10. Examination of biolectrical signals accompanying brain function (EEG)
10.3. Single-channel human EEG recording using Biopac Student Lab system
Preparations for the measurement
Attach one electrode to the earlobe of the subject for grounding purpose. Make sure that the electrode has appro-priate contact with the skin. Electrode should contain approappro-priate amount of paste for solid electrical contact.
Grounding electrode can be placed on the neck instead of the earlobe. According to Figure 10.4A, place two addi-tional electrodes on the scalp for recording EEG. Place supportive rubber cap or elastic gauze bandage to press electrodes against the scalp. Plug electrode lead set (SS2L) according to Figure 10.4B. Pay attention to the color code!
Figure 10.4. Electrode positions on the head and their color-coded connections (A), electrode lead set connection with the Biopac hardware (B)
Make sure the Biopac MP30/35/36 unit is turned off. If not, turn it off. Plug electrode lead set to channel one (CH1). Turn the MP30/35/36 unit on.
Examination of biolectrical signals accompanying brain function (EEG)
During the recording, subject should be in supine position. Comfortable position is a must as subject should be motionless throughout the recording session.
Recording protocol:
1. Demonstration of the EEG components belonging to different frequencies. Effect of visual information on the parieto-occipital EEG
Start Biopac Student Lab software and choose Lesson 3 (EEG I).
Start the calibration process by pressing theCalibratebutton. Follow the instructions on the pop-up windows.
During the calibration, 8 seconds of EEG signal is recorded. In case of a successful calibration, there should be a low-amplitude signal (around the zero value). If the recorded signal is different, redo calibration by clicking the Redo Calibrationbutton. If the calibration succeeded, recording can be started.
Tasks:
• At the beginning of the recording, subject should close her/his eyes. PressRecordbutton to start recording.
• After 20 seconds, subject should open her/his eyes. The observer should place a marker in the file by pressing theF9/F4button.
• After another 20 seconds long period, subject should close her/his eyes again. The observer should place a marker in the file by pressing theF9/F5button.
• After 20 seconds of recording, terminate the measurement by pressing theStopbutton.
If the recording was performed well, recording should resemble the picture seen on Figure 10.5. The software shows EEG components belonging to different frequency bands if buttons representing these bands (alpha-theta) are clicked (see Figure 10.5.B).
Figure 10.5. The recorded EEG signal (A) and the signal decomposed into its frequency components (alpha-theta) (B)
The recording was succesful if alpha activity decreased after eye opening. If it is not the case, then some error might have occurred, and the recording should be repeated by pressing theRedobutton. By pressing theDone button, recording can be terminated.
2. Changes of the EEG alpha rhythm
Start Biopac Student Lab software and choose Lesson 4 (EEG II).
Start the calibration process by pressing theCalibratebutton. Follow the instructions on the pop-up windows.
During the calibration, 8 seconds of EEG signal is recorded. In case of a successful calibration, there should be a low-amplitude signal (around the zero value). If the recorded signal is different, redo calibration by clicking the Redo Calibrationbutton. If the calibration succeeded, recording can be started.
Examination of biolectrical signals accompanying brain function (EEG)
2. performingmental mathwith eyesclosed
3. recovering fromhyperventilationwith eyesclosed 4. relaxedstate with eyesopen
The software shows the recorded EEG, the alpha frequency band of the EEG signal and alpha power (as alpha-rms - root mean square). Alpha-rms is also visualized by a column graph.
Tasks:
• Start recording by pressing theRecordbutton. Record a 15-20 seconds long segment when the subject is relaxed with eyes closed. Press theSuspendbutton to stop the recording.
• The observer should prepare the math task. The task should be neither too easy nor too difficult. It should contain operation with fractions.
• Continue recording by pressing theResumebutton. The observer should give the subject the math task. Subject sould perform mental arithmetic but the result should not be said aloud.
• After 20-30 seonds, recording should be stopped (press theSuspendbutton). Result of the mental arithmetic should be discussed.
• Suject should hyperventilate for a minute with eyes closed. Immediately after the end of the hyperventilation, recording sould continue again (Resume). Subject should be in a relaxed, supine position with eyes closed.
• After 10 seconds, recording should be stopped (Suspend).
• Subject should open her/his eyes and should be in a relaxed, motionless position for 2-3 minutes.
• Start recording again by pressing theResumebutton then pressSuspendafter 15 seconds of recording.
• PressDoneto terminate recording.
Data analysis:After opening the file recorded with Lesson04 (EEG II), the following signals can be seen on the screen: Channel 1 – recorded (raw) EEG signal; Channel 40 – alpha frequency band (after digital filtering);
Channel 41. – alpha rms. The alpha rms (root mean square) value is the square root of the mean values (calculated in 0.25 s long time windows). It is a good indicator of the actual amount of alpha waves.
1. Determination of the amplitude and frequency of alpha waves
During the analysis, channel 1 (raw EEG) data is examined. Zoom in on a small segment then set the amplitude for optimal viewing by theDisplay/Autoscale waveformsfunction. Set the following measurement windows:
first window - channel 1 (raw EEG) – p-p (peak-to-peak amplitude); second window – channel 1 (raw EEG) – delta T. Identify consecutive alpha waves in the recording with sinusoid shape then select an alpha wave by the I-Beamcursor from the onset of wave to the peak of the wave. This selection marks the amplitude of the wave. Note the amplitude value seen in the first measurement window (channel 1 –p-p). Following this, select the same wave from beginning to end. In the second measurement window (channel 1 – delta T), period time of the wave appears.
During the calculation of the delta T parameter, selection order matters as delta T value is positive when the selection performed from beginning to the end while negative if the direction of the selection is the opposite. Repeat the above mentioned process in case of another 7 different alpha waves then calculate the frequency of the waves using the period time values. Calculate the average of the amplitude, period time and frequency values of the analyzed waves.
2. Determination of the amplitude and frequency of beta waves
During the analysis, channel 1 (raw EEG) data is examined. Steps of the analysis are similar to ones described in the previous section. Calculate the average of the amplitude, period time and frequency values of the analyzed waves and indicate the averages in the table.
3. Effect of different treatments on the amount of alpha waves
Examination of biolectrical signals accompanying brain function (EEG)
During the analysis, Channel 41 (alpha rms) data is examined. Set the following measurement window: channel 41 (alpha rms) – mean. Zoom in on the recording then set the amplitude for optimal viewing by the Display/Auto-scale waveformsfunction. Based on the signal in channel 1 (raw EEG), select 2-second long segments from the recording of the consecutive tasks, then note the alpha rms values displayed in the measurement window.
During the selection of the analyzed segments, the first, control segment should contain a significant amount of alpha waves as it can be seen in channel 1 (raw EEG). During the selection of the second segment (mental arith-metic), select the segment recorded immediately after the subject got the math task. It can be hypothesized that the subject concentrated most on the task during this period. Selected segments must be free of artefacts. Alpha rms value of the first segment (relaxed state with eyes closed) serves as control. Compare alpha rms values of the second, third and fourth segments (hyperventilation, opened eyes) to the control one and indicate them in a table.