James W. Montgomery, Julia L. Evans &
Ronald B. Gillam
What is SLI?
Children with specific language impairment (SLI) have language learning difficulties that cannot be attributed to clinically significant medical, neurological, sensory or environmental factors. However, various risk factors related to heredity and neural development are likely to contribute to their developmental lags in language development (Bishop, 2009; Law et al., 2004; Rice, Oetting, Marquis, Bode, &
Pae, 1994). Within the language domain, children with SLI may exhibit expres - sive language impairment, receptive impairment or mixed receptive–expressive impairment relative to same-age peers. These children demonstrate normal-range nonverbal intelligence; however, they tend to perform in the lower range than same-age typically developing (TD) peers (Gallinat & Spaulding, 2014). Despite their normal-range nonverbal intelligence, children with SLI exhibit a range of cognitive limitations, including memory and Executive Functioning (controlled attention). In this chapter, we summarize what is known about the memory and Executive Functioning of children with SLI. We also summarize their language limitations and their impact on academic and vocational outcomes as well as typical intervention approaches.
Diagnostic features
Children with SLI exhibit demonstrable language impairment (receptively, expressively, or both) relative to their same-age peers. They may have deficits across a range of language areas such as lexical, morphological, syntactic, and narrative.
Epidemiologically-derived diagnostic criteria for defining SLI have been provided by a series of longitudinal studies of kindergarten children with SLI by Tomblin and associates (Catts, Bridges, Little, & Tomblin, 2008; Fey, Catts, Proctor- Williams, Tomblin, & Zhang, 2004; Leonard, Ellis Weismer, Miller, Francis, Tomblin, & Kail, 2007; Tomblin, Zhang, Buckwalter, & O’Brien, 2003). Tomblin,
Records and Zhang (1996) reported excellent sensitivity/specificity (values above .90) for (a) a standard score cut-off of 83 on a global language test that assessed receptive and expressive vocabulary, syntax, and discourse abilities or (b) per- formance at least –1.25 standard deviations (SD) from the mean on two or more composite scores indexing language comprehension, language production, vocabulary, morphology, grammar, and narration. Other investigators define SLI as performance that is –1 SD or more from the mean on several tests or subtests of receptive–expressive language (Leonard, 2014; Spaulding, Plante, & Farinella, 2006). Researchers have also argued that low performance on verbal memory tasks such as sentence imitation is a reliable marker of SLI (Archibald & Joanisse, 2009;
Conti-Ramsden, Botting, & Faragher, 2001; Stokes, Wong, Fletcher, & Leonard, 2006). There is less agreement, however, on the sensitivity/specificity of nonword repetition as a reliable marker (Stokes et al., 2006).
The term SLI does not appear in the Diagnostic Statistic Manual (DSM-5, APA, 2013). Communication disorders (including language disorder) instead are categorized as neurodevelopmental disorders along with intellectual disability, autism spectrum disorder, attention-deficit/hyperactivity disorder, Specific Learning Disorder, and motor disorders. This group of disorders has similar genetic risk factors, shared neural substrates, and similar clinical features such as cognitive processing problems, higher rates of comorbidity with each other, and a develop mental course into adulthood (Andrews, Pine, Hobbs, Anderson, & Sunderland, 2009). The current neurologically-based characterization of language disorder in some sense harkens back to a similar characterization prevalent in the 1960s, albeit with much stronger scientific grounding (see Current Debate about the Term SLI, on page 30).
Working Memory (WM) and related Executive Function (EF) deficits
Numerous models of Working Memory (WM) have been proposed, including the multi-component model (Baddeley, 1999, 2012), the embedded processes model (Cowan, Saults, & Blume, 2014), the dual store model (Unsworth & Engle, 2007), and the time-based resource-sharing model (Barrouillet, Gavens, Vergauwe, Gaillard, & Camos, 2009) to mention a few. Despite theory dependent differ- ences in architectural and functional details, all models assume WM to be a multi-mechanism construct with the principal function that enables individuals to store and maintain information (verbal, visual) in an active and accessible state while performing a cognitive task (e.g., listening to a story, calculating a mathematical problem, or reading a passage).
