Neuropsychological Assessment A.E.M. van Wordragen (u789251)
4: The Rational of Deficit Measurement
One distinguishing characteristic of neuropsychological assessment is it emphasis on the
identification and measurement of psychological (cognitive and behavioral) deficits, for it is in
deficiencies and dysfunctional alternations of cognition, emotionality, and self-direction and
management that brain disorders are manifested behaviourally.
Brain damage always implies behavioral impairment. Even when psychological changes after a
brain injury or concomitant with brain disease are viewed a improvement rather than
impairment, as when there is a welcome increase in sociability or relief from neurotic anxiety, a
careful assessment will probably reveal an underlying loss.
Although the effects of brain disorders are rarely confined to a single behavioral dimension or
functional system, the assessment of psychological deficit has focused on cognitive impairment
for a number of reasons. First, some degree of cognitive impairment accompanies almost all
brain dysfunction and is a diagnostically significant feature of many neurological disorders. In
addition, psychologists are better able to measure cognitive activity than any other kind of
behavior, expect perhaps simple psychophysical reactions and sensorimotor responses.
Summary scores that are created by averaging individual test scores in a battery may be within
some average range, but deviations between tests can be substantial, even within the typically
developing, healthy population. 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
The concept of behavioral deficit presupposes some ideal, normal, or prior level of functioning
against which the patient’s performance may be measured. This level (comparison standard)
may be normative (derived from a appropriate population) or individual (derived from the
patient’s history or present characteristics) depending on the patient, the behavior being
evaluated, and the assessment’s purpose(s). both normative and individual comparison
standards need to be used to make a good examination.
Normative comparison standards
The normative comparison standard may be an average or middle (median) score. In
neuropsychological assessment, population norms are most useful in evaluating basic cognitive
functions that develop throughout childhood. Many tests of memory, perception, and attention
and those involving motor skills fall into this category. Typically, performances of these
capacities do not distribute normally; the proportions and score ranges of persons receiving
scores above and below the mean are not statistically similar as they are normal distributions.
Functions most suited to evaluation by population norms also tend to be age-dependent,
particularly from the middle adult years onward, necessitating the use of age-graded norms.
,Neuropsychological Assessment A.E.M. van Wordragen (u789251)
When the behavioral concomitants of brain damage are mild or subtle, particularly when
associated with widespread or diffuse rather than well-defined lesions, few if any of these
rudimentary components of cognitive behavior will be demonstrable impaired on the basis of
species-wide norms.
Normative comparison standards can only be used in functions that are normally distributed in
the population. Vocabulary level, for example, is dependent of social class and education and
therefore not normally distributed over the whole population. For vocabulary level, there need
to be an comparison with people in that same ‘class’ of social class and education.
When examining patients for adult-onset deficits, only by comparing present with prior
functioning can the examiner identify real loss.
The first step in measuring cognitive deficit in an adult is to establish the patient’s premorbid
performance level for all of the functions and abilities being assessed. For those functions with
species-wide norms, this is easy. Although an average score may be, statistically, the most likely
score a person will receive, statistical likelihood is a far cry from the individual case.
Individual comparison standards
As a rule, individua comparison standards are called for whenever a psychological trait or
function that is normally distributed in the intact adult population is evaluated for change. The
use of individual comparison standards is probably most clearly exemplified in ‘rate of change’
studies, which depend solely on intraindividual comparisons. In child psychology the
measurement of rate of change is necessary for examining the rate of development.
Measurement of deficit
For most abilities and skills that distribute normally in the population at large, determination of
deficits rests on the comparison between what can be assumed to be the patient’s characteristic
premorbid level of cognitive functioning as determined from historical data (including old tests
scores when available) 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 of neuropsychological impairment.
Direct measurement of deficit
Deficit can be assessed directly when the behavior in question can be compared against
normative standards. Direct deficit measurement using individual comparison standards can be
a simple, straightforward operation: the examiner compares premorbid and current behavior in
question and evaluates the discrepancies. The direct method using individual comparison
standards requires the availability of premorbid test scores, school grades, or other relevant
observational data. In may cases, these will be non-existent or difficult to obtain. Therefore,
more often than not, the examiner must sue indirect methods of deficit assessment from which
individual comparison standards can be inferred.
