In my ERP study, where I implicated mixed design "oddball - passive perceptual training (aka classical conditioning) - oddball", I obtained strange pattern of correlation: changes in MMN were straightly correlated to changes in P3a. It means the bigger the MMN became after training, the smaller the P3a became.
Honestly, I don't know how to interpret this... Absolute majority of studies showed that after discrimination training increase in MMN associated with increase in P3a.
Will appreciate any ideas/links!
Two thoughts: It's possible that this is directly related to your passive training design in which your treatment makes your specific stimuli more familiar, and thus the contrast between better suited to driving MMNs, and also less relevant or surprising (due to the passive nature of the training) , and thus rendering them less able to drive P3 components. I would also recommend plotting the full topography of both effects, and examining the power of each component using GFP estimations.
Numerous endogenous factors influence amplitudes of the MMN and the P3a. Among all possible the relevant factors are attention, saliency/novelty, adaptation or fatigue - these factors change with a prolonged observation of stimuli. Try to answer how novelty or attention change in your experiment it might give you an answer.
P.S. Figure without description does not help much ;-)
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