[pymvpa] Update on: Suspicious results

[Nynke van der Laan]

Hello all,

thanks for all the very useful suggestions, it's nice to have so many
people thinking with me!
I have tried a few suggestions with for each returning the same
results (approximately):
- I have tried a linearNuSVMC, this gives approximately the same results.
- I tried a Odd/even splitter instead of a NFold splitter --> approx
the same results
- Detrending the data --> approximately the same results.
- I have randomized the labels of the two categories. This resulted in
the same distribution of accuracies as with the correct labeling (peak
of histogram at 0.6 accuracy)..... Does this mean that there is
contamination across chunks???

The design of the fMRI task is as follows: The task exists of 38
trials = 38 chunks
One trial consists of the following sequence of events:
- 4 sec category 1
- 2 sec fixationcross
- 4 sec category 2
- 2 sec fixationcross
- 4 sec other event (of no importance)
- random inter trial interval between 2 and 12 sec

Thus total trial duration is between 18 and 28 seconds.

So, in each trial/chunk both category events are presented once. The
order of category 1/2 in the trial is randomized. So in some trials
first 1, then 2 or viceversa. Onsets of events category 1 and 2 are
thus 6 seconds apart, but order is randomized so I would not expect
problems. Between chunks is also enough time I would expect...

Indeed, I have used blockaveraging. For this the functional scans
between (approx) 3.6 and 6 seconds after onset of the event are
averaged. TR is 0.61 secs.

Does anyone have any additional suggestions?

Thanks in advance!

Best regards,
Nynke