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Fmri overview

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Fmri overview

  1. 1. Functional MagneticResonance Imaging (fMRI): a quick overview Matthew Baggott, Ph.D. Matthew@baggott.net
  2. 2. Section OverviewGoal is to understand fMRI well enough to make some sense of studiesWe will discuss the basics of fMRI – Indirect measure of neural activity – (Actually measures local amount of de-oxygenated blood) – Slow – Noisy – Often analyzed by subtracting different conditions (or by correlating data)In the next section, we will use what we learn to evaluate recent workon the mechanisms of visuals in ayahuascaThen we will discuss more general theories about psychedelic visuals
  3. 3. If you put someone in a veryhomogeneous magnetic field… (image source: Wikipedia)
  4. 4. The protons in their body line up Protons = hydrogen nucleus, abundant in water and fat (like spinning tops aligned with gravity)
  5. 5. …then you can send a RF pulse, and get the protons to briefly turn 90degrees away from the magnetic field
  6. 6. before they „relax‟ and realign like spinning tops that pop back up after a push(Their relaxation and return to equilibrium can be divided into thecomponents that are parallel and perpendicular to the magnetic field,which take different times (called T1 and T2) and can be used to makeslightly different images
  7. 7. Their movement gives off a signal that you can pick up (Head coil for measuring signal from head)(They emit energy at the same radio frequency they received until theyreturn to their equilibrium state)
  8. 8. If you want to know where the signalis coming from… Gradient coils let us create magnetic fields in any direction Z Coil Y Coil X Coil fnord transceivermake the field uneven and use pulses of different strengths and directions
  9. 9. Proton density and uneven magnetic fieldsalter the image intensity Proton density: density of fat and water (important for structural scans) Uneven magnetic fields: If each proton experiences a slightly different magnetic field, the energy they give off when they „relax‟ partly Image from Harvard cancels out whole brain Atlas
  10. 10. Magnetic field irregularities fromhemoglobin Deoxyhemoglobin is a significantly more paramagnetic (with four unpaired electrons) than oxygenated hemoglobin The amount of deoxy- hemoglobin in each part of the image alters the image. Hemoglobin, which carries oxygen in the blood (image source: Wikipedia)
  11. 11. The magnetic difference between deoxy- andoxygenated hemoglobin is the basis of theBlood Oxygen Level Dependent (“BOLD”)signal, used by almost all fMRI
  12. 12. The magnetic difference between deoxy- andoxygenated hemoglobin is the basis of theBlood Oxygen Level Dependent (“BOLD”)signal, used by almost all fMRI This is what we indirectly measure
  13. 13. The Blood Oxygen Level Dependent(“BOLD”) signal is slow ON = Checkboard shownVisual cortex activity changes in response to a flashing checkboard in an earlyevent-related fMRI study (Blamire et al. 1992)
  14. 14. The Blood Oxygen Level Dependent(“BOLD”) signal is slow ON = Checkboard shownVisual cortex activity changes in response to a flashing checkboard in an earlyevent-related fMRI study (Blamire et al. 1992)
  15. 15. The Blood Oxygen Level Dependent(“BOLD”) signal is slow ON = Checkboard shown Response peaks around 6 seconds after stimulusVisual cortex activity changes in response to a flashing checkboard in an earlyevent-related fMRI study (Blamire et al. 1992)
  16. 16. The Blood Oxygen Level Dependent(“BOLD”) signal is slow and ‘noisy’ ON = Checkboard shownSeemingly random fluctuations are ~40% as big as the response to the stimulusVisual cortex activity changes in response to a flashing checkboard in an earlyevent-related fMRI study (Blamire et al. 1992)
  17. 17. Most fMRI images show statistical mapsrather than raw changes in the BOLD signalThese statistical maps take into account the fact that different parts of the brainhave more variable signals. (Beauregard & Paquette 2006)
  18. 18. Blurry BOLD signal is projected onto highresolution structural data or an averagebrainResolution of functional data Voxels with statistically(volume pixel or voxel initially significant changesaround 2 cm wide)
  19. 19. Blurry BOLD signal is projected onto highresolution structural data or an averagebrainResolution of functional data Voxels with statistically(volume pixel or voxel initially significant changesaround 2 cm wide)
  20. 20. Blurry BOLD signal is projected onto highresolution structural data or an averagebrainResolution of functional data Voxels with statistically(volume pixel or voxel initially significant changesaround 2 cm wide)
  21. 21. Blurry BOLD signal is projected onto highresolution structural data or an averagebrainResolution of functional data Voxels with statistically(volume pixel or voxel initially significant changesaround 2 cm wide)
  22. 22. Blurry BOLD signal is projected onto highresolution structural data or an averagebrainResolution of functional data Voxels with statistically(volume pixel or voxel initially significant changesaround 2 cm wide)
  23. 23. fMRI analyses are usually based onsubtraction of conditionsMost fMRI studies use a task-activation approach: – participants do a task – scientists look for which areas become more activeBut “more active” compared to what? (the brain is always active)Best comparison is usually another similar taskEarly studies compared Tasks vs. “Quiet rest” – glossing over the fact that “Quiet rest” actually involves very active minds
