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On Tue, Aug 7, 2012 at 9:50 AM, Ming-Tsung Tseng <[log in to unmask]> wrote: >> In my fMRI paradigm, I have 4 conditions (1~4, and conditions 3 and 4 are the main EVs of interest) and 1 time slot of baseline of doing nothing. >> >> Model 1: >> I model the 4 conditions separately. To get activations of the main EV of interest, the contrast is (0 0 1 0) for condirion 3 and (0 0 0 1) for 4. >> >> Model 2: >> I model 1 and 2 conditions together as 1 EV (because they are both EV of no interest) and conditions 3 and 4 separately. To get activations of the main EV of interest, the contrast is (0 1 0) for condirion 3 and (0 0 1) for 4. >> >> I found the activations of both models are very identical, but it seems that activation in model 2 is slightly larger (and with slightly higher statistics) than that in model 2. >> > > > Thanks and sorry I forgot to say it is a first-level analysis. So I think I should model conditions 1 and 2 separately, and now I model 4 conditions. > > My another question is, in another situation, I have 3 conditions (1~3, and condition 3 is the main EVs of interest) and 1 time slot of baseline of doing nothing. > > In addition to know the activation related to condition 3, I use the contrast (0 0 1) (let's code this Result A). > > However, I'm also interested in the response to early versus late phase of condition 3. In another model, I then model 4 EVs: condition 1, condition 2, first half of condition 3, last half of condition 3. Make sure you code the duration of the events. > > My question is: > > (1) To know activation related to early phase of condition 3, the contrast should be (0 0 1 0) > (2) To know activation related to late phase of condition 3, the contrast should be (0 0 0 1) > (3) To know activation related to the entire condition 3, the contrast could be (0 0 1 1) (I know 1 1 means the average activation of early and late phases of condition 3) 0 0 1/2 1/2 would be the average. 0 0 1 1 is twice the average. Both will give the same statistics and group maps though. > > Are (1) and (2) correct? > Does the result of (3) equal to Result A? No. Result A will only equal (3; 0 0 1/2 1/2) when the activity is constant over the whole event, which is unlikely since you are telling me that there is a early phase and late phase of the response. > > Thanks > > Mark