Cognitive Aspects of Mental Retardation Research Paper

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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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