Biofeedback pages - Spectrogram

Spectrogram

Simple oscilloscope / spectrogram

The first example shows channel 1 connected to a scope and spectrogram. Attach your channel 1 electrode sensor to a site towards the rear of the 10-20 map, for example Pz or Oz. This is so that we can see strong alpha with eyes closed. Connect your Bias and Reference (SRB) sensors to opposite earlobes or mastoid areas.

Connect your sensors to the pins closest to the circuit board, IN1N, IN2N, etc. And use the SRB on that same side, which is actually SRB2, for your reference sensor. Both BIAS pins are connected to the same 'bias' or 'ground' source.

Download the design file to your desktop (right-click link). After installing and starting BrainBay, use the Design / Load Design menu item to load the .con file you just downloaded, called scope-spect.con. Con files are configuration files for BrainBay and contain the layout and interconnection of the elements.

The other term used for a con file is simply a design, this is common terminology used with VPLs to refer to the source files containing the programming. The window should show a view similar to the below image, but without any scope or spectrogram output just yet. [Right click and open image in another tab if you want a higher resolution view.]

The Design 'source' pane in the lower right of this window shows the circuit (signal flow) diagram. Channel 1 of the EEG is first connected to a bandpass Filter. Then the output of that filter goes to the scope and to the FFT (spectrogram). The top pane and lower left pane in the window are the outputs of the scope and spectrogram after the design has been receiving EEG data for brief time.

All the design element blocks "open up" for inspection by right clicking on them. These dialog boxes contain various parameters that can be adjusted for the block you have right clicked. Examples are shown below.

Plug in your dongle and turn on the OpenBCI. Blue LEDs should light on each. You should already know your COM port number for the dongle, from previous operation with the Processing GUI.

Adjusting EEG element block (COM port)

Right click the EEG element block, select your COM port number from the pull down list and press the Connect button. The check box should come on, indicating the serial port is open.

Left clicking anywhere in the Design pane will cause the previous element dialog box to close. Right click will also do this if on an empty section, or open another element box. Blocks are interconnected with left clicks on their output pins, then dragging the 'wire' over to the input pin of the next block. Note for the examples shown here, this has already been done, so the wires are in place.

If you use the Design / Save Design menu item you can save another copy of the this same design which includes any of your modifications, such as the COM port. This also saves the "checked" / opened status of that COM port so that you do not have to recheck / connect it on every startup.

BrainBay currently saves the COM port information in each individual .con file. So this means .con files are tied to the input device and COM port that were active when the .con was saved. Any .con files you import from others will need to have the COM port adjusted, then can be resaved to remember your configuration.

Filter element block

Looking inside the Filter element by right clicking on it, we see:

This is the bandpass region of the filter element. It cuts off frequencies outside the range of .5 Hz to 40 Hz. There are different types of digital filters, Butterworth [Wikipedia link] is one of them. The 'Filter Order' controls how tight the edges of the bandpass are, higher orders have sharper edges at the expense of taking longer to compute. (Order 4 is usually sufficient for most purposes.)

Note that with the current version of Brainbay you should only use even numbered filter orders such as 4, 6, or 8. This is because the odd orders result in erroneous computation / display of the bandpass. (You would see the odd shaped bandpass if you put in an odd filter order value.) And it is best to stay with the Butterworth type filters, versus the Bessel type.

On the graph above, the horizontal axis is frequency, and the vertical shows filter output amplitude at that corresponding frequency. "1.0" or the top of the graph indicates no attenuation at that frequency. Minimum and maximum values of the horizontal axis can be set with the text boxes at lower left/right corners.

It is necessary to use a bandpass filter as the first element following any output channel of the OpenBCI. This serves two functions:

it removes the DC or very low frequency components of the EEG. For background on these components see pages 14-46 of this pdf.
it filters out any remaining AC power line noise at 50 or 60 Hz.

Oscilloscope element

Similarly, if you right click the Oscilloscope element, you can see various adjustable controls. The primary one is the Gain, which controls the values shown on the upper and lower limits of the scope graph pane. As distributed the gain is set so the scope range is about 100 microvolts (uV). You can make it more or less sensitive by moving the slider there. As you move the gain slider, watch the scale shown on the scope pane. Typically for watching the raw EEG signal a gain is set somewhere between 60 and 100 microvolts.

FFT (spectrogram) element

Finally we show here what the FFT dialog box looks like. FFT stands for Fast Fourier Transform, which takes a time varying signal and turns it into a frequency map (how much amplitude at each frequency). FFT can be displayed in various ways such as bar charts or spectrograms:

You don't need to change any of these settings. Only a few of them were adjusted to be different than that created as a default FFT. We changed the Gain to 1000% (from 100%) to increase sensitivity. The Style was changed to Spectrogram; there are also bar graphs and 3D spectrograms available. (You can use the Style pull down menu to see what those look like with the design running.) Lastly the Color Palette was changed to use the "red-yellow.pal" black / red / orange / yellow gradient.

If you look at the first image on this page (which is the full window view of this design) the bottom left pane is showing the spectrogram output. This is a rolling graph with time on the horizontal axis. And the vertical axis represents frequency, as you can see with the scale on the left in Hz. The color is "brighter", more towards the yellow end as the amplitude at that particular frequency increases. (Amplitude increases in this palette from black [no amplitude], through red, orange, then yellow.] So the first image above shows a small segment where my eyes were closed and the frequencies around the alpha range of 8 to 12 Hz, increased, as you can see by the brighter band there.

When you run this yourself you will be closing your eyes for 15 to 30 seconds, then opening them to see what the spectrogram looked like over the past 10 to 20 seconds. The spectrogram rolls over to the left after it completes on the right.

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Running the design:

If the bottom window button / status area ever becomes hidden, you can re-expose it with F6 function key.

Pressing the Stop button at lower left of the main window will result in a short blip of red LED on the dongle. That red represents what the OpenBCI is sending to your computer. It will light up solid / flickering when the data starts to flow.

Pressing Play will begin the design, and you should see the panes inside the main window fill in with their displays. It takes 10 seconds or so for the EEG to settle down. When you close your eyes you should see evidence of that on both scope and spectrogram graphs. You can experiment with adjusting the gain on the scope or playing with the various 'Style's of the FFT, for example 3D or bars.

After you are complete with this design, you can press Stop. Reset will clear the displays and reset the design back to it's initially loaded defaults. Remember if you want to save any changes you make to use the Design / Save menu before you close the design or open another.

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Next page: Alpha Neurofeedback