How to trigger a stimulus based on alpha?

[Nethanjan]

Hi

I'm newbie to Openvibe. I'm using the Emotiv Epoc device to capture the EEG and i'm trying to identify if there is any alpha waves in the signal. I just need to detect alpha and output a message there is alpha for 'this' time frame. Is there any way to output a message?If this worked can anyone please show me a example with how the boxes are use in a scenario.

[fabien.lotte]

Dear Nethanjan,

This is indeed possible to do something like that. However, note that you will always find some alpha in the EEG signals, the question is more how much there will be.
So typically you would need to use a threshold on the power in the alpha band, and if the alpha band power exceeds this threshold, you would send an openvibe stimulation to play a sound or display an image:
- to compute the power in a frequency band you can have a look at what is done in the motor imagery scenarios: http://openvibe.inria.fr/motor-imagery-bci/
- to detect if the power exceeds a threshold, you can subtract your alpha band power value using the simple DSP box (http://openvibe.inria.fr/documentation/ ... leDSP.html) and then use the sign change detector box (http://openvibe.inria.fr/documentation/ ... ector.html) to send a stimulation if the sign changes (i.e., if the band power exceeds the threshold)
- you can then display a message, but e.g., playing a sound when the stimulation is received using the Sound player box (http://openvibe.inria.fr/documentation/ ... layer.html) or display an image using the display cue image box (http://openvibe.inria.fr/documentation/ ... Image.html)

I hope this helps,

Best,
Fabien

[Nethanjan]

Hi Fabien,

Thanks so much for the reply. Yeah motor imagery scenario is quite good, I think it will work for me. I have created a working scenario for my case. However i have a small question what is the threshold value i should set? I just can't set any random value right, so how can I justify this is why i put 'this value' as the threshold? It will be really helpful if you can help me with that, a book or research paper to refer anything will work for me.

Thank you
Nethanjan

[fabien.lotte]

there are actually no universal threshold you could use as this threshold would be different from one user to the next, and also depends on your application. For instance you can measure the alpha band power at rest on your user, and determine the threshold as the mean alpha power + 2 (or 3) times the standard deviation of this power. As such you would trigger an event when the alpha power is significantly different from what it is at rest. But I guess this threshold really depends on your application and objective.