The vast majority of WM research on children with SLI has been conducted within Baddeley’s (2012) multi-component model, with a primary focus on storage abilities because of their immediate relevance to language. This model comprises four separable yet interactive components. The domain-general central executive (controlled attention) was originally conceived of as an attentional supervisor, controlling activities across the WM system. It was fractionated into various attention
functions such as focus, dividing attention, and switching, as well as an interface with long-term memory (LTM). The second and third mechanisms corres pond to domain-specific memory storage devices, one for the retention of verbal material (phonological loop) and the other for visuospatial input (visuo spatial sketch pad).
The episodic buffer, the fourth component, was incorporated later to account for a variety of findings showing individuals’ ability to engage in simultaneous informa - tion processing and storage, and to account for the interface between WM and LTM. The buffer is regarded as a storage device for holding cross-modal (verbal, visual) inputs that have been bound together into coherent chunks or episodes (e.g., integrated pictorial–linguistic representations), which are part of LTM.
Working Memory
Memory storage
There is a long history of research on the basic memory skills of children with SLI dating back to the 1960s. However, Gathercole and Baddeley (1990) were the first to tie the evidence of SLI verbal memory difficulties to a well-specified cognitive model (Baddeley, 1986; Baddeley & Hitch, 1974). Specifically, they proposed that children with SLI have a primary deficit in storing phonological representations, which interferes with language development and use. Memory storage has been conceptualized and measured in two different ways—simple (phonological) storage and complex storage. Children with SLI consistently demonstrate significantly reduced simple and complex verbalstorage than TD peers.
Simple verbal storage
Simple verbal storage reflects children’s ability to temporarily hold in mind phonological material in the absence of performing any explicit processing activity.
For example, children hear or see strings of items and are asked to remember and recall the strings in serial order. Compared with same-age TD peers, children with SLI exhibit significant deficits, indexed by reduced item recall (Archibald &
Gathercole, 2006, 2007; Conti-Ramsden, 2003; Dollaghan & Campbell, 1998;
Edwards & Lahey, 1998). Results of a meta-analysis (Graf Estes, Evans, & Else- Quest, 2007) indicate that children with SLI demonstrate a significant deficit (–1.27 SD) in simple verbal memory storage relative to same-age TD peers.
Simple visuospatial storage
Findings are inconsistent as to whether children with SLI exhibit significantly poorer nonverbal storage compared with age-matched TD peers. Some investi gators have reported a significant deficit (Bavin, Wilson, Maruff, & Sleeman, 2005; Nickish
& Von Kries, 2009) while others have not (Archibald & Gathercole, 2006, 2016).
Some of these differences may owe to variations across studies in the criteria used
to define children as SLI and the visual stimuli that have been used. The mixed findings and reduced disproportionate deficit in visuospatial storage in SLI is supported by the results of a meta-analysis reported by Vugs, Cuperus, Hendricks and Verhoeven (2013) showing that children with SLI perform just .67 SD below the mean relative to same-age peers.
Complex verbal storage
Complex verbal storage reflects children’s ability to coordinate verbal storage with concurrent information processing. A common complex storage measure is listening span. In one kind of task, children are presented blocks of sentences and are asked to judge the truth value of each sentence while also remembering the last word in each sentence. Immediately after the last sentence in a block, children recall in order as many sentence-final words as they can. Compared with same-age peers, children with SLI reliably exhibit more-limited storage, indexed by reduced item recall (Archibald & Gathercole, 2006; Ellis Weismer, Evans, & Hesketh, 1999;
Montgomery & Evans, 2009).
