depend on brain systems that are at risk for varying rates of change with aging. In the field of sensory processing, a major development has been the identification of a hierarchy of functionally specific visual processing areas in the human brain (Sereno et al., 1995), which may be homologues of areas that have been studied extensively at the cellular level in nonhuman primates. Not only does this knowledge provide a solid foundation for the interpretation of a large body of existing neuropsychological data on human brain function (with concomitant clinical relevance), but it sets the stage for the first direct and in-depth comparisons between human and nonhuman primate data. In the field of memory, several recent studies have identified brain regions that are active while volunteers hold specific types of information in short-term memory (e.g., Smith and Jonides, 1998) Other studies have demonstrated changes in the nature of the fMRI response—expansion of the zone of activation, for example—associated with the learning of new sensorimotor tasks (e.g., Karni et al., 1998). These observations provide direct evidence for selective plasticity of neuronal structures as a substrate for learning. Finally, language has become a special focus of fMRI investigation, as it addresses issues that are difficult if not impossible to approach using animal models. Several studies have explored the functional modularity of language processing areas by these means (e.g., Posner and Pavese, 1998).

WHAT IS THE RELEVANCE OF FMRI TO UNDERSTANDING THE AGING BRAIN?

Inasmuch as an understanding of the relationship between the aging process and the brain must be built on a general understanding of brain organization and function, all of the functional imaging applications cited above are of relevance to aging research. In addition, there are specific areas of research in which functional imaging can make a unique contribution. Perhaps the most obvious of these is the use of imaging technology to compare brain organization and function across different age groups, as a complement to psychological and neurological characterization of normal and pathological states. Indeed, this is an area of tremendous potential owing to the unprecedented ability offered by fMRI to identify specific brain abnormalities associated with age-related sensory, motor, memory, and language dysfunction.

LIMITATIONS OF FMRI FOR STUDIES OF THE HUMAN BRAIN

The reasons for the rapid development of fMRI techniques and their application in studies of human brain function are obvious. In spite of the many realized and potential gains, however, and the fact that fMRI stands to significantly advance research on aging of the brain, there are some notable limitations to this approach when it is applied strictly to humans. These



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