Effects of aging on low luminance contrast processing in humans.

Luminance contrast is a fundamental visual cue. Using a dedicated neuroimaging framework, we sought to characterize typical Blood Oxygen Level Dependent (BOLD) responses in two subcortical regions, the superior colliculus (SC) and the lateral geniculate nucleus (LGN), and V1, the primary visual cortex area, and how they change over the lifespan. For imaging subcortical activity related to luminance contrast modulation, specific measurements were introduced to rule out possible signal contamination by cardiovascular activity and vascular alterations with age that could hamper the BOLD signal interpretation. Clearly, BOLD responses increased in these three regions with luminance contrast, with a statistically significant diminution in LGN and V1 for older compared to younger participants, while basal perfusion remained unchanged. Additionally, perceptual responses, as assessed with psychophysical experiments, were highly correlated to BOLD measures in the three studied regions. Taken together, fMRI and psychophysics results indicate an alteration of luminance contrast processing with normal aging. Based on this knowledge we can better recognize when age-related brain changes vary from these expectations especially during neurodegenerative diseases progression where the functioning of subcortical structures is altered. The proposed fMRI-physchophysics methodology allows performing such investigation.