Your filter should be 2*interval between block As and block Bs.

If the time from beginning A to A is 126 seconds, the filter should be at least 252 seconds.

If you have random block orderings, then the onset of one block to the next can be used instead. 

In your case - either approach leads to increasing the high-pass filter.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Sat, Jun 21, 2014 at 11:22 AM, M.Momenian <[log in to unmask]> wrote:
Dear all

I am work hard through my data, and the question of setting the right high pass filter is getting on my nerves very recently. I have also been shifting through the different emails in the list to come up with a remedy, but I couldn't. My work is a block design consisting of two conditions A and B, every condition consisting of four different activation blocks (48 seconds each) and four rests (15s each). Should I leave the high pass filter in its default value? or should I change it due to the slightly long nature of my blocks? I recognize that when I changed the default high pass to 400, I had more activations? Are these new activations false positives? How can I make sure?

Thanks in advance
Best wishes
Mohammad
PhD of Applied Linguistics
Tehran, Iran