Dear André,
Yes. For proper statistics I would rely on separate one-sample t-tests for each of the groups though.
To get a better idea about the direction & magnitude of the average contrast estimates it can also be useful to set up an effects of interest F contrast, in your case (due to the 2 columns being the relevant ones, there are no other ones anyway) it would be eye(2). For any of the contrasts, e.g. [1 -1], you could then move to e.g. a peak voxel, select "plot" in the lower left SPM GUI, "Contrast estimates and 90% C.I." and choose the previously specified effects of interest F contrast.
Best
Helmut
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