You need to set up the psdwin to span the duration of a hemodynamic response to your event as if it were presented in isolation. For a 3sec event, you can probably use something like 24 sec (so min=0, max=24). Since your TR=2, but your event duration is 3sec, I'd set dpsd=1 (so that both the TR and the stimulus duration will be divided equally by the dpsd). To control the first-order counter balance, use --focb 100. This will cause optseq2 to compute the FOCB matrix fover 100 sequences, and keep the one that is closest to the optimum.

doug

Linh Dang wrote:

Hi,

I have 2 questions.

1) How do I calculate psd min, max, and dpsd. Is there a formula for calculating these values? I have 4 kinds of stimuli, stim duration = 3s, stim repetition = 25, TR =2 so:

optseq2 --ntp 180 --tr 2 --psdwin [?] --ev A 3 25 --ev B 3 25 --ev C 3 25 --ev D 3 25 --o 4types --tsearch 10 --nkeep 4

What should my psd min, max, and dpsd be?

2) My task is history-dependent. I would like to control the probabilities that stimulus B follows A, C follows A, A follows D... Is there a way for me to do that with optseq?

Thank you so much for your help.

Cheers,
Linh
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