Dear Panagiotis,
In the merging function there is a quite sophisticated relabelling
facility based on regular expressions that is described in the
function header. So if you average within sessions you can then merge
the averages, relabel the conditions from each file in any way you
want and then average the merged file. The only problem is that this
way the original number of replications will not be taken into
account. If the numbers of trials in all of your sessions are more or
less the same, this is not a big problem. If not, then your should use
grand mean and relabel the trials using the conditions method.
Vladimir
Sent from my iPad
On 26 Aug 2010, at 16:47, Panagiotis Tsiatsis
<[log in to unmask]> wrote:
> Hello everybody,
>
> I have a stupid question: I am running an experiment where for the stimuli are different across sessions (let's say session A has only condition A and session B has only condition B - of course it is not as simple as that). For the purposes of my analysis I want to compare these conditions and up to now I was first concatenating the sessions and then averaging the concatenated file so everything was working fine. The thing is that when moving to the Time Frequency domain, the size of the concatenated file is large (~12gb) so averaging is *extremely* slow.
>
> Since averaging works much much faster when applied to each session separately, I would prefer to first average each session and then apply the grand mean averaging across sessions. The problem which now appears is that. as I said before, the sessions have different conditions and Grand mean averaging complains about that. I know that I could maybe bypass the problem via more elaborated scripting, but I think that simplicity is a virtue and I thought to ask you whether you had any ideas that will enable me to average each session separately and then apply the grand mean averaging - maybe I could somehow alter the condlist of each session to be the union of all possible conditions (while in this session actually only one subset of this conditions appears) and then apply the GM averaging without a problem?
>
> Thanks and best,
> Panagiotis
>
> PS: I am using the latest update of SPM
>
> --
> Panagiotis S. Tsiatsis
> Max Planck Institute for Biogical Cybernetics
> Cognitive NeuroImaging Group
> Tuebingen, Germany
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