  24. 24. What wascompared towhat? (Beauregard & Paquette 2006)
  25. 25. What wascompared towhat?Mystical: Memory of Intense Closeness to GodBaseline: Memory of Intense Closeness to a Person(This could go wrong in a lot of ways -- though you have tostart somewhere) (Beauregard & Paquette 2006)
  26. 26. So much data & so many comparisons, you need tomake sure you aren‟t finding activity due tochance Neural correlates of interspecies perspective taking in the post- mortem Atlantic Salmon: An argument for multiple comparisons correction (Bennett, Baird, Miller, and Wolford 2009 poster) See Craig M. Bennett‟s blog post here: http://prefrontal.org/blog/2009/09/the-story-behind-the- atlantic-salmon/
  27. 27. Visual stimulation is often used to identify visualmaps in an individual‟s brain (Dougherty et al. 2003)
  28. 28. Specialized visual areas(You can make identical images using WebCaret, online software provided by theVan Essen lab at Washington University)
  29. 29. Really specialized visual areas Bodies Faces Houses Other objects(After Op de Beek, Haushofer, & Kanwisher 2008)
  30. 30. Specialized areas can be used to study neuralcorrelates of conscious perception Face area House areaLeft: When this image is viewed with red-green glasses, awareness switchesrandomly between the face and house.Right: BOLD signal in face and house sensitive areas change along withconsciousness (Tong, Nakayama, Vaughn, Kanwisher 1998)
  31. 31. The brain is 2% of the body but uses 20% of the energy(Shulman et al. 2004; Raichle and Mintun 2006; Photo by Ben Chenoweth)
  32. 32. The brain is 2% of the body but uses 20% of the energy Task-related fluctuations are a small part (<5%) of the brain‟s overall activity(Shulman et al. 2004; Raichle and Mintun 2006; Photo by Ben Chenoweth)
  33. 33. The brain is 2% of the body but uses 20% of the energy Task-related fluctuations are a small part (<5%) of the brain‟s overall activity Differences between normal and pathological populations in task- related changes are even smaller (often <1%)(Shulman et al. 2004; Raichle and Mintun 2006; Photo by Ben Chenoweth)
  34. 34. What is the rest of the activity?(Shulman et al. 2004; Raichle and Mintun 2006; Photo by Ben Chenoweth)
  35. 35. Most of the energy used by the brainreally is used to support ongoingneuronal signaling (Atwell & Laughlin 2001; Shulman et al. 2004; Raichle and Mintun 2006)
  36. 36. Large decreases in brain activity are produced by anesthesia Awake Anesthesia (Isoflurane) mg/100gm/min Awake AnesthetizedCerebral metabolism measured by 18FDG-PET (Hot colors indicate higher glucose use) (Alkire et al. 1997)
  37. 37. Can we find a way to analyze seeminglyrandom signal fluctuations? (Blamire et al. 1992)
  38. 38. Yes, instead of subtracting activitybetween tasks, you can correlate fMRIsignal between voxelsA “Default network” is active when research participants aren‟t told what to do.Blue shows regions most active in “passive tasks” in a meta-analysis of PET data (Buckner, Andrews-Hanna, & Schacter 2008)
  39. 39. This default network is Default Networksimilar to networksactive in internallyfocused tasks Autobiographical Memory Thinking about Others‟ Beliefs Envisioning the Future Moral Decision Making (Buckner, Andrews-Hanna, & Schacter 2008)
  40. 40. Default network is anticorrelated with anexternally focused network Regions that negatively correlate with the default network are shown in cool colors; those that positively correlate are shown in warm colors (Buckner, Andrews-Hanna, & Schacter 2008)
  41. 41. Default network is anticorrelated with anexternally focused network (When one network gets more active, the other gets less active) % Signal Change (Buckner, Andrews-Hanna, & Schacter 2008; time course from Fox and Greicius 2010)
  42. 42. Looking for correlated activity between brainareas is a powerful way to identify coordinatedbrain networks. Default L. FEF Parietal Attention Ventral AttentionFrontal-Parietal Task Control (Power et al. 2011)
  43. 43. Correlated activity during movie viewingSimilar colors indicate brain regions that respond similarlyto natural movies (Nishimoto, Huth, Vu, and Gallant 2011)
  44. 44. Take homeThe BOLD signal is an indirect, slow measure of neuralactivityMiraculously, it works. Results are consistent with directelectrocortical measurements, studies of brain injury, etc.Always ask what conditions are being compared andhow/why brain activity might differ between them –thestudy may not be measuring what it is trying to measure
  45. 45. Some tools and resourcesThe Whole Brain Atlas at Harvard is just what it sounds like.http://www.med.harvard.edu/AANLIB/BodyParts3D is an online tool for browsing (Creative Commonslicensed) gross anatomy diagrams. http://lifesciencedb.jp/bp3d/NeuroSynth is an online platform for large-scale, automated synthesisof functional magnetic resonance imaging (fMRI) data extracted frompublished articlesbrainSCANr is an online engine to search and visualize co-occurrenceof terms in the scientific literature. http://www.brainscanr.com/WebCaret is an online tool for visualizing a database of surface andvolume fMRI data. http://sumsdb.wustl.edu/sums/

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