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Dear Noelia, > We have an experiment with two groups performing the same task (Group 1 and > 2). Preiovus literature have evidenced that group 2 tipically has slower > reaction times than group 1 in this kind of tasks (although in our study the > RT followed the expected direction, they were not significant). Regarding > our fmri data, we have obtained significant differences in the two sample > t-test between groups (Group 1 > 2) in a specific ROI(striatum). Moreover, > in the one sample t-test, each group showed significant activations in the > same ROI. Therefore we wanted to find out why there were differences between > them (due to temporal or magnitude factors) and obtained the PSTH response. > The problem is that we are not sure about how to interpret these results > (see enclosed image). > 1.-Can we conclude from the graphic that there is a delayed response in the > Group 2 in comparison to Group 1? In order to conclude something definitive about the timing differences between groups, you'll have to find a way to statistically test the timing. This is not necessarily straightforward to do, which is probably one reason you don't see it done a lot in the literature. One approach, from an FIR model, is to identify the peak time bin for each subject, and then do a 2-sample t-test to see if these peak times differ across group. The advantage of this is it's fairly straightforward. At least one example of this is found in Davis et al. (in press). Alternatively, you could include the temporal derivative in your model, and use that to estimate the timing of the response (e.g., Henson et al., 2002; Steffener et al., 2010). > 2.- Can we relate the PSTH response in the striatum to this slower RT in > group 2 in comparison to group 1? This is a bit of a tricky situation. If your groups differ significantly in both RT and timing of response, there wouldn't be any way to attribute the difference to RT (because there are other things that differ between your groups). However, the fact that they don't differ may help you in this regard. Once you have a measure of fMRI response latency, I would do a correlation of RT and latency across all subjects (not split by group). A significant correlation would suggest some relationship between the latency of response in the striatum and RT. You may want to plot the two groups separately, or conduct additional correlations within each group, to further investigate the issue. References: Davis MH, Ford MA, Kherif F, Johnsrude IS (In press) Does semantic context benefit speech understanding through "top-down" processes? Evidence from time-resolved sparse fMRI. J Cogn Neurosci. Henson RNA, Price CJ, Rugg MD, Turner R, Friston KJ (2002) Detecting latency differences in event-related BOLD responses: Application to words versus nonwords and initial versus repeated face presentations. NeuroImage 15:83-97. Steffener J, Tabert M, Reubemn A, Stern Y (2010) Investigating hemodynamic response variability at the group level using basis functions. NeuroImage 49:2113-2122. Hope this helps! Best regards, Jonathan -- Dr. Jonathan Peelle Department of Neurology University of Pennsylvania 3 West Gates 3400 Spruce Street Philadelphia, PA 19104 USA http://jonathanpeelle.net/