Electroencephalography (EEG) and Hemispheric Asymmetry

Transcription

1 Updated BSL PRO Lesson H10: Electroencephalography (EEG) and Hemispheric Asymmetry Derived from original work created by Pr Jean-François Lambert and Nicole Chantrier, Institut d'enseignement à Distance / Université Paris 8- Saint-Denis France Abstract Translated from original work created by Pr Jean-François Lambert and Nicole Chantrier Institut d'enseignement à Distancez / Université Paris 8- Saint-Denis France Electroencephalography (EEG) is an electrophysiological investigation technique used to record bioelectric activity of the brain at the scalp. It is a non-invasive method that acquires measures of instantaneous activities within the cerebral hemispheres (in particular in the cortex). Computerized versions of EEG have permitted new developments in frequency analysis, cartography, and spatialtemporal resolution of the brain. With limited, easy to use equipment that implicates noninvasive recording techniques, the EEG is without a doubt the most popular technique used in clinical psychophysiology. Still, it encompasses a variety of constraints that can make usage difficult. For example, electrodes must be placed on the head, and while some are painless, all generally require the use of a fixation device to restrict the subject from moving (unless you have has access to telemetry). In addition, the recording is sensitive to artifacts produced from surrounding equipment, such as video monitors and any nearby metal, that influence the electromagnetic environment. Despite these constraints, EEG analysis from both qualitative and quantitative approaches provides information about fluctuations in awareness associated with variable attention focus during a task. Inter-hemispheric relationships are particularly interesting elements to consider. Research has shown that the left, verbal, hemisphere is associated with positive affect while the right, spatial, hemisphere is associated with negative affect. At the neuron level, brain function can be detected as variations in potential due to ionic changes that occur in order to cross membranes. When an extra-cellular electrode is placed on any point of the head, it records the sum of electric activity produced by surrounding cells. In human EEG studies, electrodes on the scalp record variations of potential that correlate primarily to the activity of the cortex below. For more information, review the notes in the Appendix. Page 1 of 10

2 Objectives 1. To demonstrate how alpha rhythm (around 10 Hz) takes place of a more rapid rhythm called beta (around 15 to 40 Hz). 2. To observe various stop reactions that represent temporary interruptions of alpha rhythm, following a sensory stimulation or change in attitude (attention focus), that give way to beta rhythm. 3. To analyze the spectrum of frequencies that constitute alpha rhythm. 4. To compare the distribution of alpha rhythm between the two cerebral hemispheres. Equipment PC running Windows or Mac computer BIOPAC Software: Biopac Student Lab PRO BIOPAC data acquisition unit (MP35/MP30) BIOPAC electrode lead sets (SS2L) BIOPAC disposable vinyl electrodes (EL503) BIOPAC electrode gel (GEL1) and abrasive pad (ELPAD) Velcro band (such as supplied for SSL5), Lycra swim cap (such as Speedo brand) supportive wrap (such as 3M Coban Self-adhering Support Wrap), or other appropriate material to press electrodes against head for improved contact CD player or other music source Varied music collection Setup Hardware 1. Plug an Electrode Lead Set into CH 1 and another into CH 2 on the MP unit. 2. Turn the MP data acquisition unit on. 3. Turn the computer on. 4. Launch the BSL PRO software on the host computer. 5. Open the EEG lab by choosing File menu > Open > choose Files of type: Graph Template (*GTL) > File Name: h10.gtl. Calibration No calibration is required. Subject Electrode Connections 1. Part the subject's hair along the sagittal plane just above the ears. 2. Lightly abrade the forehead and scalp along the parted hair. 3. Carefully place two frontal electrodes laterally, on the top of the forehead. 4. Apply some electrode gel to two occipital electrodes. 5. Place the occipital electrodes about 2 cm apart from each other, along the sagittal plane intersecting the top edge of each ear. 6. Place a ground electrode behind each ear. 7. Attach the electrode lead from CH 1 to the left hemisphere electrodes (left frontal-occipital derivation) and the electrode lead set from CH 2 to the right hemisphere electrodes (right frontal-occipital derivation) as follows. Frontal Electrode White Ground Black Occipital Electrode Red 8. Hold the electrodes in place and secure them with a head strap (such as the Velcro band from SS5L) or swim cap. Page 2 of 10

