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The changes are relative to the psychological regressors. That is to say, the interaction of region*psychological regressor is in addition to both the psychological induced BOLD response and the seed regressor. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Postdoctoral Research Fellow, GRECC, Bedford VA Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School 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, Jun 27, 2011 at 3:43 AM, Phil Yoss <[log in to unmask]> wrote: > Thanks, Donald. I still don't quite understand this though. Sure, the seed > region causes up and down fluctuations -- but relative to what? Relative to > baseline. So in seed region X, when task A is happening, there is an upward > fluctuation, and when task B is happening, there is a down-ward fluctuation. > Shouldn't I be seeing the same pattern in the regions the PPI pulls out, > precisely the regions that supposedly correlate with the seed region? > Having looked at my data more carefully, I think I am now essentially seeing > just two patterns in the regions produced by the PPI: > 1) task A > task B > task A = +ve (above baseline), task B less +ve (but still above baseline); > same as task A =+ve, task B = 0 or -ve; > [this is consistent with the PPI, since task A is more active than task B] > and > 2) task A<task B: > task A = +ve (above baseline), task B even more +ve; same as task A = 0, > task B = +ve; same as task A = -ve, task B = +ve. > (in all of these task B is basically more active than task A) > So I'm now wondering about a new issue: does the PPI correlation pull out > both *positive* and *negative* correlations with the seed region? But in > this case, how come I see both positive and negative correlations for a > positive threshold, e.g. z=3.1? Wouldn't I have to set z = -3.1 to see the > anticorrelated areas? > Many thanks, > Phil > > ________________________________ > From: "MCLAREN, Donald" <[log in to unmask]> > To: [log in to unmask] > Sent: Sunday, June 26, 2011 11:33 PM > Subject: Re: [FSL] PPI results question > > PPI is fundamentally different the task activity. > > PPI explains the covariation in signal that is in addition to the > task. One way to think about PPI is to think of the BOLD response as > smooth and then think of another region cause fluctuations up and down > from that smooth response and that these fluctuations are the result > of the activity in the seed region. > > Thus, the task activity is separate from the PPI activity. > > Best Regards, Donald McLaren > ================= > D.G. McLaren, Ph.D. > Postdoctoral Research Fellow, GRECC, Bedford VA > Research Fellow, Department of Neurology, Massachusetts General Hospital and > Harvard Medical School > 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 Sun, Jun 26, 2011 at 1:48 PM, Phil Yoss <[log in to unmask]> wrote: >> Hello all, >> is it possible to obtain highly significant PPI results (i.e. areas that >> significantly correlate with a psycho-physiological interaction regressor) >> which show a very different response profile than the seed region going >> into >> the PPI regressor? I.e. if my PPI regressor shows high activity on task A >> and low activity on task B, why is it that I am getting regions >> correlating >> with this PPI regressor, but which show e.g. negative activity on task A >> and >> B, or negative activity on task A but positive on task B, or positive on >> both task A and B? (I am plotting the % signal change for the areas pulled >> out by the PPI). >> I don't understand why the BOLD response profile varies so much in areas >> correlating with the PPI regressor, and why the response profile (% signal >> change by condition) doesn't look like the PPI regressor (i.e. high on >> task >> A, low on task B). The areas in question survive thresholding at p < 0.005 >> or lower, corrected. Am I doing something wrong? >> Thanks much, >> Phil > > >