Persons over 65 years of age have increased more than eight fold since 1900, from approximately 3.1 million persons to 26 million in 1980. This represents a rise from 4.1% to 11.3% of the total population (U.S. Department of Health and Human Services, 1984). The U.S. Bureau of Census (1980) projects that the number of persons over age 65 may reach 36.6 million by the year 2000, accounting for 13% of the total population. Further, the proportion of old people who will survive beyond the age of 75, the “old-old” age stratum, will account for 43.3% of the total aged population in the year 2000. In 1970 and 1940, the figures were 38.2% and 29.5%, respectively (Wang, 1977).

Senile dementia Alzheimer's type (SDAT) is a major pathology of old age (Emery, 1985; Pincus & Tucker, 1985; Shamoian, 1984). Estimates of the prevalence of dementia from population studies in Europe and the United States range from 3.9% to 22%; the mean estimate tends to be around 7% across age, rising to more than 22% in the age stratum of 80 years and older (Kay & Bergmann, 1980; Nielson, 1963; Pincus & Tucker, 1985; Primrose, 1962; Woods & Britton, 1985).

Given the increasing number of older people in our society, understanding the nature and causes of SDAT must rank high on research priorities. My goal in this chapter is to elucidate one aspect of SDAT, the deterioration of higher-order cortical processes which is at the center of the symptom complex that characterizes senile dementia Alzheimer's type (Emery, 1984, 1985).