Dear FSL experts,
I'm new to TBSS analysis and I would be very grateful for your input. We are interested in seeing if there is a correlation between white matter microstructure and performance in a motor task in young and older adults in two different conditions.
We have 4 independent groups: Young - Condition A, Young - Condition B, Older - Condition A, Older - Condition B.
First we wanted to see if there was an age effect so I put the all the subjects of the same age together (condition A and B) to get only two groups, like this:
EV1 EV2 EV3 EV4
1 0 Perf 0
1 0 Perf 0
...
0 1 0 Perf
0 1 0 Perf
...
And the contrasts looked like:
C1: Old 0 0 1 0
C2: Young 0 0 0 1
C3: Old > Young 0 0 1 -1
C4: Old < Young 0 0 -1 1
This analysis gave no significant results.
We then looked into the differences between the 4 different conditions separately, using this design matrix:
EV1 EV2 EV3 EV4 EV5 EV6 EV7 EV8
1 0 0 0 Perf 0 0 0
1 0 0 0 Perf 0 0 0
0 1 0 0 0 Perf 0 0
0 1 0 0 0 Perf 0 0
0 0 1 0 0 0 Perf 0
0 0 1 0 0 0 Perf 0
0 0 0 1 0 0 0 Perf
0 0 0 1 0 0 0 Perf
and redid the same contrasts (amongst others):
C1: Old 0 0 0 0 1 1 0 0
C2: Young 0 0 0 0 0 0 1 1
C3: Old > Young 0 0 0 0 1 1 -1 -1
C4: Old < Young 0 0 0 0 -1 -1 1 1
We were expecting to get the same results in both analysis, but this time, we found highly significant results in the 4th contrast. Can we consider this second result valid? I would really appreciate if someone could guide us with a possible explanation.
Thank you very much for your help,
Catherine Vien
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Candidate au Ph.D. R/I option neuropsychologie
Département de psychologie
Université de Montréal
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