Hi,
Sorry to insist on that issue, but
there seem to be a controversy around the method to calculate the cut-off period
in fMRI studies. Lets take a paradigm in which A, B are active tasks (7 scans),
N being a neutral task (7 scans) and r being the rest period (3 scans). One
scans last 5.8 seconds. Subjects will be exposed to rArBrNrArBrN (notice that
ABN order, called the experimental period, is repeated twice). There are 3
methods that have been suggested to me.
1) Take the longest on-off cycle (rest +
task): 2 * (3+7) * 5.8 seconds = 116
2) Take the longest on-off cycle used in the substraction (task + neutral):
2 * ( 7+7) * 5.8 = 162.4
3) Take the repeated experimental period ( in this case it is r+ A+B+N): 2
* (3+7+7+7) * 5.8 seconds = 278.
In our case, we ARE NOT using the rest period. We are doing A-N and B-N. Is
the method 2 more appropriate?
Even if method 1 is still recommended,
what's the origin of those alternative methods? What's the rational behind
them?
Claude Breault Ph.D.
