Back to PLS HelpPosted on 03/24/17 20:31:16
Number of posts: 1
Hello,
I am new to PLS and have a couple of questions.
1. First, how do I interpret the LVS? The way I set up my session.mat file, I have 4 conditions (Condition 1 Hit, Condition 1 Correct Rejection, Condition 2 Hit, Condition 2 Correct Rejection) and I have 4 runs. I also have two groups (Young and Old). From the manual, it appears that LV1 looks at group differences. Is this correct? If so, how would I look at differences between Condition 1 and 2 and between HIts and Correct Rejections?
2. In each run, I only have either Condition 1 or Condition 2 stimuli. However, if you click the "merge across runs" for conditions, there is no way to leave any of the conditions blank. Is there a way to get around this? I would like my condition 1 hits to be merged across run 1 and 2. However, runs 3 and 4 only have condition 2 stimuli.
3. Finally, is there a way to export the activation map produced so that it can be viewed in a program like MRIcron in order to determine which regions of the brain are active?
Any resources or suggestions would be much appreciated!
Molly
Replies:
Posted on 03/25/17 08:30:45
Number of posts: 394
Hi Molly - thanks for your quesitons. I would really recommend reading carefully through the 2004 McIntosh & Lobaugh paper to better understand some of the details on how the method works (https://www.ncbi.nlm.nih.gov/pubmed/15501095)
for other answers see belwo
1. First, how do I interpret the LVS? The way I set up my session.mat file, I have 4 conditions (Condition 1 Hit, Condition 1 Correct Rejection, Condition 2 Hit, Condition 2 Correct Rejection) and I have 4 runs. I also have two groups (Young and Old). From the manual, it appears that LV1 looks at group differences. Is this correct? If so, how would I look at differences between Condition 1 and 2 and between HIts and Correct Rejections?
If you use the default settings for mean centred PLS you will be condition main effects and the interaction of group by condtion. The mean centering removes group main effects, which in the case of fMRI are hard to interpret.
2. In each run, I only have either Condition 1 or Condition 2 stimuli. However, if you click the "merge across runs" for conditions, there is no way to leave any of the conditions blank. Is there a way to get around this? I would like my condition 1 hits to be merged across run 1 and 2. However, runs 3 and 4 only have condition 2 stimuli.
There is no easy way to do this, but I would advise against such selective averaging as it introduces an order effect bias in your analysis. For the HITS condition you average over presentations, which removes the order differences, but for CORRECT REJECTIONS you do not and thus differences between HITS and CORRECTION REJECTIONS can be driven by order effects.
3. Finally, is there a way to export the activation map produced so that it can be viewed in a program like MRIcron in order to determine which regions of the brain are active?
In the results window there is an option to save the bootstrap ratio image (BSR) to an IMG file, which can be read into other programs. There is also a cluster report in the results window that gives you the local maxima for the BSR image in coordinates.
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