Hi,

I've always believed that it's necessary to replicate the design of an experiment in every run of a scanning session, given the problem of scanner drift across runs. For example, if an experiment has a 2x2 design, then all 4 conditions should be included in every run, to help minimize problems with drift.

Some colleagues have suggested to me that replicating all conditions in each run is not necessary for 2 reasons. First, the polart function removes drift within a run. Second, converting to percent signal change removes differences between runs. Once these two steps have been taken, the runs have been equated in terms of average BOLD intensity.

I realize that there are behavioral reasons for why one might want to include every condition in every run (e.g., adaptation, fatigue, learning to learn, etc.). In some experiments, however, it's desirable to have some conditions come after others to minimize demand, mask manipulations, etc., such that different runs might contain different conditions.

For the sake of my continuing education, and for the design suggestions I give others, I'm interested to know whether my colleagues' argument is correct. Does applying the polart function and computing percent signal change completely remove the problem of scanner drift? Or are there additional factors associated with scanner drift that the polart function and percent signal change fail to control?

Much thanks in advance for your advice!

Best regards, Larry Barsalou