[Erppcatoolkit-support] Cross validation

[Andreas W. <wi...@un...>]

Dear Joe,

may I ask three (hopefully not too stupid) questions with respect to cross-validation?

(1) I successfully computed a temporal PCA for an ERP dataset (consisting of 4 conditions and 24 subjects each averaged over the individual trials). Now a reviewer wants us to perform a single trial analysis. My initial idea was to somehow "apply" the PCA pattern resulting from the grand-average PCA to the single trials (as we sometimes do "apply" ICA weights to other datasets filtered differently). My naive understanding is that this is what cross-validation does. Is this correct?

(2) In case yes, I’m not sure how standardization of factor scores should be performed. Should standardization (as in line 812 of ep_doPCA.m) be done including all trials from all subjects and conditions? Or should standardization be done per subject? I tried the former. The resulting factor scores were (averaged across trials per subject and condition) about factor 4 smaller than the factor scores from the grand-average PCA. Is this expected/plausible?

(3) Finally, I tried cross-validation of the same dataset the PCA originally was computed from. The resulting factor scores were similar but not identical. Is this expected/plausible?

Thanks a lot for your help! Best,
Andreas