NIH research Grant DC00111 and NIH T32 training Grant DC00012 from the NIDCD to Indiana University, Bloomington. We thank Dr. Ann Geers and the staff at Central Institute for the Deaf in St. Louis, Missouri for testing the cochlear implant children and making data available for our use. We also thank Dr. Emily Tobey and the staff at the Callier Advanced Hearing Institute at The University of Texas-Dallas for data measurements. We also extend our thanks to Dr.

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