Language Learning Impairment Research Paper

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Early reports of children who failed to learn language at or near the expected age, but with no history or evidence of acquired brain lesions, began to emerge only in the mid -1960s. Such children were first diagnosed as ‘developmentally aphasic.’ The diagnosis of ‘developmental aphasia’ emphasized the congenital or developmental, rather than acquired, nature of the language learning impairment. Similarly, it was emphasized that the specificity of the deficit must lie in the linguistic domain. Language learning problems secondary to other developmental disabilities such as hearing loss or deafness, mental retardation, evidence of frank neurological problems (such as seizure disorder or hemiplegia), or structural or motor deficits effecting the oral musculature (such as cleft palate or cerebral palsy), were traditionally excluded from the classification of developmental aphasia. One exception to this diagnosis by exclusion was notable deficits, not only in speech and language, but also in higher order central auditory processing.

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Over the years the term ‘developmental aphasia’ has been replaced by other terms such as ‘congenital aphasia,’ ‘specific language impairment’ and, most recently ‘language learning impairment.’ The most recent classification, ‘language learning impairment,’ acknowledges the frequent continuum between oral and written language learning impairments in children. More recently, research has moved away from a focus on exclusionary definitions of specific language impairments to a broader perspective that focuses on better elucidating the linguistic, cognitive, and neurobiological concomitants of developmental language learning impairments.

Developmental language learning impairments (LLI) can occur in children for a wide variety of reasons, and are one of the most prevalent of all developmental disabilities. It has been estimated that approximately 20 percent of all children have some form of language learning impairment. The most common known causes of developmental language learning impairments are hearing loss (including intermittent hearing loss resulting from chronic otitis media), general mental retardation, and motor or structural defects affecting the oral musculature. Approximately 7.5 percent of children have developmental language impairments of unknown origin.

1. Diagnosis

The differential diagnosis of developmental language learning impairments is based on behavioral evaluations that include audiological, neurological, psychological, educational, speech, and language assessments. Developmental oral language learning impairments are divided into two basic categories; Expressive Language Disorder, affecting language production, and Mixed Receptive–Expressive Language Disorder, a disorder encompassing both language comprehension and production deficits. Comprehension and/or production problems may occur within one or more of the components of language; including phonology, morphology, semantics and/or syntax. Problems with pragmatics (interactive conversational skills) also occur frequently. A high proportion of children with developmental language learning impairments also have difficulty with speech articulation; that is, difficulty clearly and correctly producing one or more of the speech sounds of their language. However, speech articulation defects and developmental language learning impairments can occur independently of each other.

Many developmental cognitive disorders, such as mental retardation, pervasive developmental disability (including autism), attention deficit disorder, central auditory processing disorder, or specific syndromes such as Down’s Syndrome, Fragile X Syndrome, Klinefelter’s Syndrome, etc., may include delay in language development. In addition to these known concomitants of developmental language impairment, a recent epidemiological study of monolingual English-speaking kindergarten children in the US found that approximately 8 percent of boys and 6 percent of girls have a specific developmental language impairment of unknown origin. This epidemiological study also showed that the clinical identification of developmental language impairments remains low. Over 70 percent of the parents or teachers of children identified in this epidemiological study as language impaired had previously been unaware that their child had a speech or language problem (Tomblin et al. 1997). These data emphasize the importance of early referral for differential diagnosis for children failing to meet language milestones.

Children with developmental language learning impairments also have been shown to be at risk for childhood behavioral disorders. For example, clinical research studies showed that children referred to child guidance clinics for a variety of behavioral and emotional problems were found to have a higher than expected incidence of developmental language learning disorders. Conversely, children diagnosed with developmental language disorders also have been found, upon examination, to have a preponderance of behavioral and emotional disorders. Over 80 percent of juvenile delinquents were found, upon testing, to be language learning impaired (Beitchman et al. 1986, Hall and Moats 1999).

Longitudinal research studies that have followed children with early developmental language impairments, from the preschool though elementary school years, consistently have demonstrated a striking link between early developmental oral language impairments and subsequent academic achievement problems, especially dyslexia, which is characterized by reading, writing, and spelling deficits. Research that has compared children classified as ‘language impaired’ with those classified as ‘reading impaired’ has shown that both groups are characterized by a variety of oral language deficits, specifically phonological and memory problems, as well as language comprehension deficits (Snow et al. 1998). Whether these common deficits derive from speech-specific mechanisms, or from more basic neural processing deficits, has been the focus of considerable research and theoretical debate (see Tallal et al. 1993, Farmer and Klein 1995, Leonard 1998 for review).

