Age dependency of the hemodynamic response as measured by functional near-infrared spectroscopy.

Aging reduces cerebral blood flow in association cortices during rest. However, the influence of age on functional brain activation is still controversial. The aim of our study was to examine age dependency of brain activation in primary and association cortices. Therefore, changes in the concentration of oxy- and deoxyhemoglobin as well as changes in the redox state of cytochrome-c-oxidase (Cyt-Ox) were measured by functional near-infrared spectroscopy (fNIRS) in the lateral prefrontal and motor cortices during an event-related Stroop interference task. Fourteen young (23.9 +/- 3.1 years old) and 14 elderly subjects (65.1 +/- 3.1) participated in the study. Data revealed two effects of aging on brain activation: (1) Elderly and young subjects used the lateral prefrontal cortex to cope with interference during the Stroop task. In young subjects, the vascular response was higher during incongruent than neutral trials in the entire examined lateral prefrontal cortex. However, in the elderly, all lateral prefrontal regions showed a hemodynamic response but not necessarily a specific interference effect. (2) The hemodynamic response was reduced in elderly subjects in the lateral prefrontal association cortex, but obviously not in the motor cortex. We propose that calculating effect sizes is the only reliable approach to analyze age-related effects in fNIRS studies, because they are independent from the assumed differential pathlength factor. In summary, our findings suggest that aging decreases the hemodynamic response in the frontal association cortex during functional activation, omitting the primary motor cortex.