The Interaction will have 3 rows. 

The null hypothesis is:
A1B1-A1B2=A2B1-A2B2=A3B1-A3B2=A4B1-A4B2

Thus, you have 3 inequalities:
A1B1-A1B2=A2B1-A2B2
A2B1-A2B2=A3B1-A3B2
A3B1-A3B2=A4B1-A4B2

Now you just need to use these to form the contrast of 3 rows.

The contrast should be:
zeros(1,6) 1 -1 -1 1 0 0 0 0
zeros(1,6) 0 0 1 -1 -1 1 0 0
zeros(1,6) 0 0 0 0 1 -1 -1 1

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 Thu, Jun 5, 2014 at 5:36 PM, Angela J <[log in to unmask]> wrote:
Dear SPMers,

I have a 4x2 within-subjects design with the following columns in my design matrix:

columns 1-4: main effect of F1 (4 levels)
columns 5-6: main effect of F2 (2 levels)
columns 7-14: Interaction F1 F2

I figured out the right contrast weights for the main effects but what are the weights for the interaction contrast?

Thank you in advance,
AJ