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See inline responses below. 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 Tue, Jan 15, 2013 at 12:05 PM, Christian Keller <[log in to unmask]> wrote: > Dear SPM experts, > > I used traditional PPI to analyze the task dependent interaction between different brain areas. I’ve got multiple seed regions and 2 conditions, both contrasted against a baseline. >>> You should really be using gPPI since you have 2 conditions plus a baseline condition. > One region is activated in both conditions, but the other region only activated in one of the two conditions of interest. I can thus identify seed regions using the contrast Condition1>Baseline in which both regions are activated in each single subject. But I am interested in the change of functional connectivity between two regions as a function of the 2 psychological factors: The change between Condition1>Baseline and Condition 2>Baseline. > > I thus ran 2 separate PPIs with the same timecourse, but different psychological variable (see above). How dependent are PPIs on the activity of a given brain region? >>> By definition, PPI are entirely dependent on the neural activity. However, you do not need a significant BOLD response for information to pass through a brain region. Thus, you can seed a "non-active" region to see if the small fluctuations in signal have different effects for different conditions. Also, if you use gPPI, you will only need 1 model. Shouldn't PPIs reflected in the change of the regression slope be independent of the amount of activity? >>> Yes and No. PPIs reflect the fluctuations of the signal, not the amplitude of the evoked response. The fluctuations contain the evoked response though. Keep in mind, the time period of the fluctuations that you are looking at is the duration of the event. If you specify the duration as 0, then you only look at the first time bin, the default is TR/16. Can I test PPIs for psychological variables that do not show a change in activity in a region of interest if I safely identified individual seed regions using another contrast? What would even happen if I see deactivation of region 2 in Condition 2>Baseline? >>>> Yes and I'm not sure, respectively. I define my regions as spheres based on group maps or based on the literature. Others will argue that you need activity for their to be a PPI effect, but simulations show that theoretically isn't a requirement. In practice, your less likely to find an effect in a region of no activity. Hope this helps. > > I would greatly appreciate your thoughts within this regard, > > Best wishes > > Christian