Neuropsychological Evaluation

Yaakov Stern

INTRODUCTION

Before the availability of high-resolution imaging, neuropsychological evaluation played a role in lesion localization. Currently, neuropsychological testing remains a valuable adjunct to neurologic evaluation, assisting in the diagnosis of dementia and in evaluating or quantifying cognition and behavior in development, brain diseases, and clinical treatment, as well as in research.

STRATEGY OF NEUROPSYCHOLOGICAL TESTING

Conditions that affect the brain often cause cognitive, motor, or behavioral impairment that can be detected by appropriately designed tests. Defective performance on a test and certain patterns of test performance may suggest specific pathology. Alternatively, patients with known brain changes may be assessed to determine how the damaged brain areas affect specific cognitive functions. Before relating test performance to brain dysfunction, however, other factors that affect test performance must be considered.

Typically, test performance is compared with normal values derived from populations similar to the patient in age, education, socioeconomic background, and other variables. Scores significantly below the mean expected values imply impaired performance. Performance sometimes can be evaluated by assumptions about what might be expected from the average person (e.g., repeating simple sentences or simple learning and remembering).

Unfortunately, comparable data may not exist for the patient being tested. This problem is common in the elderly and those with language and cultural differences. This situation may be addressed by collecting local, normal characteristics that are more descriptive of the local clinical population or by evaluating the cognitive areas that remain intact. In this way, the patient guides the clinician in terms of the level of performance that should be expected in possibly affected domains. Other factors that also influence test performance include the patient’s current mood, anxiety, depression or other psychiatric disorders, medication, and the patient’s motivation to participate fully.

Patterns of performance, such as strengths in some cognitive domains and weaknesses in others, have been associated with specific conditions based on empiric observation and knowledge of the brain pathology associated with those conditions. Observation of these patterns may aid in diagnosis.

TEST SELECTION

Neuropsychological tests in an assessment battery come from many sources. Some were developed for academic purposes (e.g., intelligence tests) and others for experimental psychology. The typical clinical battery consists of a series of standard tests that have been proved useful and are selected for the referral issue. These tests should have established reliability and validity. A tradeoff exists between the breadth of application and ease of interpretation, available from standard batteries, and the ability to pinpoint specific or subtle deficits offered by more experimental tasks that are useful in research but have not yet been standardized.

Most tests are intended to measure performance in specific cognitive or motor domains, such as memory, spatial ability, language function, or motor agility. These domains may be subdivided (e.g., memory may be considered verbal or nonverbal; immediate, short term, long term, or remote; semantic or episodic; public or autobiographic; or implicit or explicit). However, no matter how focused a test is, multiple cognitive processes are likely to be invoked. An ostensibly simple task, such as the Wechsler Adult Intelligence Scale (WAIS) Coding subtest (previously called Digit Symbol Coding), which uses a table of nine digit-symbol pairs to fill in the proper symbols for a series of numbers, assesses learning and memory, visuospatial abilities, motor abilities, attention, and speeded performance. In addition, tests may be failed for more than one reason: Patients may draw poorly because they may not appreciate spatial relationships, because they plan the construction process poorly, or because they are distractible or lack motivation. Relying solely on test scores may lead to spurious conclusions.

TESTS USED IN A NEUROPSYCHOLOGICAL EVALUATION

INTELLECTUAL ABILITY

Typically, a test such as the WAIS-IV or the Wechsler Intelligence Scale for Children-IV is used to assess the present level of intellectual function for adults and children respectively. These tests yield a global IQ score and index scores that are standardized, so that 100 is the mean expected value at any age (with a standard deviation of 15).

The WAIS-IV consists of 10 core and 5 supplemental subtests (Table 30.1). The 10 core subtests comprise the full scale IQ. In previous WAIS versions, verbal and performance IQ estimates were calculated. In contrast, the WAIS IV emphasizes a set of index scores based on groups of subtests, each assessing a broad, separate class of cognitive abilities. These include Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed. The WAIS-IV also adds a General Ability Index, which consists of selected subtests from the Verbal Comprehension and the Perceptual Reasoning subtests. The intention of this index score is to assess cognitive abilities that are not as susceptible to changes in processing speed and working memory. For the individual subtests, scaled scores range from 1 to 19, with a mean of 10 and a standard deviation of 3; the average range for subtest scaled scores is from 7 to 13.

The overall IQ and index scores supply summary information about the level of general intelligence and broad ability categories. The neuropsychologist is often more interested in the “scatter” of subtest scores, which indicates strengths and weaknesses.
The subtests are often better considered as separate tests, each assessing specific areas of cognitive function. There are many other tests of general intelligence, including some that are nonverbal.

TABLE 30.1 Subtests of the Wechsler Adult Intelligence Scale-IV Arranged by the Four Index Groups

Verbal Comprehension	Verbal Concept Formation, Verbal Reasoning, Knowledge Acquired from Environment
Similarities	Derive relevant superordinate category or similarity for word pairs; abstract verbal reasoning
Vocabulary	Name pictures and define words; often used to assess “premorbid” level of ability
Information	Answer questions that address a broad range of general knowledge topics; ability to acquire, retain, and retrieve general factual knowledge
(Comprehension)	Answer questions based on understanding of general principles and social situations
Perceptual Reasoning	Perceptual and Fluid Reasoning, Spatial Processing, Visual-Motor Integration
Block design	Arrange blocks with red, white, and half-red and half-white sides to form 14 designs; spatial perception, visual abstract processing, and problem solving
Matrix reasoning	Identify the picture that completes a pattern using pattern completion, classification, analogy, or serial reasoning; classic measure of fluid intelligence
Visual puzzles	View a completed puzzle and select three response options that, when combined, reconstruct the puzzle; visual perception and organization, nonverbal reasoning
(Picture completion)	Determine the missing feature in pictures; visual perception
(Figure weights)	View a scale with missing weight(s) and select the response option that keeps the scale balanced; quantitative and analogical reasoning
Working Memory	Attention, Concentration, Mental Control, Reasoning
Digit span	Standardized assessment of digits forward and backward. In addition, hear a sequence of numbers and recall those numbers in ascending order; attention, concentration, mental control
Arithmetic	Verbal arithmetic problems; mental manipulation, concentration, attention, short- and long-term memory, numeric reasoning ability
(Letter-number sequencing)	Listen to a combination of numbers and letters and recall first the numbers in ascending order and then the letters in alphabetical order. Sequential processing, mental manipulation, attention, concentration, memory span, short-term auditory memory.
Processing Speed	Ability to Quickly and Correctly Scan, Sequence, and Discriminate Simple Visual Information, Short-Term Visual Memory, Attention, Visual-Motor Coordination
Symbol search	Scan a search group and indicate whether one of the symbols in the target group matches; processing speed, visual discrimination
Coding	Using a key, copy symbols that are paired with numbers; processing speed, short-term visual memory
(Cancellation)	Scan a structured arrangement of shapes and mark target shapes; processing speed, visual selective attention
Supplementary subtests are indicated by parentheses. Comments describe the subtests and identify some select cognitive features that they tap.