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Study Design and Efficiency

Study Design and Efficiency. Tom Jenkins Catherine Mulvenna. How the class will go…. Choosing the right study design for your experimental question (Catherine) How the design affects your efficiency (Tom) . What is involved in study design?. How you present the data

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Study Design and Efficiency

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  1. Study Design and Efficiency Tom Jenkins Catherine Mulvenna

  2. How the class will go… • Choosing the right study design for your experimental question (Catherine) • How the design affects your efficiency (Tom)

  3. What is involved in study design? • How you present the data • Block Vs event-related design • How you present the experimental question • Psychological validity • fMRI is as worthwhile as the design of the experiment using it.

  4. The most commonly used fMRI paradigm is blocked design • A BLOCK: a series of trials is presented during a discrete epoch of time. • BLOCKED DESIGN: different conditions are assigned to different blocks. The signal acquired in different blocks is contrasted.

  5. Blocked design • e.g localise a specific brain region showing the response to one type of stimulus (e.g. faces vs houses) • For good reviews see Binder & Rao 1994; D’Esposito et al., 1999 • Assumptions of the Block Design: "Neural structures supporting cognitive and behavioural processes combine in a simple additive manner." • Also you may not be able to test certain things with such a design…

  6. Event Related Design • Types of trials are interleaved and each trial is modelled separately as an ‘event’ e.g. AABABBAB

  7. 10 20 30 40 50 60 70 80 1 2 3 4 5 6 7 8 Blocked Alternating Random Sequencing of events Deterministic designs: the occurrence of events is pre-determined e.g. a blocked design or alternating design (all the probabilities are zero or one ) Stochastic designs: the occurrence of an event depends on a a specified probability e.g. random or permuted design Stochastic designs can be stationary or dynamic Permuted

  8. Advantages of efMRI • Can randomise or counterbalance trial order to reduce contextual bias and minimise differences related to cognitive ‘set’ or strategy use. • Some experimental designs cannot be blocked e.g. oddball designs. • Can use post-hoc classification of trials e.g. separately model trials with correct or incorrect responses, following post-scanning testing or depending on subjective perception. • Improves temporal resolution such that you can look at events on a shorter time scale.

  9. Disadvantage of efMRI • Typically less efficient than blocked designs - Tom.

  10. Psychological validity “the brain may be gray and nondescript on the outside, but on the inside it's a well-practiced liar.”

  11. Psychological Validity in fMRI Studies • Validity: Is the test measuring what it is supposed to measure? • Ask others what they think alternative explanations for your results are. Incorporate these into your study. • E.g. of poor validity…

  12. When deciding on the tasks… • A study’s design is only as good as the tasks that have been chosen. • There should be as few explanations for your resulting data as possible. • If a result is surprising, look back to your task. • http://www.wilderdom.com/personality/L3-2EssentialsGoodPsychologicalTest.html#Validity

  13. Over to Tom…

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