Cognititve Function Assessed

Reasoning, problem solving, concept formation

Executive function

Processing speed

Reasoning, problem solving, concept formation

Executive function (sequencing)

Processing speed

Construction

Praxis

Spatial reasoning Motor function Processing speed Construction Praxis

Spatial reasoning

Motor Function

Processing speed

Attention/concentration

Executlve function (sequencing)

Motor function

Processing speed sample of IQ scores (Verbal, Performance, and Full Scale) are a mean performance of 100, with a standard deviation of 15. In addition, individual subtest performance can be converted to standard scores with associated variability. Thus, the clinician is able to classify individual performance for both summary IQ measures and individual subtest performance against these values and provide a relative ranking of performance.

The summary IQ scores are extremely useful in educational settings. They are highly correlated with educational attainment in adults (with an approximate correlation of .79), and are often used to predict academic abilities in children. Although these scores are useful, care needs to be exercised in their use. Because IQ scores represent the culmination of cognitive abilities, they are extremely susceptible to brain damage. nd The presence of a potentially reversible impairment in one of the basic elements of cognitive function (e.g., attention/vigilance) will provide artificially low IQ performance. Therefore, IQ scores need to be interpreted in the context of a complete neuropsychological examination.

One particularly difficult issue in intellectual performance is the diagnosis of profound intellectual impairments, or mental retardation. First, this diagnosis has major consequences for an individual. Once assigned with the label of mental retardate, educational and occupational opportunities are limited. The individual is guided away from mainstream educational settings and provided with specialized training. Second, there are no universally accepted diagnostic criteria for mental retardation. The usual criterion is IQ performance greater than two standard deviations below the mean (e.g., an IQ score of less than 70 on the WAIS-R). Because the performance level for mental retardation is so low, however, statistical reliability of the standardization sample is also low. As seen in FiQuie 2Z-.1, as the scores decrease (or increase) toward the tail of the distribution, the number of sample scores drops dramatically. The representativeness of cases at these extreme levels is limited and hence it is problematic to generalize the results to the population of interest. Because of these issues, it is incumbent on the examiner to base the diagnosis of mental retardation on multiple assessments of intellectual performance and not on a single measure.

The use of IQ scores in neuropsychological assessments is limited to providing a basic measure of cognitive functional integrity. Comparison of Verbal IQ performance to Performance IQ performance has been suggested to provide an assessment of dominant (verbal) and nondominant (performance) hemispheric function. Actuarial studies of this Verbal IQ/Performance IQ comparison, however, have not supported its use in this capacity. y Verbal IQ measures do provide some indication of dominant hemispheric integrity, but Performance IQ measures do not reliably indicate nondominant hemispheric integrity. This lack of reliability is due to basic differences between verbal and performance measures. Whereas most subscales comprising Verbal IQ are not timed, all Performance IQ subscales are. This difference adversely affects the Performance IQ scores of individuals with compromised motor and cognitive speed (e.g., patients with basal ganglia damage). In addition, there are basic differences in sensitivity to brain damage between verbal and performance subscales. The subtests of Verbal IQ tend to require overlearned abilities such as vocabulary knowledge and general information. These abilities are, in general, less affected by brain damage. The subtests of Performance IQ tend to require novel abilities, such as reproducing visual patterns using colored blocks. These abilities are, in general, more affected by brain damage. This difference between overlearned and novel abilities is evident in aging, and reflected in the performance of the standardization sample.

In contrast to the use of summary IQ scores for neuropsychological assessment, the analysis of subscale performance is very useful. As shown in Ta,bie,...2Z-1 , each subscale assesses specific (and overlapping) cognitive abilities. The pattern of performance on the subtests can provide useful information in forming hypotheses as to possible impairment in various cognitive abilities. These hypotheses can then be tested with additional, more focused, assessments. The use of subscale performance patterns has resulted

in various interpretative methods for specific neuropsychological use. PREMORBID ESTIMATES OF INTELLECTUAL FUNCTION

In order to provide useful information on general cognitive functioning, current measures of intelligence need to be compared with premorbid level of functioning. Because neuropsychological testing results are typically not available for the period prior to neurologic trauma, neuropsychologists are forced to estimate premorbid abilities.y There are three basic approaches to estimation of premorbid intellectual functioning: estimates derived from life history variables believed to be correlated with measures of cognitive functioning (e.g., years of education); measures of best current cognitive functioning, or "present abilities," which are used to estimate a minimum level of premorbid functioning; and comparison of an individual's test results to population-based estimates of normal performance (norm- based comparisons as discussed above). Each of these approaches has associated error that needs to be considered in the interpretation of neuropsychological testing data.

The first method, estimates based on life history variables, offers a potential wealth of information that is thought to be related to the cognitive performance. Estimates derived from life history variables may include basic demographic information such as age, education, and gender but may also include information from developmental history, medical history, occupational history, and academic history. The application of these data to an estimate of premorbid functioning can be accomplished in either qualitative or quantitative methods. The qualitative approach allows the examiner to form a "best-guess" of premorbid functioning. y For example, the estimated premorbid functioning for an individual with a normal developmental history, outstanding academic history, a college degree, and an occupational attainment of CEO of a major corporation would be higher than an individual with a normal developmental history, poor academic history, a high school diploma, and an occupational attainment of sales clerk. Although this information may be helpful, it does not provide an actual estimate of premorbid functioning.

