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John,There isn't a 1:1 relationship between number of trials and number of blocks. You are right that 30 blocks of 16-20 seconds would be excessive and you don't need that much data. I think that if 6-10 blocks of 20 seconds you'd end with enough data; however, with a block design you need to be concerned about habituation within and across blocks as well. More variability within a block means you need more blocks.Magnetic inhomogeneity really isn't an issue, what is an issue with longer runs is patient movement and how the movement can alter the relaxation time between when slices are collected. Generally, I aim for 4:30-6:00 minute long runs. Short runs are bad because of the high-pass filter not being able to be used and you need a high-pass filter in block designs. In a block design, I'd try for a run closer to 6 minutes (although some paradigms require shorter runs).Hope this helps.Best Regards, Donald McLaren
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D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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406-2464 or email.On Mon, Jan 5, 2015 at 11:53 AM, John adler <[log in to unmask]> wrote:another thing is that i think long scans would have negative effect on patient performance and i think magnetic inhomogeneity become worse in long runs, am i right ?it is a matter of fact that we cannot have 30 repeats of each conditions which take 16-20 seconds. ( i have 3 conditions and i want rest also )i read the second article for the number of trials you mentioned. i was thinking of block design instead of even-related design since they are more suitable for activation detection. so in this case what is the number of trials means in block design?Dear Anderson and Donaldthanks for your responsesOn Mon, Jan 5, 2015 at 8:16 PM, MCLAREN, Donald <[log in to unmask]> wrote:John,In addition to what Anderson mentioned, there are trade-offs between the number of subjects and number of runs. See this article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2423281/Instead of focusing on the number of runs, I tend to focus on the number of trials. In my experience, studies with at least 30 trials of each event type work well for fMRI. Decreasing the number of trials will increase the variability of the response from the actual subject mean if you had many trials. See this article: Huettel, S. A., & McCarthy, G. (2001). The effects of single-trial averaging upon the spatial extent of fMRI activation. NeuroReport, 12(11), 2411–2416.Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (773)
406-2464 or email.On Mon, Jan 5, 2015 at 7:42 AM, Anderson M. Winkler <[log in to unmask]> wrote:AndersonMore runs are better, but the uncertainty associated with fewer runs is modelled, so there's no need to repeat lots of times in general. It's hard to tell how many would be needed (if more than one) as the variability of depends on the reliability of the task, on the effects of learning/familiarity with the task, on the cooperation of the subjects, and other factors.Dear John,It's possible to combine just 2 runs with fixed effects. With the other estimation options, at least 3 are necessary given that at least two parameters are estimated.
All the best,On 5 January 2015 at 12:51, John adler <[log in to unmask]> wrote:thanks in advancehow can we estimate how many runs should we have in an experiment ?i noticed that fsl does not allow combining lower than 3 runs in FEAT. can't we combine 2 runs?Dear all FSLerswhy is it beneficial to do more runs for a subject? does combining runs in group FEAT means averaging?