Published online 12 July 2001 | Nature | doi:10.1038/news010712-13

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Neurophysiological investigation of the basis of the fMRI signal

At long last neuroscientists know what they're looking at.

© N. Logothetis

When we think, our brain lights up - or so we have been led to believe by the now-familiar pictures of the brain in action, which depict a glow around the active area. Now neuroscientists in Germany have finally worked out what these pictures are telling us.

Scans obtained using functional magnetic resonance imaging (fMRI) show brain cells (neurons) receiving and processing electrical signals, say Nikos Logothetis and colleagues at the Max Planck Institute for Biological Cybernetics in Tübingen1.

Previously, no one knew whether the bright parts of fMRI images showed the input to or output from nerve cells, or something else entirely. The great advances advertised in our understanding of brain function in recent years have until now been based largely on an ill-understood act of faith.

What the fMRI technique actually measures is not neurons' electrical signals at all, but changes to the flow of blood in the brain. The fMRI signal increases in proportion to the amount of blood flow. When a group of neurons becomes active, it needs more blood, and so generates a signal in an fMRI scan.

Because fMRI is non-invasive, it is ideal for monitoring the brain activity of a person conducting mental or physical tasks. But it has previously been a 'black box', because a change in neuron activity, indicated by greater blood flow, could mean one of several things.

Logothetis's group measured two things simultaneously: the fMRI signal and the electrical activity of neurons in the primary visual cortex of monkeys watching rotating checkerboard patterns. They tracked electrical activity and magnetic fields with needle-like probes inserted under anaesthetic into the animals' brains.

The group looked at the relationship between the size of the fMRI signal and three types of electrical activity in neurons - the slowly changing electrical fields produced by input signals to neurons and by their signal-processing activity, the rapid output pulses that individual neurons generate in response, and the output signals from collections of neurons.

The fMRI signal was most strongly related to the input-processing signals, the team found. This, they say, makes sense: processing inputs is the most energy-consuming part of a neuron's job, and so uses more fuel. This fuel is glucose, burnt up by oxygen - both of which are carried in the blood.

Interestingly the increase in blood flow is much greater than that apparently needed to enhance the brain's energy supply, implying there is some additional reason for it. As yet, no one knows what that reason might be. Brain scans might now be less of a black art, but they retain some of their mystery. 

  • References

    1. Logothetis, N. K., Pauls, J., Augath, M., Trinath, T. & Oeltermann, A. Neurophysiological investigation of the basis of the fMRI signal. Nature 412, 150 - 157 (2001). | Article | PubMed | ISI | ChemPort |