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date: 22 October 2017

Specific Language Impairment

Summary and Keywords

Children with specific language impairment (SLI) have a significant deficit in their ability to acquire language that cannot be attributed to intellectual disability, neurological damage, hearing loss, or a diagnosis of autism. These deficits can be long-standing, and adversely affect other aspects of the affected individual’s life. There seems to be a genetic component to SLI, but the disorder is not likely to be traced to a single gene. The problem appears to be universal, but symptoms vary depending on the language being learned. Current attempts to account for SLI have increased our understanding of the most salient symptoms of the disorder, but a full understanding of SLI is not yet within reach.

Keywords: language disorder, primary language impairment, developmental language impairment, grammar, morphosyntax

Introduction

Since the mid-1800s, the scientific literature has included articles about children who had significant difficulties in learning to speak, yet these children were surprisingly normal in other areas of development. Over the many decades, these children have been given a wide variety of clinical labels. In today’s literature, these children are most frequently referred to as children with “specific language impairment” (SLI).

Children with SLI show a significant deficit in their production (and, usually, comprehension) of language—a problem that is present from the outset (Leonard, 2014a). This significant deficit cannot be attributed to any documented difficulty in other areas, for the children exhibit normal hearing, their scores on nonverbal tests of intelligence are above the mild intellectual disability range, and neurological examination reveals no evidence of damage or disease. Furthermore, these children fall clearly in the non-autistic range on the autism spectrum. Their problems may include speech sound difficulties, but the diagnosis of SLI requires significant weaknesses in vocabulary and/or grammar.

From this description, it is clear that children with SLI could not be appropriately classified as showing a hearing loss, an intellectual disability, or a neurological impairment. However, many—perhaps most—children with SLI show signs of subtle weaknesses in areas such as motor development (Hill, 2001) and nonlinguistic information processing (Miller et al., 2006). Furthermore, it is not unusual to see their average scores fall slightly below those of typical peers serving as the comparison group (Spaulding, Plante, & Vance, 2008).

How Specific is the Language Impairment?

The fact that many children with SLI show subtle weaknesses has prompted some scholars to consider alternative labels for these children (Reilly et al., 2014). This is understandable, given that the term “specific language impairment” suggests a pure deficit, in which language is the only type of ability that is adversely affected. However, a suitable alternative has not been easy to come by. For example, one reasonable alternative is “primary language impairment” (Ebert & Kohnert, 2009). This term avoids the connotation that the deficit is “pure.” However, this alternative implies there is a secondary impairment; yet, when there is an additional weak area, it rarely reaches the threshold that would justify the term “impairment.” Furthermore, if the weakness in another area reached a sufficient level of severity to be regarded as an impairment (e.g., motor impairment, intellectual disability), the scientific basis for regarding the language deficit as primary would not be clear. The Diagnostic and Statistical Manual of Mental Disorders (DSM-5; American Psychiatric Association, 2013) uses the term “language disorder” to refer to children that meet the criteria for SLI. This term has its own disadvantages (Reilly et al., 2014). The criteria spelled out by the DSM-5 seem appropriate, but the term itself does not seem to clearly distinguish children with SLI from children with other diagnoses who may also have an impairment in language.

For now, SLI is probably best regarded as a term that applies to children with a significant deficit in spoken language who fall clearly outside the boundaries of conditions such as intellectual disability, hearing impairment, autistic spectrum disorder, or brain injury. For example, SLI researchers have already included children with below average nonverbal IQ test scores provided that these test scores (e.g., 75) are above the level of mild intellectual disability even when the test’s standard error of measurement is taken into account (Spaulding, Plante, & Vance, 2008). There is good reason to loosen the IQ criterion; children whose scores fall somewhat below the “average” range cutoff of 85 show essentially the same linguistic profile as children with scores above the cutoff (Tomblin, Zhang, Catts, Ellis Weismer, & Weiss, 2004), and show similar gains in treatment (Bowyer-Crane, Duff, Hulme, & Snowling, 2011).

The absence of a suitable alternative to the term “specific language impairment” is certainly one reason to maintain this term. Another is that a search of the scientific literature reveals that it is by far the most frequently used term. In a Google search of research from 1994 to 2013, Bishop (2014) found that SLI brought 18,850 “hits” but only 3,509 hits for the next most frequent term (“developmental language disorder”). When the search is confined to more recent citations (2003–2013), the dominance of the term SLI becomes even greater (Leonard, 2014b). In an era of key word searches, the promotion of an alternative term could risk losing an important link to previous research.

