125:315
BME Measurements and Analysis Laboratory
Fall 2004
Laboratory #2:
Evoked Potentials
I. Objectives
The objectives of this experiment are to show how:
1. Bioelectric signals are measured, (using EEG as an example);
2. Frequency analysis can be used as a tool for interpreting biological activity;
3. How the response to a stimuli is reflected in frequency content of a bioelectrical signal.
II. Introduction
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 spatial-temporal resolution of the brain. With limited, easy to use equipment that implicates non-invasive 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 influences 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.
II.1. 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.
II.2. 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.
II.3. 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 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
When humans are at rest (muscular relaxation), there is a region in the brain that produces 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.
The EEG measured at Vertex-points which are at the top of the head (vertex) are associated with auditory stimulation of varying intensity. They represent a state of alert frequently associated with a psycho-galvanic skin response (GSR) or Electrodermal Response (EDR).
Lambda waves are measured at occipital points and 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
III. Purpose
1. To analyze the spectrum of frequencies that constitute alpha rhythms.
2. To compare the distribution of alpha rhythms between the two cerebral hemispheres.
3. To demonstrate how alpha rhythms (around 10 Hz) take the place of more rapid rhythms called beta rhythms (around 15 to 40 Hz).
4. To observe various stop reactions that represent temporary interruptions of alpha rhythms, following a sensory stimulation or change in attitude (attention focus), that give way to beta rhythm.
IV. Materials and Methods
IV.1. Apparatus
Hardware
· PC running Windows
· BIOPAC Data Acquisition Unit (MP30)
· 2 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
· Headphones
Software
· BIOPAC Software: Biopac Student Lab
· MP3 Sample Music File (Installed on desktop)
· Windows Media Player
IV.2. Setup
1. Plug an Electrode Lead Set into CH 1 and another into CH 2 on the MP30 unit.
2. Turn the MP30 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 > go to Desktop> choose Files of type: Graph Template (*GTL) > File Name: H10.gtl.
6. From the bars menu, go to MP30 >Data Acquisition >Set the acquisition length to 15 minutes.
IV.3. Calibration
No calibration is required.
IV.4. 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 / WhiteGround / 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.
Hints for minimizing measurement 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 "F9" 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
IV.5. Measurements and Data Collection
Preparation
1. Have the subject close his eyes and relax, leaving his thoughts to stray and not fixing his 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 "F9" to insert a marker while telling the subject to open his eyes.
5. Wait a few seconds, 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
1. Find the mp3 file named ‘AuditorySampleLab2’ on the computer desktop, but don’t start to play it.
2. Instruct the subject to put on the earphones.
3. Tell the subject to close his eyes and relax.
4. After alpha rhythm is sustained, simultaneously start to play the auditory stimulation and press "F9" to add a marker.
5. Don’t stop to play until the end of the file.
6. Insert another marker at the end of the file (209 seconds).
IV.6. Data Analysis and Interpretation
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.
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 43.
5. 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 1.13. 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. /
Fast Fourier Transform (FFT)