Initially BOLD signal was thought to be correlated with these action potentials.
That the more action potentials were present the greater the BOLD signal.
And this is some of the earlier publications that focused on that
by studying both BOLD activation in a monkey brain
in combination with single cell recording, trying to record the number of
action potentials that are occurring during that BOLD activation.
But a little bit more recently Logothetis and others and colleagues in 2001, did
more extensive experiments using both BOLD signals and electrophysiological data.
They've recorded activation from a number of neuronal units from
a number of neurons, referred to as multi-unit activity.
And they also recorded local field potentials,
which reflect a summation of post-synaptic potentials.
Now if you recall from one of our previous modules, local field potentials
are measured by measuring directly from the extracellular space.
If there's a lot of activity in neurons,
they draw ions from the extracellular space causing a depolarization, or
a reduction in voltage in the extracellular space.
And thus a dip in the voltage signal that you're measuring.
That's obviously not generated by a single neuron, but
a group of neurons locally will draw in these ions.
So local field potential are essentially measuring an aggregate
of action potentials that are occurring in that area,
in that group of neurons where that electrode is located.
When they did this study, they looked at field potentials and
these multi-unit activity.
And what they saw is that
the BOLD activation is most closely correlated with the local field potential.
So you can see the BOLD signal in the right hand graph in this pinkish color,
the local field potentials are indicated by the black line,
and the MUA the multi unit activity is indicated by the green line.
And even though there's a temporal off set in that the BOLD signal is visible later
than the local field potentials,
obviously there's a delay that is caused by the influx of blood
to that particular location that is much slower than the local field potentials.
The correlation between the BOLD signal and
the local field potentials is most significant more so
than with the action potential number that can be measured there.
In a follow up study, as you can see here,
the blue colored areas indicate local field potential measurements.
The red line indicates the BOLD signal that was measured there.
And the grey lines are an estimated or a predicted BOLD signal that is
calculated on the basis of the local field potentials in blue.
So if you predict what a BOLD signal would look
like based on these local field potentials, you would get the grey line.
And as you can see the grey line very closely matches the red line which is
the actually observed BOLD response.
So these studies together show that BOLD activity's more correlated with local
field potentials than it is with multi unit activity or
other measures of neural activity.
And BOLD activity is thought to reflect the input to a neural population and
remember me talking about the post synaptic action potential
which is the input to a particular neural population.
And the information processing that happens in this post-synaptic or
receiving neural population more so than anything else.