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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 intracortical 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
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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
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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.
2.
3.
4.
5.
Part the subject's hair along the sagittal plane just above the ears.
Lightly abrade the forehead and scalp along the parted hair.
Carefully place two frontal electrodes laterally, on the top of the forehead.
Apply some electrode gel to two occipital electrodes.
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 frontaloccipital derivation) as follows.
Frontal
Electrode
Ground
Occipital
Electrode
White
Black
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
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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
Find the mp3 file named ‘AuditorySampleLab2’ on the computer desktop, but don’t start to play it.
Instruct the subject to put on the earphones.
Tell the subject to close his eyes and relax.
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).
1.
2.
3.
4.
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
3.
4.
5.
6.
7.
8.
9.
Waveforms to show all channels
on the same scale.
For each stop reaction, use the
cursor
to select an area rich
in alpha rhythm (e.g. VSR - eyes
open).
Choose the measurement
parameter “p-p” in the results bar
for channels 40, 41, 42, and 43.
Select a new area weak in alpha
rhythm (e.g. VSR - eyes closed).
Read the new results.
Note the eventual presence of
alpha after the eyes are opened
(escape phenomenon)
Observe the recovery of alpha,
generally very clear, when the
eyes are closed (off-effect).
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
11.
> 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).
Return to the subject's
plots.
Artifact Elimination
12. Choose CH 1.
13. Select
14.
15.
Transform >
Digital filters
> FIR > Band
pass.
Input “Low
Frequency”:
2.5 Hz, “High
Frequency”:
45 Hz and
“Number of
coefficients”:
150. Click “O
K”.
Choose CH 2
and repeat
the settings.
Fast Fourier Transform (FFT)
16. Choose CH 1. Hints for minimizing error:
1. Select an area with samples just less than, or in the power of 2
17. Using the
18.
19.
20.
21.
22.
cursor, select
a region rich in
alpha rhythm.
Select
Transform >
FFT to obtain
the spectrum
of different
frequencies
composing the
selected area.
Click “Linear”
in the FFT
dialogue, then
“OK”.
Double click
the channel
label
"Magnitude"
on the left side
of the screen.
Rename the
channel "Left
Hemisphere".
Do the same
for CH 2, but
rename the
channel "Right
Hemisphere".
(e.g. 1024, 2048, 4096, 8192...)
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
29.
30.
31.
The spectrum corresponding to the right hemisphere, will be inserted under the spectrum
corresponding to the left hemisphere.
Choose Display > Compare Waveforms to calibrate the vertical scales of the two spectra.
Select Transform > Waveform math….
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 >
36.
37.
38.
Select All.
In the results bar,
choose "integral" for
CH 1 and CH 2,
select “max F” for
CH 3.
Record the values
corresponding to
each channel.
Reduce the selection
to the alpha band (814 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
41.
obtained for each
hemisphere.
Compute the alpha
Integral/total Integral
ratio for each
hemisphere.
LEFT
Alpha integral
Total integral
Alpha/total integral
a) What do you notice?
b) What can you conclude from that?
RIGHT
V.
Lab Report Guidelines
V.1.
Report Format
Introduction: Motivate lab in your own words (personal assessment of topic importance); Give
justification for methods used in the laboratory experiment.
Methods:
Description of data collection and measurements. Describe experimental protocol and care
taken to perform the laboratory experiment.
Results:
Data (Objective summary of results). Focus on the organization of the data.
Analysis:
Meaning of the results, how data processing such as filters and data analysis such as FFT can
help to better interpret your results. What conclusions can or cannot be made and why?
Include results from IV.6: Data Analysis and Interpretation.
Summary & Conclusions: Include only overall trends or findings from Analysis Section. There need be
no more than 3 or 4 main points. Base your discussion on the questions in V.2, below.
References: (quote all sources used, including web sources, no plagiarism)
V.2.
Analysis Guide Questions
1. Does the data present alpha rhythm characteristics well? Justify your answer based on your data
analysis.
2. Are the visual stop reaction plots affected by the duration that the eyes were opened?
3. Compare the auditory stop reactions. Analyze and explain any differences based on the type and
intensity (or volume) of the music?
4. Explain the term `Habituation`. Do you observe any habituation patterns in your data?
5. Compute the alpha Integral/total Integral ratio for each hemisphere.