3 Running the Experiments Hints for minimizing data error The subject should remain as relaxed as possible. The subject must avoid excessive extraneous movement. Ensure that alpha rhythm is constant before giving any activation instructions. Press "Esc" to add a marker every time you give the subject an instruction. Add text to describe all interventions. During comparative data analysis, ensure that all scales are the same for every channel. Remove all jewelry or other metal objects. Check all cable connections. Preparation 1. Instruct the subject to relax with eyes closed, not thinking about anything in particular (not fixing attention on any particular mental object). 2. Click "Start" to begin recording. 3. Continue recording and observe the corresponding plot, primarily expressing alpha rhythm, for one or two minutes. Visual stop reaction 4. Press "Esc" to insert a marker while telling the subject to open his eyes. 5. Wait a few seconds and then insert another marker while telling the subject to close his eyes. 6. Repeat the above two steps with variable intervals of eye opening. Only give the signal to open the eyes after alpha rhythm is sustained. Auditory stop reaction 7. Instruct the subject to relax with eyes closed. 8. After alpha rhythm is sustained, simultaneously present a discrete auditory stimulation and press "Esc" to add a marker. 9. Repeat the auditory stimulation with varying intensity and nature, each time adding a marker. Sample Auditory Stimulation Series: 1. Soothing (classical) music, low volume. 2. Soothing (classical) music, high volume. 3. Intense (rock) music, low volume. 4. Intense (rock) music, high volume. Attention stop reaction 10. Tell the subject to close his eyes and relax. 11. Inform the subject that he will be asked to perform mental math. Example: Count backward from 30 by subtracting increasing odd numbers (e.g., 30, 29, 26, 21). 12. Tell the subject to let his thoughts stray after thinking the response and not speaking it. 13. After alpha rhythm is sustained, simultaneously present a math problem to the subject verbally and press "Esc" to add a marker. 14. At the end of this sequence, click "Stop" on the data screen to end the recording. Data Analysis Quick Tips: To hide selected channels or view hidden channels, hold the Ctrl key and click the appropriate channel. To preserve data from the results bar, click to open the Journal and select Edit > Journal > Paste Measurement or simply press Ctrl + M. To preserve wave data select Edit > Journal > Paste Wave Data or simply press Ctrl + D. Page 3 of 10

4 Calculation of alpha and beta rhythm amplitude and frequency 1. Select the calculation channels only. 2. Choose Display > Compare Waveforms to show all channels on the same scale. 3. For each stop reaction, use the cursor to select an area rich in alpha rhythm (e.g. VSR - eyes open). 4. Choose the measurement parameter p-p in the results bar for channels 40, 41, 42, and Select a new area weak in alpha rhythm (e.g. VSR - eyes closed). 6. Read the new results. 7. Note the eventual presence of alpha after the eyes are opened (escape phenomenon) 8. Observe the recovery of alpha, generally very clear, when the eyes are closed (off-effect). 9. Manually estimate alpha and beta rhythm frequency for each channel by counting the number of cycles per second on the plot. Sample analog representation of cognitive processes 10. Select File menu > Open > Samples > File Name: Eeg. o The first channel is a sample of EEG containing a sequence of active awareness following alpha rhythm. o The following channels represent the decomposition of global EEG activity into four components of frequency bands (alpha, beta, delta, and theta). 11. Return to the subject s plots. Artifact Elimination 12. Choose CH Select Transform > Digital filters > FIR > Band pass. 14. Input Low Frequency : 2.5 Hz, High Frequency : 45 Hz and Number of coefficients : 150. Click OK. 15. Choose CH 2 and repeat the settings. Page 4 of 10

5 Fast Fourier Transform (FFT) 16. Choose CH Using the cursor, select a region rich in alpha rhythm. 18. Select Transform > FFT to obtain the spectrum of different frequencies composing the selected area. 19. Click Linear in the FFT dialogue, then OK. 20. Double click the channel label "Magnitude" on the left side of the screen. 21. Rename the channel "Left Hemisphere". 22. Do the same for CH 2, but rename the channel "Right Hemisphere". Hints for minimizing error: 1. Select an area with samples just less than, or in the power of 2 (e.g. 1024, 2048, 4096, ) 2. Use the same area for all channels. Hemispheric Comparisons 23. Recall the spectrum corresponding to the right hemisphere. 24. Select Display > Autoscale Waveforms to fit the entire file on the screen. 25. Choose Edit > Select All then Edit > Cut. 26. Close the screen. 27. Recall the spectrum corresponding to the left hemisphere. 28. Select Edit > Insert wave form. o The spectrum corresponding to the right hemisphere will be inserted under the spectrum corresponding to the left hemisphere. 29. Choose Display > Compare Waveforms to calibrate the vertical scales of the two spectra. 30. Select Transform > Waveform math. 31. Input CH 2 in the first box, subtraction "-" in the second, CH 1 in the third, and New in the last. 32. Click OK to get a new plot resulting from the subtraction of the two originals. 33. Return to Display > Compare Waveforms to have the three plots on the same scale. 34. Observe the distribution of frequencies in each hemisphere. Spectral Analysis 35. Select all data Edit > Select All. 36. In the results bar, choose "integral" for CH 1 and CH 2, select max F for CH Record the values corresponding to each channel. 38. Reduce the selection to the alpha band (8-14 Hz). Hints for selecting the alpha band In the results bar, select "x-axis: F" for CH 3 and use the displayed values as a guide to locate points closest to 8 and 14 Hz. Decrease the number of Hz/div in the horizontal scale for improved visual accuracy. 39. Record the new integral measures. 40. Compare the results obtained for each hemisphere. 41. Compute the alpha Integral/total Integral ratio for each hemisphere. Page 5 of 10

