fMRI Study Design
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fMRI Study Design

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A simple introduction to fMRI study design for social science and other researchers outside the field who might want to design a study using fMRI brain scanning technology

A simple introduction to fMRI study design for social science and other researchers outside the field who might want to design a study using fMRI brain scanning technology

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fMRI Study Design fMRI Study Design Presentation Transcript

  • fMRI Study DesignThe basic frameworkRussell James, J.D., Ph.D.Texas Tech University
  • Goals of fMRI Study Design 1. Create a 2. Detect brain desired signals cognitive associated state with that state (standard (fMRI-specific experimental experimental design issues) design issues)
  • We want to estimate the likelihoodthat a voxel, or group of voxels, is responding to the stimulus
  • But, fMRI data is not like thisActivation
  • fMRI data is like thisActivation
  • The signal change is small.The signal is noisy.Activation
  • The signalchange is small½% to 3%change inintensity witha 1.5 T scanner
  • The signal is noisy1. The brain is noisy2. The scanner is noisy
  • The brain is noisy The brain is constantly active, constantly firing, constantly receiving input, constantly sending instructions
  • The brain is noisyEven consciousthought is scattered.Did you think aboutsomething otherthan fMRI in thelast 3 minutes?
  • How do we designfor noisy brains?1. Contrasts 2. Repetition
  • Think in contrasts
  • A single image A contrast can contains much subtract out unrelated brain the noise activations Task A-Task A Task B Task B
  • Think of study design in terms of contrastsImage Image of Image task A-of task of task A Image of B task B
  • The comparison task can be “rest”Image Image of Image task A-of task of task A Image of B task B
  • We can use a “cognitive subtraction”comparison to isolate an activity - =
  • Cognitive subtraction: the comparison task isidentical, except for one variation of interest
  • Cognitive subtraction: View famous faces v. non-famous faces
  • How might you improve cognitivesubtraction in this picture selection? Match gender? Color pallet? Expressions? Clothing choice?
  • Levels of cognitive subtraction • Basic cognitive subtraction:Stronger Results Rational argument of validity • Cognitive conjunction: Two cognitive subtraction designs show same activation difference • Parametric design: Increasing levels of a factor correspond with increasing levels of activation
  • Cognitive conjunction Memorizing letters v. Memorizing v. Reading numbers numbers
  • Parametric design Increasing activationMemorize 9 numbers ● ●Memorize 6 numbers ● ●Memorize 3 numbers ● ●Memorize 1 number
  • Design for BIG contrastsfMRI is toonoisy to begood atsubtle Contrast extremecognitive differences firstdifferences
  • How do we designfor noisy brains?1. Contrasts 2. Repetition
  • Design forrepeatedactivations
  • In social sciences we may get 1,000 people to make one decision on a survey In fMRI we get 25 people to do a task 40 times for 1,000 activations
  • Two common design types Event RelatedBlock DesignDesign
  • Block DesignTime
  • Stacking the HRFs Within a certain range, repeating a stimulus will cause the HRF to stack linearly. This causes large total signal change in block designs.
  • Event Related Design
  • Block design advantages• Easiest way to detect differences among two comparison states• Largest HRF activation• More robust to unexpected HRF shapes
  • BOLD response inblock v. event related (slow)
  • Event related design advantagesBlock designs may not work• Modeling incorrect responses (can’t know in advance)• Habituation may prevent activationsCan precisely observethe actual HRFPermits self-pacedtrials
  • Barriers to repeating the activation
  • Barriers to repeating the activation • Repetitions can get boring or predictable, reducing activation • Emotional states may not be induced or changed quickly • Some decisions are difficult to repeat • Some biases can be added once, but not removed
  • Repetitions can get boring or predictable, reducing activation+ + ++ + ++ + +
