Print

Print
See inline responses below.

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 Thu, Apr 21, 2011 at 4:36 AM, Ben Becker <[log in to unmask]> wrote:
Dear SPMers,

I’m analyzing an event-related working memory task with three levels of wm-load and two groups (placebo vs. verum), the conditions are:

1.      Baseline (fixation-cross)
2.      wm-load 1
3.      wm-load 2
4.      wm-load 3


I would only model your conditions, then the betas are implicitly with relation to your fixation cross. Otherwise your beta are relative to an unknown value -- not to mention that you probably begin your study with a fixation cross and have a BOLD response that is predicted to increase right away even though the cross was there before some of the data.
 

To compare groups on the second level I used a full-factorial design (factors: group (2-levels) x load (3-levels).
My questions:

1.      I ran the analysis with con-images (wm-load 1-Baseline / wm-load2-Baseline / wm-load3-Baseline) and beta-images (from load1 / load2 / load3). The analyses reveal different results. Which analysis reveals the most reliable results?


Subtractions.

 
2.      Both analyses reveal a main effect of group, however no group x load interaction. How can I be sure, that the effects are working-memory specific? Would it be valid the mask the contrast of the main group effect with the main effect of task?

In a mixed-design the main effect of group is invalid (see below for correct method). The main effect of task and interaction are valid.
 

3.      For a more sensitive analysis I’m planning to compute the contrast for:
1/3 wm-load 1 + 1/3 wm-load2 + 1/3 wm-load3  vs baseline for the whole sample. Than compute a 1-sample t-test with these contrast images. Significant clusters from this analysis are thought to represent the working memory networks. In a next step individual beta values will be extracted using these significant clusters as ROIs. Does this analysis lead to valid results?

This contrast should also be used to compute the main effect of group in a two-sample t-test or a one-way ANOVA.
You want to extract the contrasts for each part, not the betas. Also, you might want to revise your first-level models to exclude the fixation condition.
See www.martinos.org/~mclaren/ftp/Utilities_DGM/peak_nii for a way to extract from all clusters and select which images to use for the extraction. A previous email provides more detail on its usage.
 

Thanks in advance & kind regards,

Ben