4 setting up regressors contrasts - university of michigan · googledoc –depending on the...
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Contrasts:It seems easy until you’re all alone setting up
your own model on your own data
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Setting up regressors in your level 1 model
• Model every task-related thing
• Typically the baseline (fixation) is not modeled– Implicit baseline in your model
• Be careful how much you split up trials– Is it necessary? Do you think the BOLD activation
will change?
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What information is needed for each regressor?
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What information is needed for each regressor?
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Simple model setup
• Task shows pos/neg/neu images to subjects, followed by fixation for 2s and then a neutral face
• What should go into our model?
4s 2s 0.5s 10s
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What your timing file looks likeHow would you split this up for SPM?
Onset time (s) Duration (s) Description
11 4 Negative
17 .5 Face
27.5 4 Neutral
33.5 .5 Face
44 4 Positive
50 .5 Face
60.5 4 Neutral
66.5 .5 Face
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Contrasts
• These combine parameter estimates from your regressors to test hypotheses– Eg. How does activation from negative images
differ from positive images?• We actually need to be more specific since we use 1-
sided hypotheses typically
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SPM example
• You’ll see this in lab today
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Example 1
o Where is the positive IAPS brain activation larger than negative IAPS brain activation?o Positive IAPS > Negative IAPS
oDo math so you end up with “stuff > 0”
o Positive IAPS – Negative IAPS >0o The coefficients of the above go into the contrast
Pos IAPS Neg IAPS Neu IAPS Face after Pos
Face after Neg
Face after Neu
PosIAPS > NegIAPS 1 -1 0 0 0 0
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Example 2
• Where is the positive IAPS brain activation larger than neutral IAPS brain activation?
Pos IAPS Neg IAPS Neu IAPS Face after Pos
Face after Neg
Face after Neu
PosIAPS > NeuIAPS 1 0 -1 0 0 0
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General rule of contrast construction
• Positive parts (if present) sum to 1• Negative parts (if present) sum to -1• Note: There are exceptions to this rule if
you’re working with group level ANOVA models– Just always make sure they are logical– We’ll cover these more later
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Example 1
o Where is both positive and negative IAPS brain activation larger than neutral IAPS brain activation?o Positive IAPS + Negative IAPS > Neutral IAPSo Positive IAPS + Negative IAPS –Netural IAPS>0
Pos IAPS Neg IAPS Neu IAPS Face after Pos
Face after Neg
Face after Neu
1 1 -1 0 0 0WRONG!
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Example 3
o Where is both positive and negative IAPS brain activation larger than neutral IAPS brain activation?o (Positive IAPS + Negative IAPS)/2 > Neutral IAPSo Positive IAPS/2 + Negative IAPS/2 –Netural IAPS>0
Pos IAPS Neg IAPS Neu IAPS Face after Pos
Face after Neg
Face after Neu
0.5 0.5 -1 0 0 0
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Example 4
• Average face activation greater than baseline
Pos IAPS Neg IAPS Neu IAPS Face after Pos
Face after Neg
Face after Neu
0 0 0 1/3 1/3 1/3
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Questions?
• Let me know if you want more practice doing this?
• Feel free to write out your examples in the googledoc– Depending on the complexity, we may or may not
get to it, but I’ll try to answer all