There is always a way in Brainstorm
It's very similar as in the post above:
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Compute the trial-average PSTH for all condition. Drag in the Process1 all your trials, and in the the Average process select
Group files: By trial group
. This will give you the one trial-average file per condition.
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Place all those trial-average files (there should be one per condition) in Process1 and use the Extract values process. Be sure to select the option Concatenate signals (dimension 1)
The resulting file will have the trial-average PSTH for each condition for a given channel