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Standard deﬁnitions of mental retardation include three criteria: (a) signiﬁcantly subaverage intelligence, (b) signiﬁcant limitations in adaptive skills, (c) onset during the developmental period. Given criteria (a) and (b), it is clear that cognitive limitations must play a major role in mental retardation. Both the extent of overall cognitive diﬃculty and the particular pattern of cognitive problems vary as a function of etiology (e.g., Down syndrome, Williams syndrome, lead poisoning).
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1. Deﬁnitions of Mental Retardation
The most commonly used deﬁnition of mental retardation is that provided by the fourth edition of the Diagnostic and Statistical Manual (DSM-IV; American Psychiatric Association 1994). According to this deﬁnition, individuals are considered to have mental retardation if:
(a) Their current IQ, based on an individually administered test, is at least 2 standard deviations below the mean (below about 70).
(b) They have signiﬁcant limitations, relative to those expected for their chronological age (CA) and sociocultural background, in at least two of the following domains: communication, social interpersonal skills, self-care, home living, self-direction, leisure, functional academic skills, use of community resources, work, health, and safety.
(c) These diﬃculties were ﬁrst evidenced prior to age 18 years.
Each of the other major organizations involved in the treatment of individuals with mental retardation—the American Association on Mental Retardation (AAMR), the American Psychological Association (APA), and the World Health Organization (WHO)—accepts the same criteria but implements them in slightly diﬀerent ways (see Luckasson et al. 1992, American Psychological Association 1996, World Health Organization 1996).
In all of these deﬁnitions, mental retardation is subdivided into four levels. The DSM-IV, APA, and WHO subdivisions are based on IQ, as follows: mild (IQ between 50–5 and about 70), moderate (IQ between 35–40 and 50–5), severe (IQ between 20–5 and 35–40), and profound (IQ below 20–5). The AAMR subdivisions are based on the intensity of support required to enhance independence, productivity, and community integration.
2. Historical Approaches to the Cognitive Aspects of Mental Retardation
2.1 Diﬀerence Approach
Mental retardation may be due to genetic causes (e.g., Down syndrome, fragile X syndrome, a bad roll of the genetic dice for genes that impact on intelligence in the general population) and or environmental ones (e.g., consumption of alcohol by the mother during pregnancy, lead poisoning, closed head injury, extreme poverty). Historically, psychologists and educators have focused on level of mental retardation rather than cause, both for educational intervention and for research. Extensive characterizations, independent of etiology, are provided in the Manual of Diagnosis and Professional Practice in Mental Retardation (American Psychological Association 1996). Both educational programs and prognosis have been based on level of mental retardation, without taking into account etiology. A standard research design for studies of cognitive aspects of mental retardation included two groups of participants: individuals with mental retardation and individuals of normal intelligence. In determining the individuals to be assigned to the mentally retarded group, etiology was characteristically ignored; individuals with cultural-familial mental retardation (bad roll of the genetic dice combined with poverty), individuals with mental retardation due to environmental causes such as lead poisoning, and individuals with various syndromes were all likely to be included. Researchers and practitioners commonly assumed that individuals with mental retardation diﬀered from individuals of normal intelligence in speciﬁc and fundamental ways, beyond simply intellectual slowness. Proposed bases for these diﬀerences included behavioral rigidity, deﬁcits in verbal mediation ability, deﬁcits in short-term memory, and deﬁcits in attention (see, e.g., Burack et al. 1998, Zigler and Balla 1982). This type of position is currently referred to as the ‘diﬀerence approach.’
2.2 Developmental Approach
In the late 1960s, Edward Zigler oﬀered the ﬁrst formal proposal of the ‘developmental approach’ (see Burack et al. 1998). Zigler argued that individuals whose mental retardation was due to cultural-familial causes should be considered to be the lowest part of the normal distribution of intelligence. As such, these individuals should follow the same developmental path as individuals of normal intelligence. In particular, individuals with cultural-familial mental retardation should acquire cognitive abilities in the same sequence as individuals of normal intelligence (‘similar sequence hypothesis’) although each step might take longer and later steps might not be attained. Furthermore, when matched to individuals of normal intelligence of the same mental age (MA), individuals with cultural-familial mental retardation should show no particular areas of cognitive strength or weakness (‘similar structure hypothesis’). Thus, in contrast to the diﬀerence theorists, proponents of the developmental approach argued that there was no speciﬁc cognitive deﬁcit associated with cultural-familial mental retardation; individuals with this type of mental retardation were simply cognitively slow.
