bci2000.org BBS

Dear all,
we are performing experiments using the BCI2000 Mu feedback cursor task. So each time we make an initial session, we extract the proper frequencies with the Offline analysis tool and we put them into the Classifier Matrix. Than we click 'Set configuration': the application runs properly but at this step, almost each time, an error message similar to the follow one, labelled in green, appears:

"Warning: Linear Classifier: Preflight: specification in physical units:
classifier (C3_OUT , input element (bin) ) = 10 Hz does not meet a single element"

Is this error message linked to something that we have to write in the "evaluation per bin" field of the filtering window (we usually fill it putting the same frequencies we used in the classifier matrix)or the problem is somewhere else?

Thank you so much in advance!

Dear Paolo,

the warning message is not related to the value entered into the "EvaluationsPerBin" field. Rather, it means that the frequency of 10Hz, as specified in the Classifier parameter, is not the center frequency of an existing bin. Thus, it is related to the values entered in the "BinWidth", "FirstBinCenter", and "LastBinCenter" fields. The background is that the AR algorithm estimates a spectrum in form of a continuous transfer function. In order to obtain a spectrum usable as features for a BCI, this transfer function needs to be evaluated over small ranges, called bins. In the "FirstBinCenter", "LastBinCenter", and "BinWidth" fields, you specify how these bins cover the continuous range of frequencies in the spectrum. The first bin will have the center frequency given in the "FirstBinCenter" parameter, the second bin will have this frequency plus the value of "BinWidth", the third bin plus the value of twice "BinWidth", and so forth, until the "LastBinCenter" frequency is reached. The warning message informs you that no bin is exactly matching the given frequency, and that the closest nearby bin will be used. If you want to get rid of the warning message, adjust the "FirstBinFrequency" value such that 10Hz will be in the list of bin frequencies.

The "EvaluationsPerBin" field should only contain a single number, typically 15. It specifies how often the continuous spectrum is evaluated within the range of a single bin. Higher values will increase the precision of the bins' amplitude values, but will increase computation time as well. 15 should be a good choice to balance between the two.

Regards,
Juergen