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Dear Anderson and Donald thanks for your responses 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? 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 ) 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 ? On 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: > >> 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. >> >> More 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. >> >> All the best, >> >> Anderson >> >> >> >> >> On 5 January 2015 at 12:51, John adler <[log in to unmask]> wrote: >> >>> Dear all FSLers >>> why is it beneficial to do more runs for a subject? does combining runs >>> in group FEAT means averaging? >>> >>> i noticed that fsl does not allow combining lower than 3 runs in FEAT. >>> can't we combine 2 runs? >>> >>> how can we estimate how many runs should we have in an experiment ? >>> >>> thanks in advance >>> >> >> >