Brain imaging
explainedAt long last
neuroscientists know what they're looking
at.
12 July 2001
PHILIP
BALL
 |
| Take a peak: fMRI shows
brain cells receiving and processing electrical
signals. |
| © 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.
. Nature 412, 150-157 (2001). |
