Neurology of Intelligence and Memory
Increasingly, as the population of elderly rises, the neurologist is consulted because an otherwise healthy person begins to fail mentally and loses his capacity to function effectively at work or in the home. This may indicate the development of a degenerative brain disease, a brain tumor, multiple strokes, chronic subdural hematomas, drug intoxication, chronic meningoencephalitis (such as caused by HIV or syphilis), normal-pressure hydrocephalus, or a depressive illness. Formerly, there was little that could be done about these clinical states, but there are now effective means of treating several of these conditions, and in some instances, of restoring the patient to normal competence. Moreover, diagnostic technologies allow earlier recognition of the underlying pathologic process, thus improving the chances of recovery or of preventing the disease’s progression.
The definitions of normal and abnormal states of mind were considered in Chap. 19, where it was pointed out that the term dementia denotes a persistent deterioration of intellectual or cognitive function with little or no disturbance of consciousness or perception. In current medical parlance, the term is used to designate a syndrome of failing memory and of other intellectual functions as a result of chronic progressive degenerative disease of the brain. Such a definition may be too narrow. The term more accurately includes a number of closely related syndromes characterized not only by intellectual deterioration but also by certain behavioral abnormalities and changes in personality. Furthermore, dementia can be the result of a static encephalopathy such as head trauma or cerebral anoxia or of a progressive degenerative disease, but it differs from the global confusional state, or encephalopathy, in its chronicity. Thus, it is not possible to determine if a confused, amnestic person is demented until some time has passed and the deficits have persisted, marking a dementia, or abated, signifying an encephalopathy. Beyond the need to properly define these terms, the two entities have different causes. There are several states of dementia of differing causes and mechanisms and degeneration of systems of cerebral neurons, albeit common, is only one of the many causes. Similarly, as discussed in previous chapters, there are myriad intrinsic (metabolic) and extrinsic (toxic) causes of encephalopathy.
To understand the phenomenon of intellectual deterioration, it is helpful to have some idea of how intellectual functions, particularly intelligence and memory, are normally organized and sustained, and the manner in which diffuse and focal cerebral lesions cause deficits in these functions. The neurology of intelligence is considered in this chapter as a prelude to a discussion of the dementias and the neurology of memory.
Intelligence, or intelligent behavior, has been variously defined as a “general mental efficiency,” as “innate cognitive ability,” or as “the aggregate or global capacity of an individual to act purposefully, to think rationally, and to deal effectively with his environment” (Wechsler), in other words, the capacity to have ideas and reason about them. It is global because it characterizes an individual’s behavior as a whole; it is an aggregate in the sense that it is composed of a number of independent and qualitatively distinguishable cognitive abilities. This topic should be of interest to neurologists because intelligence is disturbed by many disorders of the brain but cannot be easily attributed to any cerebral region or particular cognitive function. Indeed, the dementias and developmental delays, affect intelligence in a way that cannot be explained except by some widely distributed aspect of brain function.