cursor task results

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paolobernardis
Posts: 40
Joined: 02 Jul 2010, 04:51

cursor task results

Post by paolobernardis » 07 Dec 2010, 05:45

Dear all,

Thank You very much for your previous responses.
We made different trials of cursor moving task (1D/2D/3D with different number of targets), always following your tutorial step by step.
Nevertheless, we observed that in all our trials the cursor began its movement independently from the user's intention (it seemed it had a movement "by chance"). How can avoid unvoluntary cursor movements? We downloaded the latest version of BCI2000.
In all the trials we had a result such as "100% correct" and bits transferred were between 0 and 12. We observed that just hit the target once (18 trials, 1 hit, 17 invalid) and we have get automatically "100%correct", although we didn't transfer any bit. Could someone explain how the calculation is done to "% correct" and transfer bit?
We read the tutorial, but we couldn't find in it any information regarding this problems.
Hopefully, you could help us in the best way!

mellinger
Posts: 1065
Joined: 12 Feb 2003, 11:06

Post by mellinger » 07 Dec 2010, 09:39

Nevertheless, we observed that in all our trials the cursor began its movement independently from the user's intention (it seemed it had a movement "by chance"). How can avoid unvoluntary cursor movements? We downloaded the latest version of BCI2000.
There will always be some amount of random cursor movement, which you cannot avoid, especially at the beginning of a run, when the normalizer filter has not adapted to the participant's signals yet. It is important that, for the first, say 10, trials, the participant performs movement imagery as indicated by the target, even if the cursor does not move according to his/her intention. Over time, cursor movement will follow the participant's intention better.
We observed that just hit the target once (18 trials, 1 hit, 17 invalid) and we have get automatically "100%correct", although we didn't transfer any bit.
The report is correct. One hit means that all valid trials were correct, which is 100 percent of valid trials. Also, there has information been transferred, amounting to selection of one target out of two (or four).
Could someone explain how the calculation is done to "% correct" and transfer bit?
% correct is simply the number of correct trials divided by the number of valid trials. A trial is invalid when no target has been hit, i.e. when the trial timed out.
The number of bits transferred is computed from the confusion matrix over valid trials, using Shannon's expression for the channel capacity, multiplied by the number of valid trials. For details, see
http://www.bci2000.org/tracproj/browser ... istics.cpp, line 82.

--Juergen

paolobernardis
Posts: 40
Joined: 02 Jul 2010, 04:51

Post by paolobernardis » 07 Dec 2010, 11:06

Thanks for your answer Jurgen.

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