1. Hi, kids! My name
is Vickie Voxel. I’m
going to tell you
● ●
about fMRI, HRF &
BOLD.
Intro to fMRI
Part I: HRF & BOLD
Dr. Russell James, Texas Tech University
2. An fMRI picture of the
brain is made up of
thousands of boxes, called
voxels, just like me!
● ●
19. 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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20. But, instead of showing pictures
in red, pinks, and blues, the fMRI
creates black and white pictures.
Differences in
magnetism are
shown as shades of
light and dark.
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21. The fMRI machine is measuring a BOLD
signal because the color is
Blood
Oxygen
Level
Dependent
● ● ● ● High blood oxygen
Low blood oxygen
● ●
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24. Because the color
tells us the blood
oxygen level, we
● ●
really don’t need
a vertical axis…
Blood Oxygen Level
● ● ● ●
● ● ● ●
● ● ● ● ● ● ● ● ● ●
● ●
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
25. We can just use
one line
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SECONDS after neurons fire
26. And, if we always
take a picture
every 2 seconds,
then we don’t even
need the seconds
on the bottom
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SECONDS after neurons fire
27. So, all of our HRF
information is
shown on the
Neuron timeline below
firing
starts
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28. But, instead of a
Neuron
firing
series of colors,
starts the fMRI
machine gives us
numbers
180 200 250 305 249 201 182 172 180
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29. The fMRI records a
Neuron number for the
firing magnetism of each voxel
starts at each time
180 200 250 305 249 201 182 172 180
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30. If we take a picture every 3
seconds for 7 minutes, we get 140
numbers for each voxel
31. And we might have 100,000 or so
voxels in the whole brain
32. 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!
33. So we have to ask
the computer to
analyze the data
34. But the computer
needs to know
WHAT to look for
and WHEN to look
for it
36. We are looking for changes in the
signal that look like this
● ●
Blood Oxygen Level
● ● ● ●
● ● ● ●
● ● ● ● ● ● ● ● ● ●
● ●
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
37. We are looking for changes in the
signal that look like this
● ●
Blood Oxygen Level
● ● ● ●
● ● ● ●
● ● ● ● ● ● ● ● ● ●
● ●
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
38. 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
180 200 250 305 249 201 182 172 180
● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
39. 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
180 200 250 305 249 201 182 172 180
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41. We are interested in the HRF-like
signals that happen right after the
subject experiences something
+ + + + +
42. Then we can estimate the
likelihood that a voxel, or group of
voxels, is responding to the
stimulus
43. This simple concept can be
difficult in practice because:
1. The signal change is small
2. The brain is noisy
44. This simple concept can be
difficult in practice because:
1. The signal change is small
2. The brain is noisy
Overcoming these barriers
requires:
Good study design
+
Good data analysis
45. Bye for now!
Next time we will
look at how to
create a good study
● ●
design.
Intro to fMRI
Part I: HRF & BOLD
Dr. Russell James, Texas Tech University
