Age dependence of hemodynamic response characteristics in human functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) studies of cognitive aging have generally compared the amplitude and extent of blood oxygen level-dependent (BOLD) signal increases evoked by a task in older and younger groups. BOLD is thus used as a direct index of neuronal activation and it is assumed that the relationship between neuronal activity and the hemodynamic response is unchanged across the lifespan. However, even in healthy aging, differences in vascular and metabolic function have been observed that could affect the coupling between neuronal activity and the BOLD signal. Here we use a calibrated fMRI method to explore vascular and metabolic changes that might bias such BOLD comparisons. Though BOLD signal changes evoked by a cognitive task were found to be similar between a group of younger and older adults (e.g., 0.50 ± 0.04% vs. 0.50 ± 0.05% in right frontal areas), comparison of BOLD and arterial spin labelling (ASL) responses elicited in the same set of structures by a controlled global hypercapnic manipulation revealed significant differences between the 2 groups. Older adults were found to have lower responses in BOLD and flow responses to hypercapnia (e.g., 1.48 ± 0.07% vs. 1.01 ± 0.06% over gray matter for BOLD and 24.92 ± 1.37% vs. 20.67 ± 2.58% for blood flow), and a generally lower maximal BOLD response M (5.76 ± 0.2% vs. 5.00 ± 0.3%). This suggests that a given BOLD response in the elderly might represent a larger change in neuronal activity than the same BOLD response in a younger cohort. The results of this study highlight the importance of ancillary measures such as ASL for the correct interpretation of BOLD responses when fMRI responses are compared across populations who might exhibit differences in vascular physiology.