fMRI terms: HRF and BOLD

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fMRI terms: HRF and BOLD

  1. 1. Hi, kids! My name is Vickie Voxel. I’m going to tell you ● ● about fMRI, HRF & BOLD. Intro to fMRI Part I: HRF & BOLDDr. Russell James, Texas Tech University
  2. 2. An fMRI picture of the brain is made up of thousands of boxes, called voxels, just like me!● ●
  3. 3. We voxelsare small – usually about thesize of one ● ●peppercorn
  4. 4. Inside each of us voxels are thousands of neurons● ●
  5. 5. When a lot of these neurons start to fire, the body rushes in● ● oxygen to help
  6. 6. This rush of oxygen comes through the blood and makes me● ● start to change color
  7. 7. As my blood oxygen increases, I get redder● ●
  8. 8. And redder● ●
  9. 9. If this keeps going, I will be totally red from all of the oxygen in my● ● blood
  10. 10. But then, if the neurons don’t keep firing, the body will stop● ● rushing oxygen to me
  11. 11. And my color will start to return to normal● ●
  12. 12. I can get a bit blue at the end if my oxygen drops too low, right before it● ● returns to normal
  13. 13. In 20 seconds after the neurons fired, I will be back to my normal● ● color again
  14. 14. This whole color change process is called my hemodynamic● ● response
  15. 15. “Hemo” means blood. “Dynamic” means change. ● ● So, hemodynamic response is my “blood-change”Blood Oxygen Level ● ● ● ● response. ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 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  
  16. 16. When we model this change with math, we call it a ● ● hemodynamic response functionBlood 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  
  17. 17. But, saying “hemodynamic response ● ● function” takes too long, so we will just call itBlood Oxygen Level ● ● ● ● the HRF ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● 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  
  18. 18. ● ● The HRFBlood 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  
  19. 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).● ●● ●
  20. 20. But, instead of showing picturesin red, pinks, and blues, the fMRIcreates black and white pictures. Differences in magnetism are shown as shades of light and dark. ● ● ● ● ● ● ● ●
  21. 21. The fMRI machine is measuring a BOLDsignal because the color is Blood Oxygen Level Dependent ● ● ● ● High blood oxygen Low blood oxygen ● ● ● ●
  22. 22. So, instead of an HRF looking 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 after neurons fire  
  23. 23. The fMRI shows an HRF that looks 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 after neurons fire  
  24. 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. 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. 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. 27. So, all of our HRF information is shown on theNeuron timeline belowfiringstarts ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
  28. 28. But, instead of aNeuronfiring series of colors,starts the fMRI machine gives us numbers 180 200         250       305        249         201       182       172         180 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
  29. 29. The fMRI records aNeuron number for thefiring magnetism of each voxelstarts at each time 180 200         250       305        249         201       182       172         180 ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
  30. 30. If we take a picture every 3seconds for 7 minutes, we get 140 numbers for each voxel
  31. 31. And we might have 100,000 or so voxels in the whole brain
  32. 32. 20 images per minute X 7 minutesX 100,000 voxels X 20 people =280 million data points We can’t track it by hand!
  33. 33. So we have to ask the computer toanalyze the data
  34. 34. But the computer needs to knowWHAT to look forand WHEN to look for it
  35. 35. So WHAT are we looking for?
  36. 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. 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. 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 firing180 200         250       305        249         201       182       172         180● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
  39. 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 firing180 200         250       305        249         201       182       172         180● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●
  40. 40. WHEN are welooking for thisHRF-like signal?
  41. 41. We are interested in the HRF-likesignals that happen right after the subject experiences something+ + + + +
  42. 42. Then we can estimate thelikelihood that a voxel, or group of voxels, is responding to the stimulus
  43. 43. This simple concept can bedifficult in practice because:1. The signal change is small2. The brain is noisy
  44. 44. This simple concept can bedifficult in practice because:1. The signal change is small2. The brain is noisyOvercoming these barriersrequires: Good study design + Good data analysis
  45. 45. Bye for now! Next time we will look at how to create a good study ● ● design. Intro to fMRI Part I: HRF & BOLDDr. Russell James, Texas Tech University

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