Figure 11. The effects of light exposure on cognition are wavelength dependent. Different colors of light distinctly influenced cognitive function in old mice. Red light exposure markedly altered the gut microbial diversity and composition and induced remarkable spleen enlargement in mice, leading to cognitive impairment in non-septic mice and exaggerating cognitive disorders in septic mice. Pseudo germ-free mice receiving fecal microbiota from red light-exposed septic mice also presented with splenomegaly and cognitive deficits. The damage depended on communication via the subdiaphragmatic vagus nerve through the “gut-brain axis” and the “spleen-brain axis.” Green light exposure had no obvious impact on cognition in mice.