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Subject:

Contrasts

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Date:

Thu, 25 Feb 1999 18:56:48 +0100

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 ```Dear Timm, >But there is one question which we didn`t ask you yesterday: accepting >the given-letters of yesterday we got a resting condition (A) and want >to compare this to an activation (B) under the reconsideration of a >covariate (performance), in this case we do not want to compare to a >second study. >Choosing the contrast we are asked to give three numbers, considering as >you told us the first two contrasts would be -1 1 , but if the object of >interest is the influence of the covariate, which number is suppost to >be the third? >We are looking forward to another satisfying solution. >thanks timm strotmann (..and after I sought clarification...) >Only during task B we could assess a subject`s performance. Now, we >would like to >use the scores as covariates. If I just compare condition B to the >covariates I am >still asked to give a contrast, and I do not know which kontrast to give >1 or -1. >i hope it makes our problem understandable >sincerely timm This sounds to be a not entirely straightforward paradigm: a hybrid of block design and parametric design. There is an immediate problem with having two different rows in your design matrix, one of which models condition B as an 'epoch', the other of which models a series of scores which only occur during condition B, with a score of zero given to the model for every scan in condition A. Clearly these two rows would be quite similar, in that both have a row of zeroes during condition A and a row of positive numbers in condition B. To some extent, therefore, the two rows will be confounded. Is it possible to adjust your scores so that the mean score is zero? In this case the parametric row will have a series of scores during condition B which will sum to zero, and a series of zeroes during condition A, and it will therefore be completely orthogonal to either of the 'epoch' rows. If you have done this, and you have three covariates of interest in your design matrix: 1. positive baseline shift during condition A (the rest condition); 2. positive baseline shift during condition B (the task condition) and 3. parametrically varying row, scores summing to zero, then the questions which you could ask are as follows: Contrast -1 1 0 This identifies voxels in which there is MORE activation during condition B (task) than condition A (rest). Contrast 1 -1 0 This identifies voxels which are LESS active during condition B (task) than condition A (rest). Contrast 0 0 1 This identifies voxels in which the signal has a significant positive correlation with the subject's performance (regardless of whether there is an overall change in baseline in condition B). Contrast 0 0 -1 This identifies voxels in which the signal has a significant negative correlation with the subject's performance. Thus when you said that you have '... a resting condition A and want to compare this to an activation (B) under the reconsideration of a covariate (performance) ...' I think that part of the reason why I didn't understand exactly what question you are asking of your data is that your question breaks down into two distinct questions (with a positive and negative contrast for each). Personally I would consult one of the spm experts (I am a relative novice) before embarking on such an analysis, as it is relatively complex. I will copy this back to the spm discussion list, and perhaps you will receive a more expert opinion, Best wishes, Richard. from: Dr Richard Perry BM BCh MA PhD MRCP(UK), Clinical Research Fellow, Wellcome Department of Cognitive Neurology, Darwin Building, University College London, Gower Street, London WC1E 6BT. Tel: 0171 504 2187; e mail: [log in to unmask] Pager: 04325 253 566. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% ```