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Hi, kids! My name is Vickie Voxel. I’m
going to tell you about fMRI, HRF &
BOLD.
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Intro to fMRIPart I: HRF & BOLD
Dr. Russell James, Texas Tech University
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An fMRI picture of the brain is made up of
thousands of boxes, called voxels, just like me!
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We voxels are small –
usually about the size of one peppercorn
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Inside each of us
voxels are thousands of neurons
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When a lot of these neurons
start to fire, the body
rushes in oxygen to help
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This rush of oxygen comes through the blood and makes me start to
change color
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As my blood oxygen
increases, I get redder
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And redder
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If this keeps going, I will be
totally red from all of the oxygen in my
blood
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But then, if the neurons don’t keep firing, the
body will stop rushing oxygen
to me
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And my color will start to
return to normal
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I can get a bit blue at the end if my oxygen
drops too low, right before it returns to
normal
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In 20 seconds after the neurons
fired, I will be back to my
normal color again
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This whole color change process is called my
hemodynamic response
“Hemo” means blood. “Dynamic” means change. So, hemodynamic response is my “blood-change” response.
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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When we model this change with math, we call it a
hemodynamic response function
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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But, saying “hemodynamic
response function” takes too long, so we will just call it
the HRF
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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The HRF
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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The fMRI machine can see my color change because blood with a lot of oxygen (red) is less attracted to magnets than blood without much oxygen (pink or blue).
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But, instead of showing pictures in red, pinks, and blues, the fMRI creates black and white pictures.
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Differences in magnetism are shown as shades of light and dark.
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The fMRI machine is measuring a BOLD signal because the color is
BloodOxygenLevelDependent
High blood oxygen
Low blood oxygen
So, instead of an HRF looking like
this…
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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The fMRI shows an HRF that looks
like this…
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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Because the color tells us the blood oxygen level, we really don’t need a vertical axis…
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS after neurons fire
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We can just use one line
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16SECONDS after neurons fire
And, if we always take a picture
every 2 seconds, then we don’t even need
the seconds on the bottom
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16SECONDS after neurons fire
So, all of our HRF information is shown on the timeline below
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Neuron firing starts
But, instead of a series of
colors, the fMRI machine gives us
numbers
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Neuron firing starts
180 200 250 305 249 201 182 172 180
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Neuron firing starts
180 200 250 305 249 201 182 172 180
The fMRI records a number for the
magnetism of each voxel at each time
If we take a picture every 3 seconds for 7 minutes, we get 140
numbers for each voxel
And we might have 100,000 or so voxels in the whole brain
20 images per minute X 7 minutes X 100,000 voxels X 20 people = 280 million data points
We can’t track it by hand!
So we have to ask the computer to analyze the data
But the computer needs to know
WHAT to look for and WHEN to look
for it
So WHAT are we looking for?
We are looking for changes in the signal that look like this
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS
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Lev
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
SECONDS
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We are looking for changes in the signal that look like this
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180 200 250 305 249 201 182 172 180
We are looking for changes in the signal that look like this…
because this looks like an HRF, meaning it was caused by a large
group of neurons firing
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180 200 250 305 249 201 182 172 180
We are looking for changes in the signal that look like this…
because this looks like an HRF, meaning it was caused by a
large group of neurons firing
WHEN are we looking for this HRF-like signal?
We are interested in the HRF-like signals that happen right after the
subject experiences something
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Then we can estimate the likelihood that a voxel, or group of
voxels, is responding to the stimulus
This simple concept can be difficult in practice because:
1. The signal change is small2. The brain is noisy
This simple concept can be difficult in practice because:
1. The signal change is small2. The brain is noisy
Overcoming these barriers requires:
Good study design +
Good data analysis
Bye for now!
Next time we will look at how to
create a good study design.
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Intro to fMRIPart I: HRF & BOLD
Dr. Russell James, Texas Tech University