The Prevalence of SLI

Early prevalence estimates were based on clinical referrals and thus were not indicative of the prevalence of SLI in the general population. Subsequent epidemiological studies suggest that the prevalence of SLI is approximately 7% among five-year-olds (Tomblin et al., 1997). An important aspect of the prevalence figure of 7% is that SLI appears to be more common than autism spectrum disorder (ASD), and at least as common as Attention Deficit with Hyperactivity Disorder (ADHD) (Bishop, 2014). There are several factors that complicate prevalence estimates of SLI for older children. On the one hand, because SLI is a developmental condition, children benefit from maturation, and some children may catch up to peers eventually. Many others will have a persisting deficit in language, but symptoms will not be as striking in simple conversational settings. Furthermore, their diagnostic label might change, even though their central problem with language has not. For example, because these children are at serious risk for reading difficulties, many might receive a diagnosis of dyslexia (Catts, Fey, Tomblin, & Zhang, 2002). In addition, during the school years, many children’s scores on nonverbal tests of intelligence begin to decline (Botting, 2005). Although this decline is rarely of the magnitude that results in the children being placed in the category of intellectual disability, it can lead to a view of these children having a broader impairment. Yet, the cause of the decline might well be language. As children enter the academic world, much of the new knowledge they acquire will occur through language, in either written or spoken form. If their language ability is weak, their intake of this new information will be hampered and therefore their knowledge base will grow more slowly than that of their peers. It is ironic that a more specific language impairment might lead to a narrowing of the gap between language and non-language cognitive ability as time marches on.

Can Subtypes of SLI be Identified?

There is a good deal of heterogeneity among children with SLI, and over the years there have been many attempts to identify subtypes of this disorder (e.g., Aram & Nation, 1975; Rapin & Allen, 1988). A common division that is made clinically is the distinction between an expressive (production) language disorder and a receptive-expressive (comprehension-production) language disorder. Although it is true that some children score higher on tests of language comprehension than on language production, it is not clear that the precise language knowledge needed for production has been adequately captured on language comprehension tests (Leonard, 2009). More broadly, the differences among children have not yielded valid and reliable subcategories. In fact, applications of sophisticated statistical procedures to large numbers of these children do not reveal separable dimensions (Tomblin & Zhang, 2006), casting doubt on distinctions such as expressive versus receptive-expressive disorder, and vocabulary versus grammar disorder.

The Long-Term Outcomes of Children with SLI

Early retrospective studies indicated that many children with SLI have long-standing problems, showing problems in reading as well as in spoken language. The appearance of longitudinal studies with a prospective design improved on this work, because the children’s initial symptoms could be described in a more uniform manner (Beitchman, Wilson, Brownlie, Walters, & Lancee, 1996; Conti-Ramsden, Botting, Simkin, & Knox, 2001; Johnson et al., 1999; Stothard, Snowling, Bishop, Chipchase, & Kaplan, 1998). According to these studies, by approximately age five and a half years, approximately half the children originally meeting the criteria for SLI perform at age level on most, though not all language measures. (In fact, even in their teenage years, the “recovered” children still perform below age level on select measures). For the remaining children, significant problems in language and reading persist, even into adulthood. The long-term difficulties of many of these children are perhaps shown most convincingly when children originally participating in epidemiological studies are evaluated at a later age (Law, Rush, Schoon, & Parsons, 2009; Tomblin, Zhang, Buckwalter, & O’Brien, 2003). These are children selected from the general population rather than being clinically referred. As a result, they are not heavily weighted toward children with the most severe problems at the outset and thus the outcomes for these children are probably not biased toward finding lingering difficulties. Yet, even in these studies, at least half of the children meeting the criteria for SLI show signs of significant difficulty in language. (When the nonverbal IQ criterion is lowered to above 70—below age level but above the level of intellectual disability—the percentage of children showing later language problems is even higher.

