Executive Function

The term "executive function'' is used to cover a broad category of neuropsychological functions concerned with the sequencing and coordination of mental activity and behavior. These aspects tend to show impairment in the early stages of AD, with considerable impairment in the middle phase. This includes impairment on a range of standard tests, such as the Wisconsin Card Sorting Test, the Stroop Test, Random Generation of Digits, and Verbal Fluency. Although some of the deficits on these types of tasks are due to impairments in other domains of functions (e.g., language in relation to verbal fluency), there does appear to be a core deficit in relation to such functions as mental flexibility, initiation, and response inhibition. It is possible to detect subgroups of AD in which executive dysfunction appears early in a more isolated form, although the impairment tends to be become more general as the dementia progresses. It should be noted, however, that progressive dysexecutive impairment as an early presenting feature is likely to occur in people with frontotemporal dementia rather than AD.

Behavioral signs of executive dysfunction at a more basic level include perseveration and utilization behavior. Various forms of perseveration are seen in AD. These can be elicited using simple tasks, such as performing movement to command or getting the patient to draw alternating squares. The unchecked repetition of movement, sometimes termed continuous perseveration, is thought to be due to a disturbance in motor output in which there is postfacilitation of motor impulses. Operating at a different level are "stuck in set'' perseverations, which involve difficulty in switching from one activity to another. A third type, which has been termed recurrent perseveration, is the most common type reported in AD and involves unintentionally repeating a response after a delay, cued by a new stimulus. For example, when a patient is asked to define a series of words, such as in the Wechsler Adult Intelligence Scale Vocabulary test, he or she may produce definition material from earlier items in response to a new item in an inappropriate fashion. It has been found that 88% of patients with AD have produced at least one recurrent perseveration within a standard test battery, whereas in normal older adults this type of error is rarely seen. In utilization behavior, responses are cued by objects in the environment in an exaggerated or inappropriate fashion —for example, being impelled to reach out and grasp proximal objects. Although this has not been studied extensively in AD, it can be observed in patients in the middle or later stages.

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