2. Etiology

Considerable research attention has been focused on trying to understand the causes of developmental language learning impairments. Given the lack of homogeneity amongst children showing delayed language development, and the diversity of symptomatologies that can accompany developmental language learning impairments, it is unlikely that a single etiology is responsible. Etiological research to date has focused on linguistic, cognitive, and neurobiological studies.

2.1 Linguistic Studies

There have been considerable advances made in understanding the specific linguistic deficits of children with language impairments. Research has demonstrated particular patterns of difficulty within the linguistic components of phonology, syntax, and morphology. Specifically, deficits in both receptive and expressive grammatical morphology have been demonstrated (Leonard 1998, Bishop 1997). However, results of longitudinal studies have shown that children with developmental language impairments develop the linguistic structures of language along a similar linguistic trajectory to that observed in normally developing younger children, albeit considerably more slowly. To date, there is little evidence that these children make linguistic errors that are deviant or substantially different from younger children at the same stage of language development. Rather, children with language learning impairments take much longer to progress through the stages of normal language development. A similar pattern of delay, rather than deviance, occurs across most populations of children with developmental language impairment, regardless of clinical diagnosis.

Looking cross-linguistically at children with developmental language impairments learning different languages in different countries, it has been shown that whichever linguistic structures are brief, unstressed, or of weak phonetic substance, and occur less frequently in a particular language, are the most difficult for children to learn, and also the most delayed in children with language learning impairments (Leonard 1998). That the same order of development occurs across populations and language environments gives strong support that there is a potent metric of sheer difficulty (whether representational or perceptual) that is imposed on the learning of linguistic structures and contents.

2.2 Cognitive Studies

The most extensive etiological studies to date have utilized behavioral methodologies and have resulted in the formulation of two prominent hypotheses: the ‘linguistic specific’ hypothesis and the ‘rate processing constraint’ hypothesis. The rate processing constraint hypothesis assumes a direct impact on the learning of the linguistic system, specifically on learning phonology and grammatical morphology. This hypothesis claims that central auditory processing constraints, particularly in the areas of auditory attention and serial memory, may result from a more basic impairment in the rate of neural information processing (Tallal et al. 1993). Tallal and colleagues have shown that many children with phonologically based speech, language, and reading disorders need significantly more neural processing time (hundreds of milliseconds instead of tens of milliseconds) to process dynamic (rapidly changing) sensory stimuli. They have hypothesized that this dynamic sensory processing rate constraint has a particularly detrimental effect on phonological processing, as many acoustic changes occurring in rapid succession within syllables and words, necessary to distinguish the individual phonological elements (speech sounds), occur within the tens of milliseconds time window. Research has demonstrated that perception of those speech syllables that incorporate brief, rapidly successive acoustic cues is most problematic for these children.

There is growing evidence from family and twin studies that developmental language learning impairments may aggregate in families and may be genetically transmitted. Twin studies have shown a particularly high concordance rate (heritability) for measures of phonological analysis (Bishop 1997). Family studies show that, as a group, infants born into families with a positive history of developmental language learning impairment demonstrate longer processing times for both nonverbal and verbal acoustic stimuli than matched infants born into families with a negative family history. When followed prospectively, nonverbal acoustic processing rates established at six months of age have been shown to predict the rate of language development, with infants showing the slowest processing rates subsequently being most likely to be delayed in language learning (see Tallal et al. 1993 for review).

In older children and adults, highly significant positive correlations between nonverbal auditory and visual dynamic sensory sensitivity and reading decoding (phonological and/orthographic, respectively) also have been found across a number of studies (see Farmer & Klein 1995, Stein & Talcott 1999 for reviews). Importantly, highly significant correlations between dynamic auditory and visual sensitivity and literacy skills has been reported in normally developing children as well (Talcott et al. 2000). These recent findings provide strong evidence that individual differences in dynamic sensory sensitivity play an important role in the development of both oral and written language, across the entire range of development. As such, children with LLI may fall at the disadvantageous end of a normal distribution, rather than representing a distinct impairment.

The proponents of the ‘linguistic-specific’ hypothesis strongly dispute these data. They exclude any role for nonlinguistic processing constraints as a possible etiology for language learning impairments. Rather, they support a modular view of language processing and propose that language learning impairments are caused by failure of innate, ‘language specific,’ modules to function normally (see Leonard 1998 for review).