When tested at a later age, children with SLI can show a variety of problems that extend beyond spoken language and reading. The degree to which they are directly caused by the language problem or serve as indirect, collateral effects from the children’s struggles with communication is not yet clear. Difficulties include poorer quality friendships (Durkin & Conti-Ramsden, 2007), greater social stress (Wadman, Durkin, & Conti-Ramsden, 2011), and being perceived by teachers and parents as having difficulty regulating their emotions (Fujiki, Brinton, & Clarke, 2002; Fujiki, Spackman, Brinton, & Hall, 2004). Upon reaching adulthood, these individuals are much more likely to be unemployed (Law et al., 2009).

Genetics and SLI

There is strong evidence that SLI has a genetic component. There is high familial concentration of language disorders, and some of the available studies verified family members’ language abilities through direct testing, in addition to using information from questionnaires (e.g., Barry, Yasin, & Bishop, 2007; Plante, Shenkman, & Clark, 1996). Addressing the genetic vs. environmental factors issue more directly, twin studies show greater concordance between monozygotic twins than between same-sex dizygotic twins (Bishop, North, & Donlan, 1995; Tomblin & Buckwalter, 1998), especially when clinical referral is included among the criteria for impairment (Bishop & Hayiou-Thomas, 2008). The early work in molecular genetics focused on a multigeneration British family who showed a high incidence of language impairment. The affected members of this family showed a mutation of the FOXP2 gene, located in the chromosome region of 7q31. However, closer inspection of the symptoms of these individuals revealed that they did not fit the usual criteria of SLI, exhibiting, as they did, striking problems in oral-motor ability and other problems. Molecular genetic studies of children matching the SLI profile more closely have identified several chromosome areas that seem to be associated with this disorder. These include chromosome areas 13q21 (Bartlett et al., 2004), 19q13 (Falcaro et al., 2008; SLI Consortium, 2004), 16q24 (Monaco, 2007; SLI Consortium, 2004), and 6p22 (Rice et al., 2009). However, it has become clear that SLI has a complex basis, with several or many genes each accounting for small amounts of variance, probably in combination with environmental factors. The same symptom can arise from variants of different genes, and a variant in a single gene can affect more than one type of ability. In addition, there can be epigenetic effects, which affect gene expression without a change in the actual DNA sequence (Rice, 2012).

The complex nature of SLI has also raised our awareness that this disorder is not wholly distinct from other conditions associated with language weakness. For example, there is overlap with ASD. This is seen not only in the overlap of some of the chromosome regions likely to be involved (Vernes et al., 2008) but also in studies of familial patterns. Children with SLI are more likely than control children to have another member of the family with a diagnosis of ASD, and vice-versa (Bailey, Palferman, Heavey, & LeCouteur, 1998; Tomblin, Hafeman, & O’Brien, 2003). This appears to be true even when the children with SLI are quite far from the autism end of the spectrum.

The apparent genetic involvement in many cases of SLI might suggest that this disorder is quite distinct from language ability that is in the low range of “normal.” However, this has not been easy to document (Dollaghan, 2011; Leonard, 1991; Rescorla, 2009). For example, studies that have identified specific ability areas showing genetic involvement have employed cutoff scores on tests that actually represent a continuum. It might be safer to say that lower levels of ability in these language areas appear to be genetically based than to conclude that an impaired ability in these areas has a genetic basis.

There are two ability areas that appear to have a genetic basis, according to twin study methodology (Bishop, Adams, & Norbury, 2006). These are weaknesses in: (1) “grammatical computation,” involving inconsistency in using grammatical morphemes that reflect tense (e.g., saying jump instead of jumped) and difficulties with sentence comprehension; and (2) “phonological short-term memory,” involving weaknesses with digit span and repeating nonsense words of varying lengths. Although these weaknesses are genetically separable, they co-occur with more frequency than assumptions of independence would predict.

The Neurobiology of SLI

The absence of neurological damage or disease in SLI should not be interpreted to mean that the children’s neurological profile is routinely typical. Structural MRI studies have revealed numerous differences between children with SLI and typically developing peers. For example, a relatively large percentage of these children show perisylvian areas in the right hemisphere that are uncharacteristically large (Plante, 1991). Some of the unaffected siblings and parents of these children have also shown this atypical configuration (Plante, Swisher, Vance, & Rapcsak, 1991), so it is not clear if this pattern is sufficient to cause language impairment. However, it probably serves as an important risk factor. Other imaging studies have reported other kinds of atypical patterns, including larger volumes of cerebral white matter in the outer zone in children with SLI (Herbert et al., 2004), an area that myelinates later. Other differences have included reversals of asymmetries in the inferior frontal gyrus and the cerebellum (Hodge et al., 2010). There are also findings of atypical configurations that apply to individual children with SLI (e.g., Trauner, Wulfeck, Tallal, & Hesselink, 2000), though these have been too isolated to be considered part of the SLI profile.

