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Chapter 4

PSYC31 - Clinical Neuropsychology Ch. 4

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Pare, Dwayne

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PSYC31 – Chapter 4: The Rationale of Deficit Measurement  Neuropsychological assessment has its emphasis on the identification and measurement of psychological deficits  Neuropsychological assessment is also concerned with the documentation and description of preserved functions  Some degree of cognitive impairment accompanies almost all brain dysfunction and is a diagnostically significant feature of many neurological disorders  Knowledge of intraindividual variations in test performances does not support the popular concept of “intelligence” as a global phenomenon which can be summed up in a single score nor does it support summing scores on any 2 or more tests that measure different functions  If one only relies on examining test scores and their deviations without taking into consideration all of the relevant clinical, historical, and observational data in evaluating a patient, misclassification can become a considerable problem Comparison Standards for Deficit Measurement  The concept of normal behavioral deficit presupposes some ideal, normal, or prior level of functioning against which the patient’s performance may be measured  Comparison standard may be normative (derived from an appropriate population) or individual (derived from the patient’s history or present characteristics), depending on the patient, the behavior being evaluated, and the assessments’ purpose  Neuropsychological assessment uses both normative and individual comparison standards for measuring deficit, as appropriate for the function or activity being examined and the purpose of the examination  Examiners need to be aware of judgmental biases when estimating premorbid abilities Normative Comparison Standards  Normative comparison standard may be an average or middle score  “norm” for many measureable psychological functions and characteristics is a score representing the average or median performance of some more or less well-defined population  In neuropsychological assessment, population norms are most useful in evaluating basic cognitive functions that develop throughout childhood  The overall distribution of scores for these capacities tends to be skewed in the substandard direction as a few persons in any randomly selected sample can be expected to perform poorly, while nature has set an upper limit on such aspects of mental activity as processing speed and short-term storage capacity  Functions most suited to evaluation by population norms also tend to be age-dependent  Education also contributes to performance on these tests and needs to be taken into consideration statistically, clinically, or both  As the number of different kinds of variables contributing to a measure increases, the more likely will that measure’s distribution approach normality  The norms for some psychological function and traits are actually species-wide performance expectations for adults  Subsequent speech development mainly involves more variety, elegance, abstractions, or complexity of verbal expression o The adult norm for speech is the intact ability to communicate effectively by speech  A number of assumed normative standards have been arbitrarily set, usually by customs and the most familiar of these is the visual acuity standard: 20/20 vision does not represent an average but an arbitrary ideal o Verbal response latency: amount of time person takes to answer a simple question (has normative value of 1-2 seconds)  Applications and Limitations of Normative Standards o In the assessment of persons with known or suspected adult-onset brain pathology, however, normative standards are appropriate only when the function or skill or capacity that is being measured is well within the capability of all intact adults and does not vary greatly o When it is known or suspected that a patient has suffered a decline in cognitive abilities, a description of that patient’s functioning in terms of population norms will shed no light on the extent of impairment unless there was documentation of premorbid cognitive levels o Comparisons with population averages do not add to the info applied in standardized test scores, for standardized test scores are themselves numerical comparisons with population norms. Thus, when examining patients for adult-onset deficits, only by comparing the present with prior functioning can the examiner identify real losses o First step in measuring cognitive deficit in an adult is to establish the patient’s premorbid performance level o For normally distributed functions and abilities for which the normative standard is an average, only an individual comparison provides a meaningful basis for assessing deficit Individual Comparison Standards  Individual comparison standards are called for whenever a psychological trait or function that is normally distributed in the intact adult population is evaluated for change  When dealing with functions for which there are species-wide or customary norms, normative standards are appropriate for deficit measurement  There will always be exceptional persons for whom normative standards are not appropriate The Measurement of Deficit  for most abilities and skills that distribute normally in the population at large, determination of deficits rest on the comparison between what can be assumed to be the patient’s characteristic premorbid level of cognitive functioning as determined from historical data and the obtained test performance scores and qualitative features of the test performance evaluated in the context of presenting problems, recent history, patient behavior, and knowledge of patterns Direct Measurement of Deficit  deficit can be assessed directly when the behavior in question can be compared against normative standards  the extent of the discrepancy between the level of performance expected for an adult and the level of the patient’s performance provides one measure of the amount of deficit the patient has sustained  direct deficit measurement using individual comparison standards can be a simple operation: examiner compares premorbid and current examples of the behavior in question and evaluates the discrepancies  a study compared scores that army veterans made on tests taken at the time of their induction into service with scores obtained on the WAIS-R post-injury approximately 13 years later o findings provided unequivocal evidence of cognitive impairment  the direct method using individual comparison standards requires the availability of premorbid test scores, school grades, or other relevant observational data Indirect Measurement of Deficit  in indirect measurement, examiner compares the present performance with an estimate of the patient’s original ability level  it is the examiner’s task to find meaningful and defensible estimates of the pretraumatic or premorbid ability levels to serve as comparison standards for each patient  historical and observational data arte obvious sources of info from which estimates of premorbid ability may be drawn directly  premorbid ability estimates inferred from historical and observational data alone can also be spuriously low  some patient self-reports may be inflated, invoking what has been referred to as the “Good Old Days” bias  neuropsychologists have devised a number of distinctive methods for making the estimates  the most techniques for indirect assessment of premorbid ability rely on cognitive test scores, extrapolation from current reading ability, on demographic variables, or on some combination of these  a common feature of estimation techniques based on test scores is that premorbid ability level is estimated from the scores themselves  of all the Wechsler tests, Vocabulary correlates most highly with education, which also can be a
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