If you look at how the fMRI signal works, what we're going to focus on in particular
is the BOLD signal, the BOLD which stands for blood-oxygen-level dependent signal.
When a neuron is active we see that it's
extracting its oxygen and energy from the blood stream locally.
This is seen if you just focus in on the signal and the current
the signal in the particular region, in that particular region of the frame.
You will see that as a drop in the signal.
The reason that we're seeing a small drop in the signal, is that oxygen and
consumption oxygen leads to a smaller amount of oxygen regionally.
And oxygen, since it has a magnetic property,
the magnetic resonance imaging machine can pick that signal change up.
So a part, particular part of the brain that is
active it is initially seen as a drop in the signal.
This is what I call the initial bit.
Now the initial bit is very, very tiny, and very, very small.
It's really hard to, to to measure.
What then happens is that in the same region that has just consumed oxygen and
glucose, for example, receives a huge influx of fresh blood
that is rich in oxygen and rich in other compounds, which is the glucose.
And it actually is an overshoot.
It's actually too much that is provided to that region,
and what we see there in the MR machine is a huge increase in the signal change.
This is the actually measure that we're using,
when we're using fMRI to measure brain activation.
And then after that we see a normalization,
actually undershoot after that.
Then in come the stabilization, over time, in the same region.
So this means that when we're measuring the fMRI, and
we're using fMRI to measure brain activation.
It's an indirect measure of brain activation because it's not
a direct measure of brain activation in but in itself.
But the overshoot of influx of oxygen-rich blood to a region that has been active.
This is a particular function of the fMRI,
which is very different from other methods.