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## SPM@JISCMAIL.AC.UK

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Re: 3 long runs or 15 short runs?

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Thu, 9 Oct 2008 21:36:16 +0100

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 ```Hi Jason I can't think of any advantages do doing 15 2-minute runs. In particular it's preferable to have all of your conditions represented equally in each run. If you have different conditions in different runs, it's impossible to tell if some effect is run 1 > run 2 (due to some weird property of the scanner on that particular run), or condition 1 > condition 2 (what you're interested in). So, I think you probably should go with 3 10-minute runs. As far as I know the noise characteristics of fMRI are unaffected by the length of a scanning session. Jonathan On Thu, Oct 9, 2008 at 7:59 PM, Jason Steffener <[log in to unmask]> wrote: > Hello. (I have sent this to both SPM and FSL so I apologize for multiple receipts) > I am in the process of designing an experiment. > > We have two options: > > 15 2 minute runs > > versus > > 3 10 minute runs. > > > Are there pros and cons of either one? > > > The background of the experiment is that we have 5 conditions: 4 task blocks and 1 rest block and each block is 24 seconds long. We are using a 5x5 latin square design to control for ordering effects. Each row of the latin square therefore consists of 120 seconds (2 min = 5 - 24 second blocks). We want to repeat the experiment three times in each subject. > > We therefore have the two options: perform each row of the latin square as its own separate scanning run to have 5 times 3 = 15 total. Or perform the entire latin square in one scanning running and repeat it three times. > > > Does 1/f noise come into play in either or both of these situations? > > Thank you very much, > Jason > ```