Aging is associated with both decreases and increases in brain activity. Whereas age-related decreases usually reflect neural decline, some age-related increases have been linked to functional compensation. Compensatory activations in older adults (OAs) are typically found in the prefrontal cortex (PFC) but in the memory domain we have also found them in the temporal lobes (see Aging effects on basic memory mechanisms project page).
Within PFC, age-related increases typically take two forms. In one form, OAs show greater activity in the PFC hemisphere that is less activated in younger adults, yielding a more bilateral activation pattern. This pattern is known as Hemispheric Asymmetry Reduction in Older Adults or HAROLD (Cabeza, 2002). In the other form, the age-related increase in PFC is coupled with an age-related decrease in occipital cortex. This second pattern is known as Posterior-Anterior Shift with Aging or PASA (Davis et al., 2008). There is substantial evidence linking HAROLD and PASA to functional compensation. For example, in one study we found HAROLD in high- but not in low-performing OAs (Cabeza et al., 2002), and in a study on PASA we found that the PFC activity in OAs was negatively correlated with occipital activity and positively correlated with performance (Davis et al., 2008).
We have also found PASA-like (Dennis et al., 2008; St. Jacques et al., 2009) and HAROLD-like (Davis et al., in press) age-related increases in PFC functional connectivity that may also reflect functional compensation. For example, we found that OAs showed not only a more bilateral pattern of PFC activity, but also stronger functional connectivity between left and right PFC in a condition that required cross-hemispheric communication (bilateral trials) and this increase in functional connectivity was associated with better performance in that condition (Davis et al., in press). The age-related functional connectivity was constrained by the integrity of the genu of the corpus callosum (see Aging effects on white matter (DTI) project page).
We are currently developing a model of age-related compensation (Cabeza & Dennis, in press). This model assumes that aging reduces neural resources (brain anatomy and physiology), which cascades as a reduction in neural supply (resources available for task performance) and cognitive processing resources. OAs try to reduce the mismatch between available processing resources and task demands by recruiting additional neural resources (attempted compensation). Some forms of neural supply increases enhance cognitive performance (successful compensation), whereas others have no effect or a detrimental effect (unsuccessful compensation). This model can be tested by assessing correlations (curved arrows) between measurable variables (ovals). Beyond correlations, transcranial magnetic stimulation (TMS) may be used to test if regions overrecruited by OAs are actually linked to successful cognitive performance.
- Cabeza, R. (2002). Hemispheric asymmetry reduction in old adults: The HAROLD model. Psychology and Aging, 17, 85-100.
- Cabeza, R., Anderson, N. D., Locantore, J. K., & McIntosh, A. R. (2002). Aging gracefully: Compensatory brain activity in high-performing older adults. Neuroimage, 17, 1394-1402.
- Cabeza R, Dennis NA (in press) Frontal lobes and aging: Deterioration and compensation. In Stuss DT, Knight RT (Eds). Principles of frontal lobe functioning. Second edition. New York: Oxford University Press.
- Davis, S.W., Dennis, N.A., Fleck, M.S., Daselaar, S.M., Cabeza, R. (2008). Que PASA?: The posterior-anterior shift in aging. Cerebral Cortex, 18, 1201-1209.
- Davis, S. W., Kragel, J. Madden, D.J., & Cabeza, R. (in press). Cross-hemispheric communication and aging: Linking behavior, brain activity, functional connectivity, and white matter integrity. Cerebral Cortex.
- Dennis, N.A., Hayes, S.M., Prince, S.E., Madden, D.J., Huettel, S.A., and Cabeza, R. (2008). Effects of aging on the neural correlates of successful item and source memory encoding. Journal of Experimental Psychology: Learning, Memory, & Cognition, 34, 791-808.
- St. Jacques, P.L., Dolcos, F., Cabeza, R. (2009). Effects of aging on functional connectivity of the amygdala during subsequent memory for negative pictures: A network analysis of fMRI data. Psychological Science, 20, 74-84.