With the advent of diffusion tensor imaging, researchers were able to examine brain connectivity in white matter. Several studies have revealed that children with SLI have white matter tracts that seem deficient for transmission of information between language-relevant cortical regions of the brain (Roberts et al., 2014; Verhoeven et al., 2012; Vydrova et al., 2015).

Neurophysiological studies also reveal numerous differences between children with SLI and their typically developing peers. When the stimuli are linguistic in nature, differences are not surprising because behavioral measures of the same phenomenon are also likely to reveal differences. For example, grammatical deficits abound in children with SLI, and one electrophysiological measure, the P600 (that is, a peak in positive polarity at approximately 600 msec after the stimulus onset) is often shown to be reduced in amplitude relative to peers when an ungrammatical sentence is presented (Purdy, Leonard, Weber-Fox, & Kaganovich, 2014; Weber-Fox, Leonard, Hampton Wray, & Tomblin,, 2010). Measures that reflect semantic processing, such as the N400, tend to show fewer group differences (Archibald & Joanisse, 2012). However, sometimes interesting group differences can be seen on electrophysiological measures that are not directly reflected in behavioral responses. For example, Fonteneau and van der Lely (2008) found that children with SLI responded to ungrammatical sentences with what appeared to be an N400 response, which is usually seen when a semantic anomaly is presented. This finding led the investigators to speculate that the children with SLI may have been employing neural circuitry usually associated with semantic processing.

Many studies have used measures (e.g., responses to tones) that reflect obligatory responses that are elicited by the physical characteristics of the signal or the interstimulus interval between them. The evidence has been quite mixed, with some studies reporting group differences (e.g., Neville, Coffey, Holcomb, & Tallal, 1993), whereas other studies have found no differences between groups (e.g., Marler, Champlin, & Gillam, 2002). The findings seem to be clearer for measures that presumably reflect a higher cognitive activity. One of these is “mismatch negativity” (MMN) in which an auditory stimulus (e.g., “ba”) that differs from a string of identical stimuli (e.g., “da”) is presented. Here, a negative wave around 150 to 250 msec is usually seen from the point at which the deviant stimulus is presented. Children with SLI tend to show a reduced amplitude MMN in these studies (Shafer, Morr, Datta, Kurtzberg, & Schwartz, 2005; Uwer, Albrecht, & Von Suchodoletz, 2002). Finally, measures at the brainstem level have been obtained in studies of SLI (e.g., Basu, Krishnan & Weber-Fox, 2010). The findings of these studies point to problems detectable at the brainstem level when stimuli involve rapid frequency changes or rapid presentation rates.

The Language Symptoms

Although genetic and neurobiological studies of SLI are relatively recent, this field has had a long history of documenting the language symptoms seen in these children. In every language systematically studied to date, children with SLI have been readily identified. There may, of course, be limits to this pattern. Languages spoken in countries where there are no established norms for language development have not been included. It is also possible that some cultures do not recognize children with slow language development as having an impairment.

Apart from some early studies on German- and French-speaking children, the great majority of early studies of this type have focused on children with SLI who were acquiring English. We have a rather clear picture of SLI in English during the preschool years. Frequently, English-speaking children with SLI exhibit a mild to moderate weakness in phonology and vocabulary, and a more serious deficit in grammar. Regarding the latter, the dominant grammatical profile is one of sentences produced with an exaggerated telegraphic appearance, with the frequent absence of “grammatical morphemes”—function words such as the and is and grammatical inflections such as past tense –ed. The grammatical morphemes most problematic for these children are those that mark subject-verb agreement and/or tense. When they are five years of age or even older, children with SLI often still produce sentences such as Yesterday I play all day at the park, Every day my brother ride his bike, and That girl running real fast. When children with SLI are compared to younger typically developing children matched according to average sentence length, the SLI group has much more difficulty with these tense/agreement morphemes than the younger peers (e.g., Hoover, Storkel, & Rice, 2012; Joseph, Serratrice, & Conti-Ramsden, 2002; Leonard, Eyer, Bedore, & Grela, 1997; Oetting & Horohov, 1997; Rice & Wexler, 1996). When children with SLI produce these morphemes, the morphemes are usually correct (errors such as I is going are extremely rare); the problem is that children with SLI often omit these morphemes in obligatory contexts. As we will see below, this type of finding was influential in shaping some of the prominent theoretical accounts of grammatical deficits in SLI.

