To visualize brain activity, neuroscientists use functional magnetic resonance imaging (fMRI) to measure blood oxygen levels, known as BOLD signals, which are considered a proxy for cellular activity. However, it has been unclear which types of brain cell contribute to these signals.

Fritjof Helmchen and his colleagues at the University of Zurich in Switzerland have developed a method that tracks the activity of neurons and glial cells — support cells that might also contribute indirectly to neurotransmission — during an fMRI scan. They found that activation of both cell types correlates with BOLD signals.

The team used an optical fibre to record the activity of dye-loaded brain cells that fluoresce when calcium enters them — an indication of cell activation. This composite method will help scientists to interpret BOLD signals, the authors say.

Nature Methods http://dx.doi.org/10.1038/nmeth.2013 (2012)