Archibald and Gathercole (2007) examined the influence of short-term verbal and nonverbal memory storage and processing abilities on the complex storage (recall on complex WM tasks) of children with SLI. Most relevant to us, they compared a group of children with SLI with a same-age TD group. They were primarily interested in speed of processing because children with SLI have exhibited domain- general slower processing than same-age peers, (Miller, Kail, Leonard, & Tomblin, 2001), which could affect the speed with which children with SLI complete the processing component of a WM task.
Children completed two simple storage tasks (verbal, visuospatial), two processing tasks (verbal, visuospatial), and four WM tasks. The individual storage and processing tasks were combined and crossed to derive the four WM tasks:
verbal–verbal; verbal–nonverbal; visuospatial–verbal; and visuospatial–visuospatial.
The design allowed the authors to examine the effects of verbal versus nonverbal storage, verbal versus nonverbal processing, and the interaction of storage-processing domains on children’s complex WM capacity. On the WM tasks, the SLI group yielded significantly poorer verbal storage (but not nonverbal storage) compared with same-age mates when the WM tasks required either verbal or nonverbal processing. The findings were taken to suggest that, relative to TD peers, children with SLI were disproportionately impaired in verbal storage but that this storage deficit by itself did not account for their reduced complex verbal storage.
Rather the children’s storage limitations were a reflection of a combination of a phonological storage deficit and slower processing.
Executive Function
Executive Functions (also known as executive control or controlled attention) refer to various controlled cognitive processes that participate in a range of other
higher-order cognitive abilities such as planning, reasoning, and problem solving (Diamond, 2013). Emerging evidence suggests that, relative to same-age TD mates, children with SLI exhibit deficits across a range of domain-general (verbal, visual) Executive Functions. Children with SLI show significantly poorer ability than same-age peers in sustaining their attention in both the auditory and visual domains (Ebert & Kohnert, 2011; Finneran, Francis, & Leonard, 2009; Victorino
& Schwartz, 2015). The ability to inhibit or suppress a response to interfering non- target stimuli is also an important Executive Function, allowing children to selectively respond to task-relevant stimuli while ignoring irrelevant stimuli.
Compared with TD children, children with SLI have significantly poorer inhibition (Im-Bolter, Johnson, & Pascual-Leone, 2006; Marton, Campanelli, Eichorn, Scheuer, & Yoon, 2014; Pauls & Archibald, 2016; Spaulding, 2010).
Another Executive Function is attention shifting, which is the ability to rapidly change attentional focus from one task to another task (also referred to as mental set shifting). Children with SLI reveal significant difficulty making global attention shifts (Im-Bolter et al., 2006). Adolescents with SLI also show evidence of poorer attention shifting than TD adolescents, indexed by slower attentional blink (Lum, Conti-Ramsden, & Lindell, 2007). Attention may also be shifted in a more micro- level fashion—within the same task—referred to as attention switching (Barrouillet et al., 2009). Switching involves rapidly alternating focal attention within the same task to different elements of the task. A good example is when individuals perform a WM task in which they must rapidly alternate their attention between refreshing items in storage and completing a processing task (Barrouillet et al., 2009). Relative to TD peers, children with SLI show significantly poorer attention switching on these types of tasks (Evans, Montgomery, & Gillam, submitted).
Finally, memory updating, another Executive Function, relates to the ability to continually add new items to a mental memory list (update list) as the individual per forms a cognitive activity. For example, for each trial in a WM task an individ - ual must update the ongoing mental list of to-be-recalled items while completing the processing component of the task. As with the other Executive Function tasks mentioned above, children with SLI exhibit unusual difficulties updating their memory with new, incoming information (Evans et al. (submitted); Im-Bolter et al., 2006).
Long-term memory in SLI
While SLI research has historically focused on these children’s WM system, memory can also be conceptualized based on different brain systems, where the key distinction is the capacity for conscious recollection of facts and events (e.g., declarative explicit memory) and a heterogeneous collection of nonconscious learning capacities (e.g., non-declarative implicit memory) that are expressed through performance and do not afford access to any conscious memory (Squire, 1994; Squire & Zola, 1996). Learning via declarative memory is characterized by rapid, single stimulus presentation of arbitrarily related information. It includes
memory for “episodes” or experiences and memory for “meanings” (semantic;
Tulving, 1991), which are acquired easily through single instances, easily applied to novel situations, and easily expressed verbally (cf. Squire & Knowlton, 2000).