Children with SLI acquiring Germanic languages such as German, Dutch, and Swedish also show difficulties with tense/agreement morphemes relative to younger typically developing children matched for average sentence length. However, compared to their English-speaking counterparts, children with SLI in these languages show higher absolute levels of appropriate use (e.g., Leonard, Hansson, Nettelbladt, & Deevy, 2004). One possibility for this finding is that these morphemes are more abundant in these languages than in English. For example, whereas in English, present tense inflections are limited to third person singular (e.g., He/she runs), in German, an inflection is used for each person and number combination (though some inflections share the same phonetic form, as in first and third person plural). In English, the same inflection is used for all person and number combinations in past tense; this is true for Swedish as well, but in Swedish there is also an inflection that is used for all person and number combinations in present tense.

However, children with SLI in these Germanic languages have great difficulty in an area that is not witnessed in English-speaking children—a problem with word order. These languages are considered “verb-second” languages, because the verb that marks tense/agreement must appear as the second constituent of the sentence, even when the sentence begins with something other than the subject. Children with SLI in this language are often too rigid in having the verb immediately follow the subject, even when the subject is not the first constituent (Hansson, Nettelbladt, & Leonard, 2000). For example, in Swedish, a child with SLI might say the Swedish equivalent of Yesterday he bought a car, when the correct order would be Yesterday bought he a car. Another noteworthy error type is seen when children with SLI fail to mark a verb for tense/agreement in German and, instead of placing the verb in second position, they place the verb in infinitive form in sentence-final position (Rice, Noll, & Grimm, 1997). Thus, instead of the German equivalent of The man plays basketball, an error equivalent to The man basketball play might be produced. This type of error has also contributed to the development of theories about SLI.

The Romance languages of French, Italian, and Swedish also differ from English in the most common profiles seen in SLI. In these languages, verb inflections that mark tense/agreement are used accurately to approximately the same degree as in younger typically developing peers matched on average sentence, although this is more difficult to document in French, given that many inflections have the same pronunciation (compare Paradis & Crago, 2001 and Elin Thordardottir, & Namazi, 2007). However, auxiliary verbs in these languages are frequently omitted, much as they are in English SLI. For example, in French, past events are frequently described using an auxiliary in present tense followed by the past participle, as in She has eaten the apple. A frequent error would be the French equivalent of She eaten the apple (Paradis & Crago, 2001).

In these same Romance languages, a major weakness is seen when a direct object of a sentence would ordinarily be a “clitic”—an unstressed pronoun form that precedes rather than follows the main verb. For example, in Italian, a direct object in noun form (Franco, in this example) follows the verb, as in Linda ama Franco “Linda loves Franco.” However, when the direct object is already understood it would be pronominalized (lo “him” in this example) and placed in front of the verb, as in Linda lo ama (with the literal translation of “Linda him loves”).

Even though direct object clitics constitute an obligatory element in the sentence, these morphemes are omitted more frequently by children with SLI acquiring French, Italian, and Spanish than by their younger typically developing compatriots (Bedore & Leonard, 2005; Dispaldro, Leonard, & Deevy, 2013; Hamann et al., 2003).

Germanic and Romance languages are certainly not the only languages revealing areas of extraordinary difficulty for children with SLI. Pockets of special difficulty have been seen in languages ranging from Cantonese (Fletcher, Leonard, Stokes, & Wong, 2005), a language with no grammatical suffixes, to Hungarian (Lukács, Leonard, Kas, & Pleh, 2009), a language that permits a string of suffixes to appear in sequence after a single noun or verb. The problems with pronominal clitics so often associated with Romance languages have also been observed in children with SLI acquiring Czech (Smolík & Vávrů, 2014) and Greek (Stavrakaki & van der Lely, 2010). Not surprisingly given the diversity of languages under study, the specific types of difficulty experienced by children with SLI acquiring these languages will vary as a function of the type of language serving as the target. Preliminary evidence is also being obtained from deaf signing children suspected of experiencing SLI. Although this area faces obstacles not usually seen in research on hearing children (e.g., developing a normative database on deaf, signing children; controlling for age and quality of sign exposure), results thus far seem to be informative. For example, as is often the case with hearing children with SLI, deaf signing children with SLI appear to be weaker in morphosyntax than in vocabulary (Marshall, Mason, Rowley, Herman, & Morgan, 2011).

