Hemispheric Asymmetry Reduction in Old Adults (HAROLD): Evidence for the Compensation Hypothesis

Cabeza R.*, Anderson N. D. ‡†, Kester J.†, & McIntosh A. R. †§

* Center for Cognitive Neuroscience, Duke University, B203 LSRC Bldg., Durham, NC 27708, USA
‡ KLARU, Baycrest Centre, 3560 Bathurst Street, Toronto, Ontario M6A 2E1, Canada
† Department of Psychology, University of Toronto, 100 St. George Street, Toronto, Ontario, M5S 1A1, Canada
§ Rotman Research, Institute, Baycrest Centre, 3560 Bathurst Street, Toronto, Ontario M6A 2E1, Canada

Whereas some older adults show significant cognitive deficits, others perform as well as young adults. To investigate the neural basis of these different aging patterns, we measured brain activity in low- and high-performing older adults using positron emission tomography (PET). In functional neuroimaging studies, prefrontal cortex (PFC) activity tends to be less asymmetric in older than in younger adults (Hemispheric Asymmetry Reduction in Old Adults or HAROLD). This finding has been observed in the domains of episodic retrieval, episodic encoding/semantic retrieval, working memory, perception, and inhibitory control. Age-related asymmetry reductions may help counteract age-related neurocognitive decline (compensation hypothesis) or they may reflect an age-related difficulty in recruiting specialized neural mechanisms (dedifferentiation hypothesis) To compare these two hypotheses, we measured PFC activity in young adults, low-performing older adults, and high-performing older adults during recall and source recognition of recently studied words. Low- and high-performing older participants were selected before scanning from a larger sample of older adults. Compared to recall, source memory was associated with right PFC activations in young adults. Low-performing older adults recruited similar right PFC regions as young adults, whereas high-performing older adults engaged PFC regions bilaterally. Thus, consistent with the compensation hypothesis and inconsistent with the dedifferentiation hypothesis, a hemispheric asymmetry reduction was found in high-performing but not in low-performing older adults. These results suggest that low-performing older adults recruited a similar network as young adults but used it inefficiently, whereas high-performing older adults counteracted age-related neural decline through a plastic reorganization of neurocognitive networks.