Declarative memory is often tested by list learning and list retrieval tasks that indicate how well participants learn word pairs after multiple exposures. Children with SLI typically recall fewer words than age-matched peers, with an average effect size difference of .90 (Lum, Conti-Ramsden, Page, & Ullman, 2012). However, the learning tasks in declarative memory measures have large attention, WM, and language requirements. Once differences in basic language knowledge and WM were accounted for, the declarative memory differences between the groups were no longer significant.
The non-declarative memory system is a collection of abilities that, in addition to perceptual motor skill learning (e.g., procedural memory), includes probabil- istic learning of categories and sequences, artificial grammar learning, statistical sequential learning and prototype abstraction, with different brain structures supporting each (Perruchet & Pacton, 2006; Squire & Zola, 1996). Non-declarative memory is often called “implicit” because learning is expressed through per formance and not available to conscious access. Learning occurs gradually. Non-declarative memory is tightly tied to the original learning situation, and best assessed in con - ditions similar to the original learning context (Squire, 1994). A growing body of work has begun to examine non-declarative—implicit learning in children with SLI and emerging findings indicate that implicit memory and learning in these children should be considered a significant aspect of the deficit profile, suggesting that children with SLI are unable to extract statistical dependencies from the input compromising lexical and grammatical acquisition (Evans, Saffran, & Robe, 2009;
Hsu & Bishop, 2011; Lum, Conti-Ramsden, Morgan, & Ullman, 2014).
According to the Procedural Deficit Hypothesis (PDH), abnormalities of brain structures underlying procedural memory largely explain the language deficits of SLI (Ullman & Pierpont, 2005). These authors have proposed that the abnormalities result in core deficits of procedural memory, which, in turn, explain SLI grammar problems. According to the PDH, deficits in procedural learning are also likely to lead to problems with other, non-procedural functions like WM that rely at least partly on the affected brain structures. Though the procedural component of implicit memory is impaired in SLI, research suggests that SLI implicit learning deficits may extend beyond the procedural system to deficits in artificial grammar learning and statistical sequential learning (Evans et al., 2009; Hsu &
Bishop, 2011; Plante, Gomez, & Gerken, 2002; Tomblin, Mainela-Arnold, &
Zhang, 2007). Declarative memory is expected to remain largely intact, and should play a compensatory role in grammar learning.
Neurological profile of SLI
By definition, children with SLI evidence no frank neurological impairments such as brain lesions. However, it seems reasonable that a connection may exist between
these children’s neural functions and language abilities. Neuroimaging research to date is relatively scarce, though some studies suggest neurological differences between children with SLI and TD peers. Structural differences between children with SLI and TD children in inferior frontal cortex have been noted (Gauger, Lombardino, & Leonard, 1997), as well as in the temporal–parietal junction (Fu, Wan, Baker, Montgomery, Evans, & Gillam, 2016; Jernigan et al., 1991). Many children with SLI also have exhibited atypical hemispheric asymmetries. The perisylvian area (inferior frontal gyrus back to the supramarginal gyrus) is larger in the right hemisphere than in the left hemisphere (Plante, Swisher, Vance, & Rapcsak, 1991; Gauger et al., 1997). This pattern is the reverse of the left lateralization typically seen in TD children. Laterality differences between children with and without SLI, however, have not been revealed by fMRI studies (Ellis Weismer, Plante, Jones,
& Tomblin, 2005) and functional Near Infrared Spectroscopy (fNIRS) studies (Fu et al.) as the children engaged in memory or language processing tasks.
