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*Global scaling*: Is used to remove global signal drifts/trends across an indidividual time-series (session). It is largely a remnant from the PET days where differences in injected activity could be a large part of the scan-scan variance. For fMRI a) most of the "global" changes are removed by the high-pass filter and b) the "estimator" of "global" intensity that is commonly used can be biased by true activations. For that reason it is typically not recommended. -Jesper Anderson 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 Thu, Aug 28, 2014 at 10:04 AM, Yang Yang <[log in to unmask]> wrote: > Dear all, > I have several questions concerning the global normalisation. I found that > the activation of my task (overt picture naming) when the option of 'None' > was choosen in global normalisation at the first level is remarkably > different from that when the 'scaling' was choosen. But I don't know why > the big difference happens. Is it appropriate to use the way of 'scaling' > to do global normalisation at the firs level for fMRI data? > On the other hand, how about the global normalisation when specify factor > design at the 2nd levle analysis? Could I choose scaling at both levels? > I wish obtain some suggestion from you. Thanks a lot! > Ivan > > >