Dear Joelle,
The contrast is alright, it's just you should make sure about the exact interpretation. You test for relationships with behavioral performance as such, not with the "learning part" of the performance = increase. Some extreme examples to illustrate the problem,
- some subjects might get tired, lose attention towards the end of the session and then their performance might drop again, leading to an inverse U-shaped performance over time
- some subjects might not learn at all, just random fluctuations
- some might learn in a roughly linear fashion, but the fluctuations around this "increasing line" might be much larger than the increase
With linearly increasing PM values you would run into a similar issue, as the PM values do not exactly match the increases in performance on behavioral level.
Best
Helmut
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