The quantitative approach to the use of life history information typically uses demographic information such as age, education, and gender to provide an estimate of the range of premorbid test scores for a given individual. y , y This approach is useful because specific ranges of test scores may be generated to be compared with current scores, but typically does not take full advantage of nonquantifiable information such as developmental and medical history.

The second method of estimating premorbid cognitive abilities, the "present abilities" approach, uses current test scores to provide a lower-bound estimate of prior functioning. The theory underlying this approach is that neurologic damage does not affect all cognitive abilities equally and that an individual's highest test score on current testing is most likely to reflect intact performance. Therefore, that highest test score can be used to provide an estimate of overall premorbid cognitive functioning. For example, if an individual is scoring at the below average range of performance on tests of verbal memory, verbal comprehension, and verbal fluency but is performing at the average range on a test of visual-spatial skills and nonverbal reasoning, the examiner may assume that the individual was performing at least at the average range on all measures. Of course, this estimate is only meant to provide an interpretative guide, as other explanations for a given pattern of intact and impaired performance are possible.

A more standardized method than the "present abilities" approach for estimating premorbid IQ is found in tests such as the National Adult Reading Test (NART) y and its variants. These tests rely on current performance of reading ability or vocabulary knowledge to estimate premorbid abilities. Both reading ability (particularly reading of phonically irregular words such as debt) and vocabulary knowledge tend to be less affected by brain damage than other cognitive abilities. By using these abilities to estimate premorbid IQ, the examiner is able to get an estimate of a lower-bound to previous iQ. y All of these approaches to premorbid IQ estimation are subject to many caveats. First, the range of scores provided by the quantitative approaches is limited. With these measures, it is not possible to get very high or very low IQ estimates. Second, these approaches overestimate low IQ performance and underestimate high IQ performance. Third, each approach is associated with differing levels of statistical error, usually measured as the standard error of estimate. For example, the quantitative approaches have standard errors ranging between 7 and 16 points. Since the standard deviation of IQ is 15 points, these estimates can at best provide a range of possible scores equal to or greater than one standard deviation. Therefore, extreme care needs to be applied to the use of these estimates. Most neuropsychologists will use a combination of quantitative and qualitative approaches to the estimation of premorbid IQ.

OTHER INTELLIGENCE TESTS

The Wechsler scales are not the only method for the assessment of intellectual abilities. Due to space limitations we cannot list all tests of intelligence. However, other tests of intellectual abilities tend to follow two different assessment methods: tests of verbal abilities and tests of nonverbal abstraction. The first method, testing of verbal abilities, is represented by tests such as the Peabody Picture Vocabulary Test-Revised (discussed in the section on Perception and Construction Abilities), in which patients are asked to choose which of four pictures best identifies a spoken word. This method is based on the high correlation between vocabulary knowledge and IQ.y The second method, tests of nonverbal abstraction, is represented by the Raven's Progressive Matrices test (discussed in the section on Reasoning and Problem Solving Through Concept Formation), in which patients must abstract general principles and rules from patterns. This method tends to correlate well with the Full Scale IQ from the Wechsler scales.

TESTS OF ACADEMIC ACHIEVEMENT

Tests of academic achievement typically assess standard academic skills such as reading, writing, arithmetic skills,

and spelling. The scores derived from these measures provide important information on neuropsychological and educational abilities. They allow comparisons of an individual patient's educational development to normative expectations. They also help identify specific areas in need of remediation. Finally, academic achievement tests provide important information in the diagnosing of specific learning disabilities. The presence of learning disabilities is suggested when there is a large discrepancy between measures of intellectual ability and measures of academic achievement. Additional testing is required to formally assign a diagnosis of learning disabilities. The most commonly used tests of academic achievement are the Peabody Individual Achievement Test-Revised (PIAT-R) y and the Wide Range Achievement Test-Revised (WRAT-R).y Both of these tests assess reading, arithmetic, and spelling. The PIAT-R has additional scales assessing the patient's fund of general information and reading recognition. Both tests provide information as to an individual patient's academic achievement in terms of grade level, standard score, and standing.

CLINICAL SITUATIONS FOR TESTS OF INTELLECTUAL ABILITIES

Intellectual assessment is very important for patients presenting with impairments of higher cognitive functioning such as memory, language, and reasoning. Because these higher cognitive functions are highly correlated with intelligence, intelligence assessment allows the examiner to interpret scores of memory, language, and reasoning performance in light of basic intellectual abilities. Intellectual assessments are not particularly helpful in patients with disturbances in level of consciousness (see Chapter.! ), disrupted sleep/wake cycles (see Chapter.2 ), or neglect and dyspraxia (see Chapter 4 ). Additionally, the comparison of intellectual abilities and academic achievement is helpful in identifying patients who may have learning disabilities. In children with developmental or neurodegenerative disorders affecting the cortex or subcortex (see Chapte.rs...2.8 , 33 , and 34 ) and those with chromosome alterations, enzyme defects, or storage abnormalities (see Chapters.30 , 31 , and 32 ), these assessments help in tailoring educational efforts and expectations. When intelligence testing is requested, the referring physician should ask for a reporting of subscale performance as well as summary IQ scores.

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