Neurologic data, while unable to locate the sources of a general factor for intelligence certainly does not exclude its possibility—one that is unavoidably measured in many different tests of cerebral functions. It is expressed if the connections between the frontal lobes and other parts of the brain are intact as attention, drive, and motivation are noncognitive psychologic attributes of fundamental importance to performance reside in this lobe. It is also possible, if not likely, that the parietal lobe associative areas of the cerebrum are engaged in the processing of sensory experiences and their manipulation in symbolic form. This applies equally to the ability to relate thoughts to each other and to stored concepts, but here, memory, symbols, and names, requiring the full function of the temporal lobes, play a central role. The interrelationships between some of these special abilities had been thoughtfully analyzed by Luria (see also the section on frontal lobes in Chap. 21). An account of the subject of IQ and intelligence can also be found in the monograph by Mackintosh.
An equivalently complex problem arises in the neurologic analysis of the highest human achievement and the method of human advancement, namely creativity. In some ways, creativity is tied to special skills along the lines of Gardner’s modality-based intelligence, particularly as it relates to artistic work, but the brain structures involved in aesthetics and abstraction are obscure, as Zeki points out. Some insight is gained from the fact that intelligence and problem-solving ability are only roughly tied to creativity and that there are congenital absences and deficiencies of appreciation of visual, artistic, or mathematical skills. The capacity to be creative may be inhibited by other functions of the brain, as exposed in the case described by Seeley and colleagues of a woman with frontotemporal dementia whose artistic abilities emerged as her facility with language deteriorated. As pointed out in the following chapter, like intelligence, traits such as creativity almost certainly do not reside in a particular lobe or structure of the brain and may depend on the overdevelopment of certain associative areas, as well as on frontal lobe drive and, of course, are fully manifest only by exposure and encouragement.
Dementia is a syndrome comprising the loss of several separable but overlapping intellectual abilities. It therefore presents in a number of different combinations. These constellations of intellectual deficits constitute the preeminent clinical abnormalities of a large number of cerebral diseases and are sometimes virtually the only abnormalities. Table 20-1 lists the most common types of dementing diseases and their relative frequency.
What is noteworthy to us about the figures in this table is the apparently high level of accuracy of diagnosis based on clinical assessment. Certainly, specialized testing that is now available improves the diagnostic accuracy but rather consistently, postmortem examination confirms the clinical diagnosis of Alzheimer disease is in excess of 80 percent when rigid research criteria are used (Table 20-2). (The high frequency of this disease in the older population makes the likelihood of correct diagnosis high). In most cases, the degenerative dementias can be differentiated by one or two characteristic clinical features, but these distinctions may be difficult to discern early in the process. In particular, a proportion of patients thought to have Alzheimer disease are ultimately found to have another type of degenerative cerebral atrophy, such as Lewy-body disease, progressive supranuclear palsy, Huntington disease, Parkinson disease, corticobasal degeneration, Pick disease, or one of the frontotemporal lobar degenerative diseases (see Chap. 38). Or such patients have a non-degenerative processes, such as multiinfarct dementia or hydrocephalus alone or in combination with one of the degenerative disorders. Of special importance is that approximately 10 percent of patients who are referred to a neurologic center with a question of dementia prove to have a potentially reversible psychiatric or metabolic disorder. There are also the earlier mentioned groups of nonprogressive dementias that are the lasting result of a single injury to the brain and do not appear in
Table 20-2.