A number of EEG and ERP studies on children with SLI exist. Results of these studies suggest that these children exhibit poor attentional processes related to the auditory processing of nonspeech and speech stimuli, as indexed by flatter or reduced ERPs (McArthur, Atkinson, & Ellis, 2009; Weber-Fox, Leonard, Hampton, & Tomblin, 2010). These children also show less robust ERPs (P600s) than TD children when detecting grammatical violations (Shafer, Schwartz, Morr, Kessler, & Kurtzberg, 2000; Weber-Fox et al., 2010), but not when detecting semantic violations (Fonteneau & van der Lely, 2008; Weber-Fox et al., 2010).
Interestingly, attentional processing of children with SLI (indexed by ERP) become more like TD children after receiving narrative language intervention (Popescu, Fey, Lewine, Finestack, & Popescu, 2009). Such findings suggest that neural processing difficulties contribute to SLI. But, more important, these processes can be positively altered through language intervention.
Impact of SLI on language and academic outcomes
Language: Preschool years
By 24 months of age, TD children are using 2-word phrases with diverse vocabulary, while children with SLI may still be pointing at objects they want or using a few single words. Children with SLI are known to lag behind their age peers in learning words, both nouns and verbs. These children are described as having difficulty with fast mapping—learning the phonological form as well as syntactic and semantic features of words (Chiat, 2001; Jackson, Leitao, &
Claessen, 2016; Rice et al., 1994). Relative to same-age peers, children with SLI overall exhibit poorer lexical knowledge (Mainela-Arnold, Evans, & Coady, 2008, 2010; McGregor, Newman, Reilly, & Capone, 2002).
Preschoolers with SLI also have difficulty learning grammatical morphology, especially those that mark tense and agreement (e.g., He walk home; She my momma). Problems with grammatical morphology represent a reliable clinical
marker of SLI (Conti-Ramsden et al., 2001; Hadley & Holt, 2006). Children with SLI also show difficulty understanding complex sentences (Contemori & Garraffa, 2010) and producing complex sentences (Contemori & Garraffa, 2010; Marinellie, 2004).
Language: School-age years
School-age children with SLI continue to exhibit sentence comprehension deficits (Friedmann & Novogrodsky, 2007; Montgomery & Evans, 2009; van der Lely &
Stollwerck, 1997) and produce less complex grammar (Gillam & Johnston, 1992;
Leonard, 2014; Novogrodsky & Friedmann, 2009). These children also show difficulties with phonological awareness (Gillon, 2005). Problems with phonological awareness may make it more difficult for these children to make connections between letters and the sounds they represent, making word recognition difficult.
Narration (storytelling) is an important form of discourse during the school-age years. TD children produce narratives that are generally grammatical and contain syntactically diverse and complex sentences, in addition to basic story elements (settings, events, actions, endings) that are organized temporally and logically.
Children with SLI, however, are likely to produce narratives that are disorganized, and that contain more ungrammatical sentences and fewer complex sentences (Gillam & Johnston, 1992). Another hallmark of SLI during the school-age years is difficulty understanding and using higher-level language skills such as inferences, assumption that are not explicitly expressed about concepts or ideas (Laing & Kamhi, 2002; Karasinski & Ellis Weismer, 2010).
Language: Secondary-school years
Adolescents with SLI may demonstrate difficulty with understanding and using figurative language, multiple meaning words, idioms and abstract or curricular vocabulary. Much of the conversational and written discourse that is encountered during the adolescent years contains morphologically complex words that require knowledge and experience with word structure as well as mastery of sophisticated metalinguistic skills (Nippold & Sun, 2008). Adolescents with SLI may exhibit poor comprehension of curricular texts presented orally or in written form identified by an inability to state main ideas, and to recall and report facts about persons, dates and details, and to make logical generalizations and conclusions (Gillam, Fargo,
& St. Clair Robertson, 2009). They may also have difficulties composing book reports, essays, and term papers that require integration of information contained in oral and written discourse.
Academic and vocational outcomes
The consequences of SLI on academic achievement are detrimental and long - standing. Children with SLI are at great risk for academic failure through