Beginning in the early 1980s, a few psychologists began to argue that the developmental approach should be extended to individuals with mental retardation due to organic causes. In one of the ﬁrst papers to take this position, Dante Cicchetti and Petra Pogge-Hesse (1982) argued that despite the wellknown organic basis for Down syndrome, the development of young children with this syndrome ﬁt the similar structure and similar sequence hypotheses. Cicchetti and Pogge-Hesse further argued that the developmental approach was likely to be appropriate for individuals with mental retardation due to other organic causes. Later studies conﬁrmed the applicability of the developmental approach, especially the similar sequence hypothesis, for individuals with mental retardation due to a wide range of organic causes (Hodapp and Burack 1990).
By the mid 1990s, however, more than 500 genetic disorders associated with mental retardation had been identiﬁed, and it was clear that many more would be found. Similarly, large numbers of teratogenic causes had been identiﬁed, and it was apparent that more would be identiﬁed. These realizations renewed concern among some researchers that etiology should be taken into account for both intervention purposes and research. Many of the same researchers who had demonstrated the applicability of the similar sequence hypothesis for individuals with various genetic syndromes began also to examine potential diﬀerences among syndromes with regard to cognitive strengths and weaknesses. These researchers were especially concerned that the similar structure hypothesis might not appropriately characterize many genetic syndromes. Although the majority of mental retardation researchers continued to group participants by level of retardation rather than etiology, an increasing number focused their research on speciﬁc etiologies of mental retardation. The ﬁndings of many of these studies strongly suggested the importance of taking etiology into account, whether one was concerned with intervention or with theory.
3. Etiological Approach to Cognitive Aspects of Mental Retardation
The etiological approach to mental retardation was founded on the assumption that the various genetic and environmental causes of mental retardation were likely to have diﬀering eﬀects on brain structure and function. The areas and functions most aﬀected were expected to vary due to diﬀerences in which genes were involved (deleted, duplicated, or mutated) and the role these particular genes, in transaction with the environment, play in development, or which aspects of the brain were growing most rapidly at the time of exposure to a particular teratogen. Because of these diﬀerences, it is likely that some aspects of cognition will be more severely impacted than others, and that the domains of most serious impact will vary across syndromes. If so, a person’s overall MA or IQ (used to assign level of mental retardation) may not accurately reﬂect that person’s abilities in speciﬁc domains. In some domains the person may be performing at a higher level than expected, whereas in others he or she may be performing below the expected level. Furthermore, this pattern would characterize the cognitive abilities of most individuals with mental retardation of the same etiology. Cognitive proﬁles associated with particular syndromes have begun to be identiﬁed (see the papers in Tager-Flusberg 1999 and Denckla 2000). As illustration, the proﬁles associated with two syndromes that have often been contrasted are described below.
3.1 Cogniti e Proﬁle for Williams Syndrome
Williams syndrome is perhaps the syndrome most identiﬁed with a speciﬁc pattern of cognitive strengths and weaknesses. This very rare syndrome (1 in 20,000 live births) is caused by a hemizygous deletion of about 1.5 megabases on the long arm of chromosome 7 (7q11.23), encompassing at least 18 genes. Early reports noted that individuals with Williams syndrome, although mentally retarded, were extremely gregarious and highly verbal. Popular interpretation of these results has led to claims that despite ‘severe’ mental retardation, the language of individuals with Williams syndrome is ‘spared,’ and may even be above the level expected for a normally developing child of the same chronological age (CA). As such, Williams syndrome is often taken to provide a strong case for the independence of language from cognition. This popular interpretation provides a caricature of individuals with this syndrome and is seriously inaccurate in some regards: individuals with Williams syndrome typically have mild mental retardation and only rarely severe mental retardation, and language abilities typically are below, rather than above, CA expectations. Furthermore, language abilities are not independent of nonlinguistic abilities. Performance on language tasks is highly correlated with performance on a variety of other types of cognitive but nonlinguistic tasks (Mervis et al. 1999). However, the popular interpretation does capture the fact, established in empirical studies of the cognitive characteristics of individuals with this syndrome (see below), that some aspects of the language abilities of individuals with Williams syndrome are more advanced than would be expected for IQ or MA.
To determine if a particular syndrome is associated with a speciﬁc pattern of cognitive strengths and weaknesses (cogniti e proﬁle), researchers must establish that most individuals with the syndrome show the same pattern of cognitive strengths and weaknesses. This is best accomplished by comparing the performance of individuals on a single well-standardized test that includes measures of many diﬀerent types of cognitive abilities. To determine if this pattern is unusual among individuals with mental retardation, researchers may include a contrast group of individuals with other forms of mental retardation in their study (see Mervis and Robinson 1999). Research following this strategy has indicated that Williams syndrome is characterized by a speciﬁc cognitive proﬁle: relative strengths in auditory (verbal) shortterm memory and language (vocabulary) and extreme weakness in visuospatial construction (e.g., pattern construction, drawing). Most individuals with Williams syndrome, but relatively few individuals with other forms of mental retardation, ﬁt this cognitive proﬁle (Mervis and Klein-Tasman 2000).
