I need a little more help with averaging group time series:

Here is my setup:

10 subjects
task 1 -> 16 timepts (i am considering already the Single Trial Averages)
task 2 -> 17 timepts
task 3 -> 19 timepts
task 4 -> 21 timepts
task 5 -> 25 timepts

Right now i have a group map that was made by averaging all the individual functional maps together, but I still have my individual Single Trial Average time series for each task, for each subject.

I would like to use the voxels in the group map to select out the Single Trial Average time series in each task, for each subject. Then I would like to get the Average Single Trial Average across each subject, for each task. That is, in the end i want:

task 1 -> Average Single Trial Average timeseries over 10 subjects
etc

Please help!
cathy