A BOLD perspective on age-related neurometabolic-flow coupling and neural efficiency changes in human visual cortex
- 1School of Behavioral and Brain Sciences, University of Texas at Dallas, Richardson, TX, USA
- 2Department of Psychiatry, University of Texas Southwestern Medical Center, Dallas, TX, USA
- 3Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, USA
Age-related performance declines in visual tasks have been attributed to reductions in processing efficiency. The neural basis of these declines has been explored by comparing the blood-oxygen-level-dependent (BOLD) index of neural activity in older and younger adults during visual task performance. However, neural activity is one of many factors that change with age and lead to BOLD signal differences. We investigated the origin of age-related BOLD changes by comparing blood flow and oxygen metabolic constituents of BOLD signal. Subjects periodically viewed flickering annuli and pressed a button when detecting luminance changes in a central fixation cross. Using magnetic resonance dual-echo arterial spin labeling and CO2 ingestion, we observed age-equivalent (i.e., similar in older and younger groups) fractional cerebral blood flow (ΔCBF) in the presence of age-related increases in fractional cerebral metabolic rate of oxygen (ΔCMRO2). Reductions in ΔCBF responsiveness to increased ΔCMRO2 in elderly led to paradoxical age-related BOLD decreases. Age-related ΔCBF/ΔCMRO2 ratio decreases were associated with reaction times, suggesting that age-related slowing resulted from less efficient neural activity. We hypothesized that reduced vascular responsiveness to neural metabolic demand would lead to a reduction in ΔCBF/ΔCMRO2. A simulation of BOLD relative to ΔCMRO2 for lower and higher neurometabolic-flow coupling ratios (approximating those for old and young, respectively) indicated less BOLD signal change in old than young in relatively lower CMRO2 ranges, as well as greater BOLD signal change in young compared to old in relatively higher CMRO2 ranges. These results suggest that age-comparative studies relying on BOLD signal might be misinterpreted, as age-related BOLD changes do not merely reflect neural activity changes. Age-related declines in neurometabolic-flow coupling might lead to neural efficiency reductions that can adversely affect visual task performance.
Keywords: fMRI, BOLD, CBF, CMRO2, hypercapnia, aging, neurometabolic-flow coupling, neural efficiency
Citation: Hutchison JL, Shokri-Kojori E, Lu H and Rypma B (2013) A BOLD perspective on age-related neurometabolic-flow coupling and neural efficiency changes in human visual cortex. Front. Psychol. 4:244. doi: 10.3389/fpsyg.2013.00244
Received: 01 January 2013; Accepted: 14 April 2013;
Published online: 03 May 2013.
Copyright: © 2013 Hutchison, Shokri-Kojori, Lu and Rypma. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: Joanna Lynn Hutchison, Center for Brain Health, 2200 West Mockingbird Lane, Dallas, TX 75235, USA. e-mail: email@example.com