The adaptive behavior proﬁle associated with Williams syndrome is consistent with the cognitive proﬁle: communication skills of individuals with Williams syndrome (which rely primarily on verbal ability) are more advanced than their daily living skills (which rely heavily on visuomotor integration or visuospatial construction). Results of magnetic resonance imaging (MRI) studies are also consistent with the Williams syndrome cognitive proﬁle: although brain volume is signiﬁcantly reduced relative to sex and CA-matched individuals of normal intelligence, the volume of the superior temporal region (important for auditory and language processing) is preserved (Reiss et al. 2000). As the authors note, the direction of the relation between relative strength in auditory memory and language and preserved superior temporal volume is not clear. Although it may be tempting to assume that the relative strength in auditory memory and language shown by individuals with Williams syndrome is due to the preserved volume of the superior temporal region, the reverse is also possible; the preserved volume could be secondary to greater use of auditory memory and language skills over time, resulting in larger cortical representation.
3.2 Cogniti e Proﬁle for Down’s Syndrome
The most common genetic cause of mental retardation (1 in 800 live births) is Down syndrome (trisomy 21). Most individuals with Down syndrome have mild to moderate mental retardation. It often has been argued that individuals with Down syndrome have a ﬂat cognitive proﬁle. In fact, results of studies ﬁnding no signiﬁcant diﬀerences between individuals with Down syndrome and individuals with mental retardation of cultural-familial origin were used as support for the diﬀerence approach in contrast to the early developmental approach. More recent studies have shown, however, that Down syndrome is associated with a speciﬁc cognitive proﬁle: relative strength in visuospatial skills (both memory and construction) and relative weaknesses in verbal shortterm memory and expressive language (Chapman and Hesketh 2000). The extent to which this cognitive proﬁle is limited to Down syndrome has not been determined. As with Williams syndrome, the adaptive behavior proﬁle for Down syndrome is consistent with the cognitive proﬁle: individuals with Down syndrome have stronger daily living skills than communication skills.
3.3 Within-syndrome Variability
Research has established that there often are major diﬀerences across syndromes both in characteristic cognitive proﬁles and in characteristic level of mental retardation. At the same time, it is important to acknowledge that there are individual diﬀerences among people with a particular syndrome. For example, 12 percent of individuals with Williams syndrome do not ﬁt the typical cognitive proﬁle for this syndrome; many of these individuals evidence cognitive strengths and weaknesses similar to those typically associated with Down syndrome. Similarly, a minority of individuals with Down syndrome do not ﬁt the typical proﬁle for that syndrome; these individuals often ﬁt the Williams syndrome cognitive proﬁle.
Another area in which there is wide within-syndrome variability is overall intelligence, or IQ. For example, on the Kaufman Brief Intelligence Test (KBIT, composed of a verbal subtest and a nonverbal reasoning subtest), the average IQ for a large sample of individuals with Williams syndrome was 66 (in the range of mild mental retardation), with a standard deviation of almost 15 (the same as in the general population) and a range from 40 (the lowest possible IQ on the K-BIT) to 106. Thus, the variability in intelligence among individuals with Williams syndrome is as great as for individuals in the general population; the diﬀerence is that the mean IQ for Williams syndrome is about 2 standard deviations below the mean for the general population. Intelligence levels for individuals with Williams syndrome range from severe mental retardation to average intelligence. This range is due to the combination of variability in the genes that impact on general intelligence (which are outside the Williams syndrome deletion region) and the transaction of these genes with varying environmental factors.
Both etiology and level of intelligence are important in understanding the cognitive aspects of mental retardation. As with the general population, large diﬀerences among individuals with mental retardation (regardless of etiology) are typically associated with large diﬀerences in both academic skills and adaptive functioning. For example, some individuals with Williams syndrome read at the high school level and hold jobs in competitive employment; at the other extreme, some cannot read at all and work in sheltered workshops. At the same time, knowledge of etiology allows for reliable prediction of patterns of cognitive strengths and weaknesses, independent of overall level of intelligence. The existence of these patterns, and the correlations among level of performance in domains of strength and weakness, is important from a theoretical perspective to our understanding of the structure of cognition. These patterns are important from an applied perspective as well: they can be used as a basis for developing intervention strategies geared to individuals with a particular etiology of mental retardation.
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