gen-free aged animals (including primates, rats, and mice) in regional centers.

This support should probably be in collaboration with other research funding agencies. It will involve a long-term commitment, especially for primates, and the support of highly skilled researchers and animal care staff. The research community should be consulted for advice on which animal strains are most important to keep available for research purposes.

The rest of this section discusses two major types of infrastructure that require support for the benefit of research on all the recommended initiatives: general-use databases, and brain imaging research capability.

Building General-Use Databases

Strategic investments in general-use data can support research on all three recommended initiatives. In our judgment, the greatest opportunity for such support lies in developing and expanding long-term, longitudinal studies.

There is little dispute that longitudinal research is essential to investigate processes of age-related change directly, rather than indirectly in the form of cross-sectional differences that may reflect a mixture of the products of change and of preexisting differences. Nevertheless, largely because of the greater time and expense, there are currently far more reports of cross-sectional comparisons of cognitive variables than longitudinal comparisons.

For many years the dominant interpretation has been that cross-sectional and longitudinal comparisons in cognition yield substantially different results, with much smaller, and later-occurring, age-related declines in longitudinal contrasts than in cross-sectional contrasts. However, some recent data call this view into question (e.g., Hultsch et al., 1998; Zelinski and Burnight, 1997; Sliwinski and Buschke, 1999). It is clear that the relation between cross-sectional and longitudinal age trends is still not fully understood, and thus it is important to have more linkages between the two major methods of investigating age differences. Longitudinal studies are certainly not the only type of research that can yield valuable information about aging, but they provide unique information that is not available from other research designs.

Several high-quality longitudinal studies focusing on cognition are currently under way in the United States (e.g., Baltimore, Seattle, Los Angeles) and elsewhere (e.g., Victoria, British Columbia; Manchester, England; Berlin, Germany). However, there are a number of features not represented in most existing studies that would be very desirable to include in future longitudinal studies.

First, at least in the initial assessment, the sample of participants should reflect major demographic variations in the population, and not just healthy middle-class whites as is the case in most current U.S. studies. Obtaining a



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