The McGill Physiology Virtual Lab

Biomedical Signals Acquisition

EEG  > alpha waves
 

The on-going electrical activity of the brain (EEG) measured from electrodes placed on the scalp is an example of a stochastic signal which may give some information about the general mental state of the individual.

   

Alpha waves are 7.5 Hz to 13 Hz relatively large amplitude(usually less than 50 microV, although this can be variable from subject to subject) EEG waves associated with a relaxed but awaken state. Is usually best seen in the posterior regions of the head on each side, being higher in amplitude on the dominant side.

Beta waves are 14 Hz to 20 Hz waves of lower amplitude than alpha waves associated with a more active mental state.

Recording the EEG: Subject preparation and equipment setup

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The subject wears the electro-cap:

       The subject holds his or her arms straight in front of them. The body harness is slipped under the armpits and around the chest with the Velcro strips to the subjectís right side. The elastic is pulled taut and  the Velcro strips are pressed together.The snaps are centered on the chest. The fit should be snug.

        One disposable sponge disk is placed around each of the Fp1 and Fp2 electrode mounts, with the paper side away from the cap material. The sponge disks absorb perspiration and prevent the spread of electrode gel onto the forehead and help positioning the electro-cap.

The circumference of the head is measured with the paper ruler from nasion to inion. 10% of this measurement (from nasion to the forehead) will coincide with the placement of the Fp1 and Fp2 electrodes of the electro-cap on the subject. The paper is removed from the foam disks.
With the fingers on the inside of the electro-cap and the thumbs on the outside

          Fp1 and Fp2 are anchored on the forehead. The electro-cap is placed onto the head by working the hands from the front to the back of the cap in a smooth motion.

         The electro-cap is attached to the body harness by holding and pulling both straps down to the body harness simultaneously. The right strap is crossed to the left side and snapped to the body harness. The left strap is crossed to the right side and is snapped to the body harness.

         The buckles tighten the straps: the electro-cap must be stretched over the head and under constant tension, otherwise numerous artifacts will result.

Fill the electrodes: Fp1 and Fp2 and O2 (Occipital) by injecting the electrolyte gel with the syringe - with a moderate amount of downward pressure - into the holes of the electrodes.



Example of recordings
Recording from the Occipital electrode arrangement
Software:
  • Channel 1 is labeled "band-pass filter"; it applies a digital filter to the raw EEG signal coming from channel 3 in order to only let the alpha frequencies through.
  • Channel 2 is labeled "alpha power" and calculates the accumulated power of the spectrum within the alpha waves frequencies present in the raw EEG from channel 3.
  • Channel 3 is labeled "raw EEG" receives the output from the cap electrodes.
To the right of the Chart View, the spectrum window (top) shows a PSD plot of channels 1 and 3 and the spectrogram window (bottom) shows a false-colour plot (i.e. 3-dimensional plot) of spectral power, frequency and time. The spectrogram  displays spectral power as a coded colour (pink) against time and frequency.
One can find specific frequencies by scrolling through the spectrogram and find "pink" bands location against the frequency (Hz) on the y-axis.
In the recording above, the raw EEG channel displays alpha waves (the subject has shut his eyes and is quite relaxed); the corresponding FFT calculation (spectrum window) shows a dominant frequency at about 9 Hz. The "alpha power channel" shows an increase in accumulated power corresponding to the presence of the alpha waves. The band-pass filter shows a higher amplitude of incoming signals corresponding to the highlighted data on channel 3 (raw EEG).
 
In the recording above, fewer alpha waves are evidenced. The alpha power shows an increased accumulated power corresponding to the segment of data which is highlighted. The PSD plot to the top right shows a dominant frequency of about 10 Hz, however the FFT calculation of the raw EEG channel also shows other frequencies (notice the lower frequencies: 1Hz-4 Hz) as opposed to the FFT calculations of Channel1 (band-pass filter).  The coloured spectrogram shows a pink area at about 10hz and at lower frequencies.
Recording from the Frontal electrode arrangement
 

 
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Gel is added to the F4 electrode.
The recording below shows a reduction in wave amplitude (raw EEG), a decreased accumulated power (Alpha power), a Band-pass filter channel which shows very small amplitude signal, a PSD plot which shows mostly frequencies below 4 Hz and a spectrogram with barely visible if not present spectral power of the selected data. This recording was made while the subject had his eyes closed, relaxed. The electrode arrangement was F4 (frontal). What is your conclusion? What are the differences between this recording and the one with the occipital electrode arrangement?