Computing alpha power

About the GUI application to design signal processing pipelines
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rkamile
Posts: 7
Joined: Mon Oct 26, 2020 2:22 pm

Computing alpha power

Post by rkamile »

hello,

I kindly want to ask you help me understand how I can compute alpha amplitude envelope (square root of the power) over 250 samples every second in OpenVibe? Can I do that with sliding window, e.g. every 100ms compute it over the last 250 sample? I read somewhere in the forum that I could use stimulation based epoching toolbox in that case? I am sorry, I am really looking for a step to step guidance, because I don't know so much about EEG signal processing and I am new with OpenVibe. I am working on sound installation. This is the picture of what I have right now:

Image


I have 250 frames per second rate from every channel, and now I am filtering out messages inside the program where I sonify brainwaves, SuperCollider. But I only then use 1 message per second. And I understand it does not give me alpha power, but rather aliasing. Could you please help me? I will be forever grateful!

Best wishes,
Kamile

Thibaut
Posts: 225
Joined: Wed Oct 31, 2018 9:14 am

Re: Computing alpha power

Post by Thibaut »

HI,
You have the time based epoching, he make an epoch of your duration and if the interval is inferior of duration you avec a moving windows as you wan't.
Thibaut
PS : in your image it's better to make channel selector before temporal filter (to avoid apply temporal filter on all channel before selection)

rkamile
Posts: 7
Joined: Mon Oct 26, 2020 2:22 pm

Re: Computing alpha power

Post by rkamile »

Hello Thibaut,

Thank you so much for your reply. I will use the time based epoching! So in my case, it is enough to firstly channel selector, then temporal filter, then time based epoching and then send the data to osc controller? In case to compute alpha amplitude envelope, what epoch duration and interval I should choose, if I have 250frames per second..?

Thank you and sorry for such basic questions.

Best wishes,
Kamile

rkamile
Posts: 7
Joined: Mon Oct 26, 2020 2:22 pm

Re: Computing alpha power

Post by rkamile »

If I am computing alpha power which is around 10Hz, then I could apply multiple oscillations to estimate its power, for example, 300, 500, and 1000ms window, right? Now my question is, in this case, do I need to use different time based epoching boxes, or should I write these times in epoch intervals? After that, I use spectral analysis by only activating amplitude and then sending to osc controller...

Help me please with time based epoching settings, I am lost!

Thank you,
Kamile

Thomas
Posts: 67
Joined: Wed Mar 04, 2020 3:38 pm

Re: Computing alpha power

Post by Thomas »

Hi Kamile,

The time based epoching allow you to take slices of your signal of the length you want (the size of the window) at the interval you want. Depending on what you chose, the box will output chunks of signal of varying size, with potential overlap of sample between chunks.

Your input signal is 250 samples per second.
So imagine the samples of one second of this signal being [S0, S1, S2 ... S249].

With a 500ms window, the first chunk output by the time base epoching would be [S0 : S124] (half the samples of a second).

If you use a 100ms interval, the next chunk will also be 500ms long, but start 100ms after the previous one.
The second chunk will therefore be: [S25 : S149]
And then, the third would be: [S50 : S174], and so on...

We can see here the overlap between windows.
Of course, if the interval is equal to the window size, then there is no more overlap.

Hope this clarifies how the time based epoching settings impact the output.

And if you need different window sizes (300ms, 500ms, 1000ms), you will need a time based epoching box for each of them.

Hope this helps.

Cheers,
Thomas

rkamile
Posts: 7
Joined: Mon Oct 26, 2020 2:22 pm

Re: Computing alpha power

Post by rkamile »

Hello Thomas,

This helps a lot!! Thank you for your response and clear explanation.

Best wishes,
Kamile

rkamile
Posts: 7
Joined: Mon Oct 26, 2020 2:22 pm

Re: Computing alpha power

Post by rkamile »

I changed the input type of OSC controller to streamed matrix and it worked.

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