2.3 Neurobiological Studies

With the recent advent of in i o brain imaging technologies, these powerful new methods have been applied to the study of the neurobiological basis of developmental language impairments. Electrophysiological studies support the results of behavioral studies showing specific deficits in phonological processing as well as acoustic analysis (Leppanen & Lyytinen 1997). Results from structural magnetic resonance imaging (MRI) show that pars triangularis (Broca’s area) is significantly smaller in the left hemisphere of children with language learning impairments and that these children are also more likely to have rightward asymmetry of language structures. The opposite pattern is seen in children developing language normally (Gauger et al. 1997).

Studies using functional neuroimaging such as magnetoenchephalography (MEG), have the advantage of providing both the spatial and temporal resolution most compatible with studying in vito the real time neural processing involved in language. Recently, MEG has been used in an attempt to help resolve the hotly debated ‘linguistic-specific’ vs. ‘rate processing constraint’ hypotheses. These MEG studies have uniformly demonstrated significant physiological processing rate differences in individuals with a history of language learning disability, compatible with the rate processing constraint hypothesis (see Tallal et al. 1998 for review). However, the precise neural mechanisms that link these physiological rate processing constraints and the phonological and other linguistic deficits that characterize individuals with language learning disabilities, remain to be determined.

New evidence from neuropathological studies as well as functional magnetic resonance imaging (f MRI) studies has suggested that reduced cell size and density in magnocellular regions of the thalamus, leading to aberrant cortico-thalamic transmission, may underlie both the rate processing constraints and phonological processing problems of individuals with language learning impairments (see Stein and Talcott 1999 for review). Furthermore, differential cortical axonal myelination thickness has recently been postulated to underlie left hemisphere specialization for language function in general (Anderson et al. 1999), as well as for language learning impairments in particular (Klingberg et al. 2000).

3. Remediation

Remediation of language learning impairments is generally provided by speech language pathologists, psychologists, or special education teachers. Traditionally, speech and language therapy has been guided by a detailed linguistic assessment of the pattern of receptive as well as expressive errors made by an individual child using both standardized testing instruments and language sampling methodologies. Remediation has focused on helping the child to use age-appropriate linguistic structures in a variety of contexts. Reading remediation has focused on explicitly training children to recognize and manipulate phonemes within words (called phonological awareness) in an attempt to improve reading decoding skills, as well as on methods aimed at improving fluency and reading comprehension. Longitudinal outcomes research has shown that children with developmental language learning impairments generally require approximately two years of traditional intervention to acquire the equivalent of one year of gain in oral and written language development (see Leonard 1998, Lyon & Moats 1997 for reviews).

Electrophysiological studies of sensory fields in the cortex of primates have shown that the underlying neural circuitry can be altered after specific, temporally cohesive training regimens, leading to ‘remapping.’ These mapping studies demonstrate the dynamic plasticity of the brain (Jenkins et al. 1990). These studies recently have led to the development of new approaches to treating language learning impairments that incorporate temporally cohesive computerized training programs. Remediation studies, incorporating for the first time explicit dynamic auditory training, have been coupled with language training using speech that has been computer-modified to enhance and extend dynamic changes within the ongoing acoustic waveform. These studies demonstrated that explicit training, aimed at enhancing dynamic auditory sensitivity, results in highly significant improvement in temporal processing thresholds, speech discrimination, and listening comprehension (Merzenich et al. 1996, Tallal et al. 1996). These new remediation studies, together with those that have focused attention on phonological awareness training (see Lyon and Moats 1997 for review), have stimulated remediation research that aims to develop more scientifically validated approaches for the treatment of oral and written language learning impairments.

4. Goals For Future Research

Future research will need to continue to focus on etiology, specifically understanding the neurobiological, genetic, and environmental influences that contribute interactively to developmental language learning disabilities. It will be particularly important to better understand how nonlinguistic neural processing mechanisms (attention, dynamic processing, memory, and other executive functions) interact developmentally with linguistic systems in the brain during both typical and atypical language development. Integrating new data from advanced imaging technologies, genetics, and cognitive methodologies should lead to a better understanding of the causes and determinants of developmental language learning impairments. Determining whether language learning impairments represent a distinct impairment or rather lie at the disadvantageous end of individual differences along a normal distribution will also remain an important focus of research. The ultimate goal will be to understanding better the multiple factors affecting the broad array of individual differences that characterize the language learning process, which should in turn lead to the development of more effective methods for accelerating oral as well as written language learning in all individuals, specifically those with developmental language learning impairments.


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