Dear Sisi,
Donald suggested to go with two conditions only (0-back, 2-back), thus not explicitely modelling the rest condition. In that case the first column of your matrix corresponds to the first condition specified, the second to the second. However, the design matrix does include a column for beta zero, usually the last one. In case of two conditions you can enter contrast vectors like [1 -1] or [-1 1] because SPM automatically fills up with zeros when necessary. The proper "full" contrast vector would be [1 -1 0] or [-1 1 0] though. If you've incorporated the realignment parameters into your design matrix it would be something like [1 -1 0 0 0 0 0 0 0].
Best,
Helmut
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