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Indirect measurement of deficit
In indirect measurement, the 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. The most techniques for indirect assessment of premorbid ability rely on cognitive
test scores, on extrapolation from current reading ability, on demographic variables, or on some
combination of these.
A way to establish premorbid functioning is by taking a word reading test. One problem with
word-reading scores is their vulnerability to brain disorders, especially those involving verbal
abilities; one advantage of demographic variables is their independence from the patient’s
neuropsychological status at the time of the examination.
Best performance method
A simpler method utilizes test scores, other observations, historical data, and clinical judgment.
This is the ‘best performance method’, in which the level of the best performance (whether it be
the highest score or set of scores, nonscorable behavior not necessarily observed in a formal
testing situation, or evidence of premorbid achievement) serves as the best estimate of
premorbid ability. The best performance method rests on a number of assumptions that guide
the examiner in its practical applications. Basic to this method is the assumption that ‘given
reasonably normal conditions of physical and mental development, there is one performance level
that best represents each person’s cognitive abilities and skills generally’. This assumption does
not deny its many exceptions, but rather speaks to a general tendency that enables the
neuropsychological examiner to use test performances to make as fair an estimate as possible of
premorbid ability in neurologically impaired persons with undistinguished school or vocational
careers. A corollary assumption is: ‘marked discrepancies between the levels at which a person
performs different cognitive functions or skills probably give evidence of disease, developmental
anomalies, cultural deprivation, emotional disturbance, or some other condition that has interfered
with the full expression of that person’s cognitive potential’. Large discrepancies do occur in
healthy controls, again emphasizing why the clinician needs to take multiple factors into
consideration when making a determination about whether a particular neuropsychological
performance reflects actual impairment or some normal variation.
Another assumption is that cognitive potential or capacity of adults can be either realized or
reduced by external influences; it is not possible to function at a higher level than biological
capacity and developmental opportunity will permit. The phenomenon of overachievement
appears to contradict this assumption; but in fact, overachievers do not exceed their
biological/developmental limitations. Rather, they expended an inordinate amount of energy
and effort on developing one or two special skills, usually to the neglect of others.
Another assumption is: ‘within the limits of chance variations, the ability to perform a task is at
least as high as a person’s highest level of performance of that task’. The poor responses do not
, Neuropsychological Assessment A.E.M. van Wordragen (u789251)
negate the good ones; the difference between them suggests the extent to which the patient has
suffered cognitive deterioration.
It is also assumed that ‘a patient’s premorbid ability level can be reconstructed or estimated from
many different kinds of behavioral observations or historical facts’. Examples of these factors are
reports from family and friends, test scores, prior academic or employment level, school grades
or intellectual products such as written letters.
In general, the examiner should not rely on a single high test score for estimating premorbid
ability unless history or observations provide supporting evidence. The examiner also needs to
be alert to overachievers. In relying solely on the highest score the Mortensen study violated an
important directive for identifying the best performance: ‘the estimate should take into account
as much information as possible about the patient and not rely on test scores alone’. In most cases,
the best performance estimate will be based on a cluster of highest scores plus information
about the patient’s education and career, and when possible, it will include school test data.
The deficit measurement paradigm
Once the comparison standard has been determined, whether directly from population norms,
premorbid test data, or historical information, or indirectly from current test findings and
observation, the examiner may assess deficit. A statistically significant discrepancy between
expected and observed performance levels for any cognitive function or activity indicates a
probability that this discrepancy reflects a cognitive deficits. Identifiable patterns of cognitive
impairment can be demonstrated by the defict measurement method. Although the discussion
here has focused on assessment of defict where a neurological disorder is known or suspected,
this method can be used to evaluate the cognitive functioning of psychiatrically disabled or
educationally or culturally deprived persons as well because the evaluation is conducted within
the context of the patient’s background and experiences, taking into account historical data and
the circumstances of the patient’s present situation.
5: The Neuropsychological Examination; Procedures
Psychological testing is a process wherein a particular scale is administered to obtain a specific
score. Two rules should guide the neuropsychological exam:
o Treat each patient as an individual
o Think about what you are doing
The neuropsychological examination can be individually tailored in two ways. Examiners can
select examination techniques and tests for their appropriateness to the patient en for their
relevancy to those diagnostic or planning questions that prompted the examination and that
arise during its course. Ideally, the examiner will incorporate both selection goals in each
examination, as tests and time permit.