6 Additional Study Apply what you have learned by completing the attached WORKSHEET. Appendix EEG notes translated from original work created by Pr Jean-François Lambert and Nicole Chantrier Institut d'enseignement à Distancez / Université Paris 8- Saint-Denis France Origin of the EEG The origin of electric phenomena recorded at the scalp does not, as one might think, depend on the action potentials produced at the neuron level. Those are only slightly or not at all implicated in the generation of the EEG (except in rare cases of synchronized activity). Instead, EEG signals result almost exclusively from electrical activity of the dendrite membrane and pyramidal cell bodies (large neurons with extensions perpendicular to the cortical surface) of the cortex and not from cortical axon activity. Thus, the sum of post-synaptic potentials affecting pyramidal neurons in the region of the cortex below superficial electrode is responsible for the potential variations detected at the scalp and recorded as an EEG signal. Analyzing EEG plots is tedious because the data results from a combination of post-synaptic inhibitory and excitatory potentials, some being near the cortical surface (close to the recording electrode) and others being deep (far from the recording electrodes). The amplitude of EEG waves is around 100 times smaller than that of an action potential, and the primary frequency is between 1 and 40 cycles per second. The origin of EEG waves remains in question, but currently experts hypothesize that nuclei of the thalamus contain some rhythm generators (oscillators) that correspond to a portion of the cortical surface (Verzeano). Some intra-cortical generators and other sub-cortical generators are also considered to contribute. Collection and applications of EEG Electroencephalography translates spontaneous, global, electrical activity from a vast array of neurons, particularly cortical neurons. That activity is collected by intermediate electrodes placed on different points of the scalp, then amplified and recorded as plots expressing temporal activities of potential difference variations that appear between two given electrodes. These potential variations can be rhythmic and constant, or transitory. They can be measured via bipolar recording that utilizes two electrodes situated above an activity zone of the brain, or via monopolar recording using an active electrode and an indifferent electrode placed in an inactive zone like the ear lobe. The plots express somewhat regular, periodic oscillations defined by frequency and amplitude, which vary as a function of the recording site, activation state of the nervous system, and quality of nervous system function. Certain plots and characteristic figures represent varying levels of awareness, ranging from focused attention to a coma, or certain dysfunction, allowing diagnosis of epilepsy, localization of cerebral tumors, or detection of insufficient circulatory or metabolic function. The EEG is thus a particularly precise tool for neurology and also for psychophysiology since it allows association of certain types of cerebral electric activity with specific psychological states. Principal Human EEG plots The EEG provides information about different types of arousal, certain sleep stages, and neurobiological correlates in dreaming. States of Awareness Attentive arousal encompasses reflexive conscious and is generally associated with desynchronized EEG activity of weak amplitude known as beta rhythm (10 to 20 mv, 15 to 40 Hz). That rhythm is constant due to the nature and intensity of mental operations produced. Thus, it does not indicate a certain type of arousal. The EEG only changes in climatic states that generally occur during convulsive episodes, such as epileptic states as shown below. Inattentive arousal characterizes states of relaxation or meditation, favorable to surges of dreamlike conscious, and is most often represented by strongly synchronized EEG activity with very high amplitude. This type of activity is alpha rhythm, which was first described by H. Berger in 1929 (8 to 14 Hz, around 50 mv). Alpha rhythm appears, in principal, as soon as the subject relaxes (particularly when the eyes are closed) and disappears as soon as he mobilizes his attention (particularly when the eyes are opened). Subjects with low alpha are generally rebels to dreamlike mental imagery while subjects with abundant, sometimes permanent, alpha (mobilization of their attention does not requires Page 6 of 10

