The effect of normal aging on the coupling of neural activity to the bold hemodynamic response

The use of functional neuroimaging to test hypotheses regarding age-related changes in the neural substrates of cognitive processes relies on assumpti ons regarding the coupling of neural activity to neuroimaging signal, Diffe rences in neuroimaging signal response between young and elderly subjects c an be mapped directly to differences in neural response only if such coupli ng does not change with age. Here we examined spatial and temporal characte ristics of the BOLD fMRI hemodynamic response in primary sensorimotor corte x in young and elderly subjects during the performance of a simple reaction time task. We found that 75% of elderly subjects (n = 20) exhibited a dete ctable voxel-wise relationship with the behavioral paradigm in this region as compared to 100% young subjects (n = 32), The median number of suprathre shold voxels in the young subjects was greater than four times that of the elderly subjects. Young subjects had a slightly greater signal:noise per vo xel than the elderly subjects that was attributed to a greater level of noi se per voxel in the elderly subjects. The evidence did not support the idea that the greater head motion observed in the elderly was the cause of this greater voxel-wise noise, There were no significant differences between gr oups in either the shape of the hemodynamic response or in its the within-g roup variability although the former evidenced a near significant trend. Th e overall finding that some aspects of the hemodynamic coupling between neu ral activity and BOLD fMRI signal change with age cautions against simple i nterpretations of the results of imaging studies that compare young and eld erly subjects. (C) 1999 Academic Press.