Stereo files

Stereo files can contain a lot of information not present in mono files. You can use stereo files in several ways

• To synchronize the hearing of different signals. It is equivalent to a polygraphic recording. One of my favorite quotes says: "the whole is more than the sum of its parts" . We can relate different signals, e.g., respiration and heart rate, spindles and ocular movements and so on
• To make a metaphor of the spatial origin of the sound. You can hear left hemisphere signals in the left loudspeaker and right hemisphere signals in the right loudspeaker. You could even use surround sound to relate anterior/posterior left/right signals. I think that the effect would be great (I said I think because when writing this I have not done it yet)
• To process signals selectively. You could identify ocular movements in a segment of the file to try to distinguish between REM sleep and wakefulness and then decrease the amplitude of this channel to focus the analysis on the EEG.

To see an example that can be used in this way, we create a stereo file containing the signals EEG Pz-Oz and EOG horizontal of the file st7121j0.rec accelerated at 400x . Wavesurfer can handle selectively the spectrograms of each signal. To do that, we have to Create Pane after right button clicking on the original pane and then choosing Spectrogram. Finally after setting in Properties.../Spectrogram the parameters FFT window length at 1024, Cut spectrogram at: 10000 Hz, Spectrogram color to color and Show channel to the channel that we want to show in this pane, we can get a screen similar to the one shown in figure 6.1

Figure 6.1: The spectrogram of EEG Pz-Oz (upper frame) and EOG horizontal (lower frame) of the file st7121j0.rec accelerated at 400x. Notice the similarity between both signals and the presence of ocular movements during wakefulness and REM periods at the lower frame

Remember the figure 2.4 containing the hypnogram of our recording. It contains a segment of wakefulness at the beginning and at the end of the recording. But we try to hear the signals not only analyzing their frequency content. Let's do it. We can follow the hearing in both spectrograms simultaneously. You can use the control Pcm and Volume of the Mixer window. At least in my system it is better to unlock the output. As an example I show the screen of KAMix in figure 6.2

Figure 6.2: The screen of KAMix unlocking the playback

We can selectively eliminate on the fly the output of one channel to hear or not ocular movements together with the EEG. In absence of alpha rhythm, wakefulness and REM sleep can be very difficult to identify. In this case ocular movements can contribute to differentiate them.

Of course, our goal is not to score sleep but to gain some insight into the sounds that can be heard after the audification process.