Thank you for your advice, Daniel. I think you may suggest the latter way I mentioned in your first piece of advice. I have two conditions with equal trials:for target(t),32*2(t15&t25); for standard(s),8*8(s11-14,21-24). Maybe my expression in the first email was not clear enough. I don't care the activation of the individual stimuli within certain conditions. I just need to balance them to make sure "I don't care" is reasonable. Now I am going to use the seqence generated by the first piece of code. But when evaluate it with seqefficiency, I would merge stimuli within conditions together. Then I would run more runs than needed, but not as much as consider the stimuli separately. Then I would be sure the two conditions were well randomized. The stimuli within condition were also reasonablely randomized.

I am not sure whether I correctly understand your second piece of advice. But I can't change the ratio because I have to combine the fMRI data with a finished EEG experiment with the same paradigm.

On 6/29/07, Daniel G. Wakeman dwakeman@nmr.mgh.harvard.edu wrote:

Hi Peng,

I'm sure Doug and others will have very valuable advice, but why not just keep the number of trials in the two conditions equal and separate out the individual stimuli in the actual events (dividing to keep the number of events per condition equal). That way you can keep your power and if you want to do the other analysis later you can take a look at that. You could of course, also fiddle with the ratios, so you use 1/2 (bottom most) the time instead of 1/4. I'm not sure if all of this makes sense, but according to this I believe the command would look like the one below (Note! I kept the ratio of trials to null the same as in your examples).

optseq2 --ntp 160 --tr 2 --psdwin 0 14 --ev s11 2 32 --ev s12 2 32 --ev 21 2 8 --ev s22 2 8 --ev s23 2 8 --ev s24 2 8 --ev s25 2 8 --ev s26 2 8 --ev s27 2 8 --ev s28 2 8

optseq2 --ntp 160 --tr 2 --psdwin 0 14 --ev s11 2 16 --ev s12 2 16 --ev 21 2 12 --ev s22 2 12 --ev s23 2 12 --ev s24 2 12 --ev s25 2 12 --ev s26 2 12 --ev s27 2 12 --ev s28 2 12

Dan

...

hi list,

I have two conditions, one contain 2 stimuli and the other 8. The difference between stimuli within certain condition is vey small, and could be omitted in further analysis if they're treated equal. Now I have two choice: optseq2 --ntp 160 --tr 2 --psdwin 0 14 / --ev s11 2 8 --ev s12 2 8 --ev s13 2 8 --ev s14 2 8 / --ev t15 2 32 / --ev s21 2 8 --ev s22 2 8 --ev s23 2 8 --ev s24 2 8 / --ev t25 2 32 / --nkeep 6 --o pl1 --nsearch 10000 or optseq2 --ntp 160 --tr 2 --psdwin 0 14 / --ev s 2 64 --ev t 2 64 / --nkeep 6 --o pl2 --nsearch 10000 Obviously, the first one would need more test runs, which in fact was nearly 2 hours for the subjects . And the second one need only 1/4 time, but is risky. I am afraid it would induce difference between stimuli within condition. Do you have any suggestions on it? Thanks a lot.

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