Colors cast long shadows on brain activity.

Light reaching the retina of the eye enables us to form images of our environment. A much older evolutionary benefit of light, though only recognized as important for humans in the past half a century, is that its 24-h irradiance profile informs the biological clock of the brain. The brain exploits this nonimageforming (NIF) information to optimally synchronize and prepare physiology and behavior to the environmental opportunities and hazards of that cycle with 24-h predictability. However, it has only been for the last decade that not only circadian NIF effects, but also acute NIF effects of light on the brain, have been appreciated. Acute NIF effects are supported by retinal projections to brain structures involved in the regulation of sleep, alertness, mood, and cognition (1). In PNAS, Chellappa et al. (2) underscore the relevance of these projections for human cognitive brain activity. The report importantly extends their previous functional MRI (fMRI) studies showing that light intensity modulates brain activation of people performing a cognitive task. Chellappa et al. now show that the effect of light on brain activity depends on the color spectrum of light people were exposed to more than an hour before. Thus, colors cast long shadows on future brain activity.