  • Emotional states may not be induced or changed quicklyThink of the happiestmoment of your lifefor 12 secondsThink of the most painfulmoment for 12 secondsThink of the 2nd happiestmoment for 12 secondsThink of the 2nd mostpainful moment for 12 seconds
  • Some decisions are difficult to repeat“If you died todaywhat percentage ofyour estate would youwant to leave to yourchildren?”Are you sure?Any second thoughts?Want to think aboutit some more?
  • Some biases are not easily removed1. How much of a $100 extra payment will you give to the United Way?2. The United Way CEO made $1,037,140 last year.3. How much of a $100 extra payment will you give to the United Way?1st comparison works great,but can you repeat this?
  • The signal is noisy1. The brain is noisy2. The scanner is noisy
  • Designing for a noisy scanner Physical issues Timing issues
  • Physical issues of a noisy scannerMachine Metal Movement
  • Machine Optimizing machine, setting parameters, region ofinterest, speed v. resolution, etc.
  • Metal: Limit any ferrous content toavoid disturbances in magnetic field
  • MovementKeep the subject still during runs • Motivated subjects • Foam packing Statistical adjustments (but, may not help task- related movement)
  • Designing for a noisy scanner Physical issues Timing issues
  • Timing issues of a noisyscanner signal
  • Scanner DriftOver longer periods of time (2-10minutes), the magnetic field of thescanner can slowly rise and fall.
  • Scanner Drift Comparison across long (>2min) periods should be avoided. Condition A Condition A Condition B Condition BA–B shows magnetic differences but not from HRF
  • What is wrong with this?30 sec block of task A (version 1)30 sec block of task A (version 2)30 sec block of task A (version 3)30 sec block of task A (version 4)30 sec block of task B (version 1)30 sec block of task B (version 2)30 sec block of task B (version 3)30 sec block of task B (version 4)30 sec block of task C (version 1)30 sec block of task C (version 2)30 sec block of task C (version 3)30 sec block of task C (version 4)
  • What is wrong with this?30 sec block of task A (version 1)30 sec block of task A (version 2)30 sec block of task A (version 3) Comparing30 sec block of task A (version 4) A to C spans well30 sec block of task B (version 1) over 12030 sec block of task B (version 2) seconds so30 sec block of task B (version 3) we can’t30 sec block of task B (version 4) distinguish30 sec block of task C (version 1) from30 sec block of task C (version 2) scanner30 sec block of task C (version 3) drift30 sec block of task C (version 4)
  • Block Design Timing IssuesIdeal timing 15-20 seconds on then15-20 seconds off (or A then B) • Long enough for HRF to relax in between presentations • Short enough for many comparison blocks within short time
  • Slow Event-Related Timing Issues Waiting 12+ seconds in between each event to allow HRF to calm down Boring and inefficient
  • Rapid Event- Related Design Timing IssuesJitter spacing to record differentparts of the HRF and to avoidcorrelation with other functions likeheartbeat and breathing
  • Rapid Event- Related Design Timing IssuesGap spacing >4 seconds, else • HRF blurring: Not enough time for noticeable HRF changes • Non-linearity: HRFs don’t stack linearly foreverOptimum jittering estimationprograms (e.g., OptSeq - Doug Greves; Genetic Algorithm - Tor Wager)
  • Session Timing Typically, studies include groups of tasks of 4-10 min. with intervening 2 min. breaks. Also need high resolution (T1) scan ~5 min.; Locater scans (~30 sec.); T2 axial scan for radiologist (~2 min.)
  • In a 48 minute 10 min. subject in and outsession, you may Locator – 20 secondsget about 24-28 Short Breakminutes of actual T2 axial: 2 minstimulus Breakpresentation T1 high resolution: 5 min Break - wake up Block 1: 8 minutes Break/Instructions: 2 min Block 2: 8 minutes Break/Instructions: 2 min Block 3: 8 minutes
  • $650/hour scanner time ~$1300/hour stimulus time ~$20+ per minute ~$1+ every 3 secondsAdd 12 sec. to 25 subjects = $100+
  • With so many, many designissues to think about, what is the best way to design your first study?
  • Stand on some shoulders!
  • Find a good prior study, copy thetechnical elements, but change youritem of interest
  • fMRI Study DesignThe basic frameworkRussell James, J.D., Ph.D.Texas Tech University