Functional magnetic resonance imaging (fMRI) is used to record activity in different brain regions. When different regions exhibit simultaneous activity, we generally conclude that they are functionally connected in a network. Functional connectivity maps derived this way have revealed networks that control things like sensory processing and others that control cognition.

But this approach has a significant limitation: it's unable to reveal which brain region within the network is influencing which. Things happen so fast relative to the time resolution of the imaging that it's impossible to tell which part of the brain was active first.

Information about the direction of signaling—effective connectivity, rather than just functional connectivity—has been difficult to obtain. But now researchers in Germany have developed a method that combines the undirected functional-connectivity information from fMRI scans with energy metabolism data from PET scans, which measure glucose use, to begin to identify this effective connectivity.

Their new method, which they have dubbed metabolic connectivity mapping (MCM), relies on the observation that it's more energy-intensive to receive a neural signal than to send one. Up to 75 percent of signaling-related energy is consumed by the target neuron.

With the new technique, fMRI is still used to reveal an undirected connectivity between two brain regions by calculating the temporal correlation between their activity. The researchers then use PET scans to reveal where sugar is being used to infer the directionality of the signal by calculating the spatial correlations between the two types of data. The region that consumes more energy is the target of the signal; the region that consumes less is its source.

The researchers tested this on brains belonging to 24 healthy subjects who were scanned under two conditions: with their eyes open and with their eyes closed. As the authors note: “Vision is the only sensation that can be interrupted volitionally in a natural way.”

In both states MCM showed bidirectional signaling between visual cortices. But, only when the subjects' eyes were open did MCM show that the early visual cortex was receiving signals from the salience network—the one that motivates and girds individuals to act in response to a salient stimulus, be it internal or external.

The authors hope that by providing a measure of effective connectivity their MCM data can be used to shed light on disorders in signaling that may occur in conditions like Alzheimer's disease and major depression.

PNAS, 2015. DOI: 10.1073/pnas.1513752113 (About DOIs).