Fundamental Assumptions

The study of clinical neuropsychology is firmly rooted in the larger field of neuroscience and, therefore, rests on the assumption that the nervous system impacts behavior and cognition. Conversely, inferences can be made about the integrity of the brain based on observable behavior. The ability to make accurate and meaningful inferences is predicated on a thorough understanding of two streams of knowledge: (a) the neural infrastructure underlying normal human cognition and behavior and (b) the characteristic profiles of neurocognitive and neurobehavioral syndromes.

Observable behavior is frequently the most sensitive manifestation of brain pathology. Such behavior can range from a person's manner of dress to his or her performance on a specific neuropsychological test. A competent neuropsychologist will sample multiple domains of behavior. Observable behavior, including

"test behavior,'' reflects an interaction between the domains of person and environment. Variables from each domain must be considered in order to arrive at an understanding of the clinical significance of a given behavior.

Individual tests used in neuropsychological practice focus on measurement of particular cognitive processes. In order to be useful, each test must be constructed according to sound psychometric principles and possess adequate validity and reliability. In addition, there must be appropriate normative data with which to compare a single patient's performance. It is the responsibility of the neuropsychologist to have a comprehensive understanding of test construction and proper usage so that it may be applied appropriately in relevant clinical circumstances. This includes choosing tests with normative data derived from subjects of similar age, education, and nationality to the patient currently being tested.

The interpretation of test results depends on an understanding of the component processes involved in any given test response. For example, the ability to name an object depends on multiple processes that are mediated by different brain systems. A person must first orient and attend to the stimulus. Second, the stimulus must be accurately registered at the level of visual perception. Third, the neural pathway that links the visual percept to meaningful recognition must be intact. Fourth, the ability to assign a phonemic/lexical label to the object must be intact. Finally, the person must be able to convey a response through speech. Because a complex cognitive process, such as naming a simple object, can be undermined by perturbation at any point along the route between these processes, a neuropsychologist must understand the component processes involved in each behavior. By examining the patient's function in multiple domains with multiple measures, the neuropsychologist can determine which processes and associated neural circuits are functioning abnormally.

Performance on a single test is not sufficient to make a diagnostic inference. A common misconception is that a poor score on a particular test denotes an impairment in the domain that the test is nominally designed to assess. For example, if a patient performs poorly on a memory test, this does not necessarily signify an impairment in memory, but could indicate difficulties with attention or language. A neuropsy-chologist will evaluate a profile of test results in a dynamic, interactive fashion in order to arrive at a diagnostic formulation.

Neuropsychological testing is simply one means of obtaining a sample of behavior. A neuropsychologist must proceed with caution in using test data to predict behavior. The testing environment is, by necessity, contrived to promote a standard approach to test administration. This contrivance constitutes a challenge in understanding how test performance corresponds to "real-life" behavior. For example, a patient who complains of difficulty with concentration and memory at work may perform quite normally in the context of the quiet, distraction-free consultation room. Discrepancy between test behavior and real-life behavior is the source of ongoing challenge for the design of an ecologically valid assessment environment.

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