Dear Payal,
if I understand your various posts to the mailing list correctly, a single 3D acquisition lasts for 2 seconds, with another 14 seconds "gap" before the next 3D acquisition. Slice timing needs to be told about this, and it therefore asks for two time values. For slice timing, TR is the time between acquisition of the 1st slice of scan N and the 1st slice of scan N+1. In your case, this would be 16 seconds. TA is the time between acquisition of the 1st slice of a scan and the last slice of a scan. If I got you correctly, it would be 2sec*(1-1/#slices).
In 1st level statistics, there is no distinction between TR and TA. TR would be 16 seconds, and you should use the 1st time bin (reference slice/microtime onset = 1) in order to model hrf correctly.
Hope this helps
Volkmar
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