Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0905509106 (2009)

Functional magnetic resonance imaging (fMRI), used to map brain activity, gives a signal when the levels of oxygenated blood increase. The signal is often preceded by a darkening, thought to indicate early oxygen absorption by the brain owing to local neural activity.

But a study by Aniruddha Das and his colleagues at Columbia University in New York casts doubt on this. They used intrinsic signal optical imaging, a technique similar to fMRI, to measure changes in blood volume and blood oxygenation in the brains of two macaques while they performed a visual task.

The researchers found that during the initial darkening, blood-oxygen levels changed little, but blood volume increased markedly. The team suggests that blood-volume change is a better signal to use in brain imaging because it seems to be more closely linked to neural activity, occurring even before changes in blood oxygenation.