Given the difficulties that children with SLI face in their acquisition of language, it might be assumed that learning more than one language would be a burden too much for the children to bear. Yet this does not appear to be the case. When these children are exposed to two languages from the outset—an important qualifier—they will show weaknesses in both languages, but will be no more impaired in either language than if they had been exposed to one language only (Paradis, Crago, & Genesee, 2005; Paradis, Crago,Genesee, & Rice, 2003).

Although children will have difficulty in each language, the chief areas of difficulty will vary according to the language spoken (Rothweiler, Chilla, & Clahsen, 2012; Stavrakaki, Chrysomallis, & Petraki, 2011). Recall that in a Romance language such as French, direct object pronominal clitics are often omitted by children with SLI. The closest counterpart in English would be the direct object pronouns produced in similar contexts (e.g., him in Linda loves him), yet this is not problematic for English-speaking children with SLI. One difference between the two types of languages is that, in English, pronouns occupy the same sentence position as the nouns they refer to. As noted above, this is not true for pronominal clitics.

Findings from studies of bilingual children with SLI have been important for at least two reasons. The first is quite practical; parents of these children might not be faced with a decision about which of two languages to continue with a child, as both probably have importance. Second, if severity of impairment does not increase with a second language, SLI would appear to be more a problem of a weakness in language aptitude rather than in language capacity.

Do the Language Symptoms of SLI Differ from Those in ASD and Dyslexia?

There is clear overlap between children with SLI and those showing symptoms of “classic” dyslexia, in that both groups will tend to score poorly on measures involving phonological skills (Bishop & Snowling, 2004). In addition, those children whose reading problems involve poor comprehension often show low scores in spoken comprehension as well. However, because many children with reading problems show only subclinical weaknesses in spoken language areas, these two deficits should not be viewed as interchangeable.

At first blush, the distinction between SLI and autism spectrum disorder (ASD) seems obvious, but there is a subgroup of children with SLI whose language symptoms include grammatical difficulties resembling those of children with SLI (Tager-Flusberg & Joseph, 2003). However, despite these similarities, the language difficulties in children with ASD and those seen in SLI appear to have separate causes (Williams, Botting, & Boucher, 2008).

Alternative Theoretical Accounts of SLI

Beginning in the late 1980s, investigators began to move beyond merely describing the language symptoms of SLI to proposing possible reasons for these symptoms. Most of these accounts concentrated on the more salient deficits in grammar seen in these children. Because children with SLI also have weaknesses in vocabulary and other areas, these grammatically based accounts cannot provide a complete explanation of this disorder, but they have represented a significant step forward.

One of the most prominent accounts of grammatical deficits in SLI was proposed by Rice, Wexler, and their colleagues (Rice & Wexler, 1996; Rice, Wexler, & Cleave, 1995). This account has been refined since its original appearance in the literature (Wexler, 2003; Wexler, Schütze, & Rice, 1998) but we refer to it here by its original name, the “extended optional infinitive” (EOI) account. According to this account, all children go through a developmental period during which a biologically based language acquisition principle has not yet matured, resulting in children treating tense and agreement morphemes as optional in their speech. The specific morphemes affected vary somewhat according to the typology of the language being learned. For languages such as English and other German languages, verb inflections and auxiliary forms expressing tense and agreement will often be replaced by nonfinite forms, as in Jill like chocolate and Jenny walking home. For Romance languages such as Italian, this pattern will be seen to a greater extent in contexts requiring auxiliaries (e.g., Gina mangiato il gelato “Gina eaten the ice cream” in place of Gina ha mangiato il gelato “Gina has eaten the ice cream”). This principle is assumed to emerge much later in children with SLI, resulting in an extended period of inconsistent use of tense and agreement morphology. Although it is not yet clear how well this account will handle languages with other typologies, it represented an important breakthrough by showing that errors are usually not failed attempts to articulate a verb inflection like –s or –ed or to include a function word such as is, but rather these errors appear to be selections of nonfinite forms, not unlike Bob play basketball in Let’s watch Bob play basketball and The girl running in We saw the girl running. Another advantage of this account is that it assumes a delay in a developmental process that is seen in typical children; thus, it does not make assumptions that children with SLI have a wholly different language learning mechanism.

