Group comparisons are not new to psychology or the neurosciences. Since the inception of functional magnetic resonance imaging (fMRI), scientists have compared the neural activity of groups of interest. In recent years, there has been particular growth in the number of studies of the aging brain using neuroimaging (Cabeza et al.
; Grady, 2008
). Scientists may compare the brain activity of younger and older adults to explore a wide range of research interests. Group comparisons across the life span can potentially reveal the effects of age-related neural decline, effects of accumulated experience, motivational differences across adult development, or the influence of global differences in time perspective (Reuter-Lorenz, 2002
; Reuter-Lorenz and Lustig, 2005
; Carstensen, 2006
). However, the inferences that can be drawn from such comparisons are highly limited unless care is taken at all stages of the research process from study design to data analysis to interpretation of the results.
Consider the hypothetical results in . This figure will be referenced throughout the review to help illustrate a number of the concerns discussed. The figure displays the results from a hypothetical cross-sectional fMRI study. The results of the study reveal a significant main effect of age group such that younger adults show greater activation than older adults across several brain regions (A). Plotting the group-averaged ‘neural signal’ (either percent signal change from the event of interest within a trial or parameter estimates of a regressor fit) for the two groups confirms the significant difference (B). Further, combining the younger and older adult samples, a simple correlation reveals that the neural signal correlates with behavior in the task (C; i.e. the number of pictures encoded in a memory task). The region of interest (ROI) data used for the analyses were extracted within individuals using a single functional mask (D–E) created from the results of the group comparison analysis in A. The results of the study may seem clear: the older adults show less brain activation and perform worse than the younger adults. The authors of this particular study might speculate further that the group differences in task performance are due to the group differences in neural signal in these regions. However, as we will discuss throughout this review, there are several reasons to be skeptical of these conclusions.
Fig. 1 This figure displays hypothetical results from a cross-sectional study examining differences in brain activity between younger and older adults. Although, the results of this hypothetical study may seem to suggest that the poorer performance of older (more ...)
Comparing age groups using fMRI is not as simple as collecting a sample of older participants and assuming that any differences between age groups are due to differences in underlying neural computations. Many neurovascular and morphological changes accompanying healthy aging can confound results. This review highlights potential problems with comparing younger and older adults using fMRI, with an emphasis on solutions. We review three areas (group differences in hemodynamics, brain morphology and variance/noise) and summarize solutions that have been proposed in the literature. Although, a number of excellent methodological papers and chapters have appeared in recent years, they are rarely cited and the suggestions for overcoming common issues are often not implemented. Our goals are to summarize this research and provide a succinct and easily accessible set of rules of thumb for conducting studies of the aging brain using functional neuroimaging.