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Sara, PM are mean-centered. This means that if you have all negative values, then some values will become positive. The PMs are also orthogonalized within each trial type before convolution (see spm_get_ons.m). 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, Jun 25, 2015 at 2:47 AM, Sara Garofalo <[log in to unmask]> wrote: > Dear all, > > I have some doubts about how SPM handles negative parametric modulators. > > If the parametric modulators (more that one, so they will be > orthogonalized) that I associate to an EV contain negative values, are > these values used in the model as they are or are they converted to > positive values? > In the second case, does that happen because of orthogonalization? > In both cases, would that affect just the intercept or are there other > consequences for the regression? > > I was trying to find my answer looking at the design matrix, but the range > of values of the parameters it's not very clear, so I was trying to plot > them but I don't understand in which variable I can find them. Can anyone > help me? > > I also have a minor doubt: is it correct to say that the parameters are > orthogonalized before the hemodynamic response convolution? > > Many thanks, > Sara > >