Recently, there have been proposals that the principle responsible for adultlike tense and agreement use may not emerge abruptly but may instead take shape on a more gradual basis. Specifically, languages differ in the degree to which they mark tense and agreement overtly; compare English I see, you see, she sees, we see, you see, they see with Italian vedo, vedi, vede, vediamo, vedete, vedono. Legate and Yang (2007) proposed that the rate at which children grow out of the optional stage of tense and agreement use depends on the degree to which overt marking of these features is available in the language. Hadley, Rispoli, Fitzgerald, and Bahnsen (2011) showed that variation of input within a language can also affect children’s use of tense and agreement morphology. These investigators found that children of parents who used a higher percentage of overt marking showed greater productivity with tense and agreement morphology in their own speech several months later.

An alternative input approach has been advanced by Leonard and Deevy (2011) and Leonard, Deevy, Fey, and Bredin-Oja (2015). According to this alternative, children with SLI are slow in recognizing that nonfinite subject-verb sequences (e.g., the boy running; the girl wash the car) contained in larger input sentences such as We saw the boy running and Let’s help the girl wash the car cannot be extracted from the larger structures. Until they develop adequate comprehension of these larger structures, the children continue to produce the ungrammatical versions as standalone utterances. Although this approach has the potential to explain some of the salient errors in German, Dutch, and Swedish SLI, it is not yet clear if it will be able to accommodate the full range of grammatical errors seen across languages.

An account by van der Lely and colleagues views grammar as the result of a biologically based and autonomous system—a system that is impaired in many children with SLI. This account has undergone revision over the years but will be referred to here as the Computational Grammatical Complexity account (Marshall & van der Lely, 2007; van der Lely, 1998; van der Lely & Pinker, 2014). This account goes beyond problems with tense and agreement by proposing that whenever some detail of grammar involves an “extended” grammatical representation by being hierarchical or abstract, it will cause special problems for children with SLI. Along with tense and agreement as problem areas, difficulties will also be seen in understanding complex sentences and producing words with complex phonology. This type of account has an advantage in providing an explanation for a broader range of deficits than the EOI account. However, it is not clear that the deficits in grammar are as isolated from other language deficits as this account assumes (Bishop et al., 2000; Tomblin & Pandich, 1999).

Several different accounts of SLI have emphasized processing limitations in these children. These have varied from an emphasis on speed of processing (e.g., Kail, 1994), working memory (Bavin, Wilson, Maruff, & Sleeman, 2005; Ellis Weismer, Plante, Jones, & Tomblin, 2005), attention (Spaulding, Plante, & Vance, 2008), and executive functioning (Im-Bolter, Johnson, & Pascual-Leone, 2006). Children with SLI have been found to show weaknesses in each of these areas. However, apart from some correlations between weaknesses in these areas and problems in sentence comprehension, it has been difficult to tie specific symptoms of SLI to processing limitations. A few other accounts assume processing limitations but focus on specific symptoms, such as tense and agreement problems (e.g., Leonard, 1989; Leonard, Eyer, Bedore, & Grela, 1997).

Ullman and Pierpoint (2005) have offered what may be the most comprehensive account of the deficits seen in SLI. These investigators distinguish between the procedural memory system and the declarative memory system and propose that children with SLI have greater problems with the former. The procedural system is associated with skills that involve learning sequences, which can be linguistic, (non-linguistic) cognitive, or motoric in nature. In neuroanatomical terms, the procedural system involves a network of brain structures involving, especially, frontal/basal-ganglia circuits. The declarative system has its basis in medial temporal lobe regions and is associated primarily with semantic and episodic memory.

As noted earlier, morphosyntax is especially weak in children with SLI and much of this area seems to operate within the procedural memory system. Vocabulary learning, though not often at age level in these children, tends to be less seriously affected. This skill appears to involve the declarative system. Recently, Ullman and Pullman (2015) reviewed evidence suggesting that, along with controlling its own domain, the declarative system is sometimes recruited as a compensatory system in children with SLI, to handle tasks that in typically developing children would be handled by the procedural system.