7 apparition of beta rhythm) often have a very rich imagination sometimes associated with strong creativity. As shown below, alpha rhythm preferentially records in the occipital region. Other EEG activities observed during arousal In the rolandic region (at the level of the Rolando fissure), humans at rest (muscular relaxation) produce rhythmic activity at 7 or 8 cycles per second with weak amplitude (mu waves or arched rhythms). This activity is not affected by attention changes, nor by the opening and closing of the eyes, but is blocked by muscular movement. Vertex-points appear at the top of the head (vertex) in connection with auditory stimulation of varying intensity. They represent a state of alert frequently associated to a psycho-galvanic skin response (GSR) or Electrodermal Response (EDR). Occipital points, lambda waves, associate visual exploration with arousal states. They appear to correspond to eye movements and depend on the subject's interest in an object being viewed. Observation and Annotation of an Epileptic Absence 9-year-old subject - Minor seizure. The child is treated regularly, and the family no longer notices an absence (short clouding of conscience). Above shows a sudden electrical attack recorded during the second hyperventilation. 1st arrow: The order to open the eyes is not executed (2 sec). 2nd arrow: The order to raise the right hand is not executed (5 sec). 3rd arrow: Execution of the first order proceeds as soon as the absence ends: it was thus perceived. (Notice the artifact due to ECG movement). Hyperventilation took place during the absence. Conclusion: an "electric absence" of 10 seconds is only accompanied by a clinical baseness of 3 or 4 seconds. This explains why the family no longer observes a change. Page 7 of 10

8 Alpha Rhythm and Visual Stop Reaction (VSR) Note the lack of alpha in the anterior regions (1-5) and its abundance in the posterior regions (6-10). 26-year-old subject - Mu waves, or arched rolandic rhythms (2,3). They are not interrupted by the opening (O) or closing (C) of the eyes. Alpha rhythm, however, decreases and increases when the eyes are opened and closed, respectively (physiological off-effect - 4,5,6). Page 8 of 10

9 (below) Motor Stop Reaction of Rolandic Rhythms 47-year-old subject - Rolandic mu waves (4,5,6) are interrupted when the fists are clenched while alpha rhythms remain constant. (The EMG corrupts the ECG during muscle contraction) 7 month old boy - Deficiency in pitch/tone detection. 55-year-old subject Lambda waves, when the eyes are open, in the medial occipital region (2). Page 9 of 10

10 Sleep stages Slow wave or classic sleep Typical sleep, or slow wave sleep (SWS) can be divided into four stages: falling asleep (stage I) then light sleep (stage II) followed by deep sleep (stages III and IV) characterized by EEG activity of increased amplitude and decreased speed (delta waves, 1 to 3 Hz, around 300 mv). That slowing of EEG activity echoes the general slowing of biological functions (cardiac rhythm, energy metabolism) favorable to the restoration of the body. Paradoxical sleep Sometimes, the sequence of all or part of the four stages in classic sleep (after about 90 minutes in man) leads to a radical change in cerebral function corresponding to a distinct state of awareness and sleep, to which Mr. Jouvet gave the name paradoxical sleep (PS). It is characterized by intense cerebral activity that expresses itself on the EEG as a return to a plot characteristic of arousal. That intense cerebral activity is accompanied by complete resolution of muscular tone (from the head down) such that most organic functions are reactivated. Hypnosis relies on these episodes of PS. Electroencephalographic recording of different sleep stages. Stages 1a and 1b, corresponding to falling asleep (alpha fragmentation, then development of theta waves) stage 2 entering the sleep zone, stages 3 and 4 progressing deeper, appearing as slow large waves. Finally, paradoxical sleep (SP) evolves with many wave characteristics similar to arousal (desynchronization). Pathological manifestations Pathological states manifest themselves with the presence of either slow waves (oxygen deficiency, metabolic insufficiency, tumors) or pointed waves (extremes pain, epilepsy) as shown below. LEFT HEMISPHERE 59-year-old male. Asymmetric plot. Basic rhythms replaced by polymorphic activity, composed of delta waves and slow, pointed, irregular theta waves, mostly in the left hemisphere. RIGHT HEMISPHERE GRAPH TEMPLATE SETTINGS Click here to open a PDF of the graph template file settings. The BSL PRO Graph Template file automatically establishes the settings shown in the table. Page 10 of 10