Specific Language Impairment is not Specific to Language: the Procedural Deficit Hypothesis

Abstract

Specific Language Impairment (SLI) has been explained by two broad classes of hypotheses, which posit either a deficit specific to grammar, or a non-linguistic processing impairment. Here we advance an alternative perspective. According to the Procedural Deficit Hypothesis (PDH), SLI can be largely explained by the abnormal development of brain structures that constitute the procedural memory system. This system, which is composed of a network of inter-connected structures rooted in frontal/basal-ganglia circuits, subserves the learning and execution of motor and cognitive skills.Crucially, recent evidence also implicates this system in important aspects of grammar. The PDH posits that a significant proportion of individuals with SLI suffer from abnormalities of this brain network, leading to impairments of the linguistic and non-linguistic functions that depend on it. In contrast, functions such as lexical and declarative memory, which depend on other brain structures, are expected to remain largely spared. Evidence from an in-depth retrospective examination of the literature is presented. It is argued that the data support the predictions of the PDH, and particularly implicate Broca's area within frontal cortex, and the caudate nucleus within the basal ganglia. Finally, broader implications are discussed, and predictions for future research are presented. It is argued that the PDH forms the basis of a novel and potentially productive perspective on SLI. INTRODUCTIONSpecific Language Impairment (SLI) is generally defined as a developmental disorder of language in the absence of frank neurological damage, hearing deficits, severe environmental deprivation, or mental retardation (for diagnostic definitions and prevalence of SLI, see Bishop, 1992;Leonard, 1998;Tomblin et al., 1997). Other terms have also been used to label such children, including developmental dysphasia, language impairment, language learning disability, developmental language disorder, delayed speech and deviant language (Leonard, 1998;Ahmed et al., 2001). Several factors have complicated attempts to provide a unified theory of SLI, or even of subgroups of SLI. First, despite the standard use of exclusionary criteria to diagnose SLI, the disorder is clearly not limited to language.Rather the linguistic impairments co-occur with a number of non-linguistic deficits, including impairments of motor skills and working memory, and with other disorders, such as Attention Deficit Hyperactivity Disorder (Hill, 2001;Leonard, 1998;Tirosh and Cohen, 1998). Second, even though SLI must be a consequence of some sort of neural dysfunction, the neural correlates of the disorder have been largely ignored. This potentially valuable information could provide important constraints on explanatory accounts of the disorder. Third, SLI is a classification that is quite heterogeneous (Leonard, 1998;Stromswold, 2000). Surveys document variation within and across subgroups in the particular aspects of language that are affected and in the types of co-occurring no...

Implicit learning seems to come naturally for children with autism, but not for children with specific language impairment: Evidence from behavioral and ERP data

“…In line with this, intracranial findings suggest that hippocampal‐frontal networks are involved in a frontal P3 component [Knight, 1996]. Thus, it could be speculated that our ERP findings in SLI reflect impaired basal ganglia learning (reflected by the absence of the N2b) which is compensated for by hippocampal learning (reflected by the frontal P3) in line with the procedural learning deficit from Ullman and Pierpont [2005]. …”

“…This might be due to the use of intentional learning to compensate for an implicit learning deficit [Ullman & Pierpont, 2005]. Interventions for children with SLI could aim at compensating for the limited implicit learning abilities, by offering more explicit training in learning statistical regularities, such as grammar and social situations.…”

“…Indeed, most studies reported impaired implicit learning on the SRT task in SLI [Lum, Conti‐Ramsden, Morgan, & Ullman, 2014; Obeid et al, 2016; Zwart, Vissers, Kessels, et al, 2017], although several studies demonstrated intact performance [e.g., Gabriel, Maillart, Guillaume, Stefaniak, & Meulemans, 2011; Lum & Bleses, 2012]. Similar to ASD, a compensatory role of explicit learning has been suggested in SLI too [Lum et al, 2014], but it is still debated whether this explicit learning is fully intact [e.g., Hsu & Bishop, 2014; Ullman & Pierpont, 2005]. Supporting this are findings that grammar abilities were associated with implicit learning in TD children, whereas they were associated with explicit learning in children with SLI [Lum et al, 2014].…”

Speeded processing of grammar and tool knowledge in Tourette's syndrome

“…In one study, TS subjects showed faster and more force-efficient performance than controls in (some, though not all) goal-directed movements, with no group differences in movement accuracy (Georgiou, Bradshaw, Phillips, Cunnington, & Rogers, 1997). In another study, mental rotation, which depends on procedural memory brain structures (Ullman & Pierpont, 2005), was found to be impaired in males with TS (relative to male controls), but was actually enhanced (more correct responses in a five minute period, thus reflecting a combination of accuracy and speed) in females with TS (relative to female controls) .Declarative memory appears to remain largely normal in TS. A number of tasks have probed learning in declarative memory.…”

Fine motor deficits in reading disability and language impairment: same or different?

“…These studies were prompted by the procedural deficit hypothesis of Ullman and Pierpont (2005) who suggested that children with LI have abnormalities in the procedural memory system, affecting the ability to learn both linguistic and non-linguistic sequences. Nicolson and Fawcett (2007) took this idea further, suggesting that dyslexia and LI might be caused by impairments in different parts of the procedural learning system, with the cortico-cerebellar system implicated in dyslexia, and the cortico-striatal system in SLI.…”