Because the procedural system crosses modalities, one might expect difficulties in areas beyond language if this proposal is accurate. Indeed, this is the case. Tomblin, Mainela-Arnold, and Zhang (2007) found that children with SLI were relatively weak in anticipating the location at which visual characters would appear on a computer screen following a period when the characters appeared in a predictable sequence. This task involved implicit learning and in this sense mimicked grammatical rule learning. Similar findings have also been reported in subsequent studies (e.g., Hedenius et al., 2011; Lum, Conti-Ramsden, Page, & Ullman, 2012). In some of these studies, investigators singled out those children with the most severe deficits in grammar and found that these were the children performing worst on the nonlinguistic procedural learning task. It is clear that the procedural and declarative systems must interact, but this fact has made it somewhat difficult to isolate one system from the other in identifying problem areas in SLI. However, this account seems to represent an important step forward in our understanding of this disorder.

SLI and Language Treatment

Research on language treatment for children with SLI has been ongoing for decades. Most of these studies have employed certain basic controls, such as comparisons of children assigned to an experimental group versus those assigned to a business-as-usual (traditional treatment approach) or those assigned to a wait list. However, often these studies did not meet all of the criteria of a randomized controlled trial—which is currently considered the gold standard in treatment research. Early reviews of studies qualifying as randomized controlled trials indicated that approaches designed to facilitate children’s vocabulary production were largely successful, more so than approaches aimed at helping children’s production of morphosyntax. Studies directed at children’s comprehension of morphosyntax, though limited in number, met with less success (see Law, Garrett, & Nye, 2004).

Recently, additional randomized controlled trials have come on the scene, including those reporting clear effects for the treatment of the production of morphosyntax (Ebbels, van der Lely, & Dockrell, 2007). Others have been studies of large numbers of children who received targeted treatment for weaknesses that cut across vocabulary and morphosyntax. Reported gains were greater by the children receiving the targeted treatment relative to those seen in no-treatment or business-as-usual groups (Boyle et al., 2007; Broomfield & Dodd, 2011). Fricke, Bowyer-Crane, Haley, Hulme, and Snowling (2013) conducted a randomized controlled trial with young children earning relatively low scores on measures of language ability who had not received a formal diagnosis of language impairment. Treatment focused on vocabulary and narrative skills. The children assigned to treatment made significantly greater gains than children in a wait-list group.

Given the recent emphasis on randomized controlled trials, it is likely that more studies meeting stricter design criteria will appear in the near future. This will be especially valuable for a full evaluation of treatment procedures that have appeared highly successful in studies whose designs did not meet the full criteria of a randomized controlled trial. For example, “conversational recasting” is an approach in which the clinician responds to a child’s spontaneous utterance with a conversational reply that contains a particular grammatical target (e.g., Camarata & Nelson, 1992; Fey, Cleave, & Long, 1997; Leonard et al., 2008; Nelson et al., 1996). Thus far, results have been very encouraging. Large-scale randomized controlled trials would seem to be the next step in evaluating this procedure.

Conclusion

Specific language impairment is a neurodevelopmental disorder with a likely multifactorial basis that affects approximately 7% of children. Language ability is clearly the most severely affected area, though many children may have subclinical weaknesses in other developmental areas. In many children there is a genetic contribution whose nature is yet to be clarified. Although these children do not show evidence of frank neurological damage or disease, neurobiological measures have revealed atypical brain configurations and electrophysiological responses in some of these children. To the extent it can be determined at this point, SLI seems to be a universal phenomenon, though the language symptoms will vary according to the typology of the language being learned. The disorder is long-standing. Although treatment appears to be generally successful, some degree of weakness in language often persists through adolescence and into adulthood, with adverse academic, emotional, and even economic consequences.

Considering the prevalence of SLI and the long-term consequences of this condition, it is surprising that SLI has not received more attention from the general public. Part of this problem is that the clinical labels for this disorder have varied over the years, and the subtle weaknesses that can sometimes be seen in areas beyond language have blurred the diagnostic boundaries between SLI and other disorders. It is ironic that in an era when there is worry that certain diagnoses (e.g., ADHD, ASD) may be overused, there is a group of well-studied children with documented long-standing problems who go relatively unrecognized.

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