Dear Sun,
There is no 'correct' way of doing things which always works. It
depends on the data. In principle we recommend inverting the
conditions you will later compare together. Since you are inverting
contrasts, how were you planning to do statistics across subjects? If
you have a way of analysing each contrast separately all the way
through then you can also invert it separately. It can happen that the
inverse solution will be dominated by a condition with strongest
deflection and will 'ignore' the other conditions. This can be
especially severe for contrasts where there is very low SNR. Perhaps
you should try inverting the original waveforms and not the contrasts
and compute the contrast on the inversion results.
Best,
Vladimir
Sent from my iPad
On 23 Aug 2010, at 03:17, Sun Delin <[log in to unmask]> wrote:
> Dear Vladimir,
>
> I found that the solutions of source localization (SPM8) for grandmean data (not group inversion) were different according to the number of contrasts that should be inverted. In my 2*2 designed ERP experiment, signals of 4 conditions averaged across subjects were gotten by using 'Grand mean' in the 'other' option. I would like to investigate the source information of the difference between conditions, therefore, contrasts were made by using 'contrast' in the 'other' option. However, I found that the solution of contrast file containing only one contrast (e.g. a1b1 - a1b2) was different from the solutions containing several contrasts (e.g. a1b1 - a1b2; a1b1 - a2b1; ....). I am confused about how many and what contrasts should be inverted simultaneously. I am looking forward to your reply.
>
> Bests,
> Sun Delin
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