The Measurement of Brain Waves
The cerebral cortex is composed of neurons that are interconnected to each other in networks and also receive inputs from other areas of the brain. Electrical activity in the form of nerve impulses being sent and received to and from cortical neurons is always present, even during sleep. Biologically, medically and legally, the absence of cortical activity signifies death.
The electrical activity you are measuring reflects both the intrinsic activity of neurons in the cerebral cortex and the information sent to it by subcortical structures and the sense receptors. This composite activity is called an electroencephalogram or EEG.
An EEG electrode will mainly detect the activity in the brain region just under it. Nevertheless, the electrodes receive the activity from thousands of neurons. In fact, one square millimeter of cortex has more than 100,000 neurons. It is only when the input to a region is synchronized with electrical activity occurring at the same time that you begin to distinguish simple periodic waveforms in the EEG.
Four simple periodic rhythms recorded in the EEG are alpha, beta, delta, and theta. These rhythms are identified by frequency (Hz or cycles/sec) and amplitude. The amplitudes recorded by scalp electrodes are in the range of microvolts (μV or 1/1,000,000 of a volt).
Alpha: The four basic rhythms have been associated with various states. In general, the alpha rhythm is the prominent EEG wave pattern of an adult who is awake but relaxed with eyes closed. Each region of the brain had a characteristic alpha rhythm but alpha waves of the greatest amplitude are recorded from the occipital and parietal regions of the cerebral cortex. In general, amplitudes of alpha waves diminish when subjects open their eyes and are attentive to external stimuli although some subjects trained in relaxation techniques can maintain high alpha amplitudes even with their eyes open.
Beta: Beta rhythms occur in individuals who are alert and attentive to external stimuli or exert specific mental effort, or paradoxically, beta rhythms also occur during deep sleep, REM (Rapid Eye Movement) sleep when the eyes switch back and forth. This does not mean that there is less electrical activity, rather that the “positive” and “negative” activities are starting to counterbalance so that the sum of the electrical activity is less. Thus, instead of getting the wave-like synchronized pattern of alpha waves, desynchronization or alpha block occurs. So, the beta wave represents arousal of the cortex to a higher state of alertness or tension. It may also be associated with “remembering” or retrieving memories.
Delta and Theta: Delta and theta rhythms are low-frequency EEG patterns that increase during sleep in the normal adult. As people move from lighter to deeper stages of sleep (prior to REM sleep), the occurrence of alpha waves diminish and is gradually replaced by the lower frequency theta and then delta frequency rhythms.
Although delta and theta rhythms are generally prominent during sleep, there are cases when delta and theta rhythms are recorded from individuals who are awake. For example, theta waves will occur for brief intervals during emotional responses to frustrating events or situations. Delta waves may increase during difficult mental activities requiring concentration. In general, the occurrence and amplitudes of delta and theta rhythms are highly variable within and between individuals.
Electrode positions: Electrode positions have been named according to the brain region below the area of the scalp: frontal, central (sulcus), parietal, temporal, and occipital. In the bipolar method, the EEG is measured from a pair of scalp electrodes. The pair of electrodes measures the difference in electrical potential (voltage) between their two positions above the brain. A third electrode is put on the earlobe as a point of reference, ‘ground’, of the body’s baseline voltage due to other electrical activities within the body.
Goal of the project: In this project you will record the EEG with your eyes closed and in a state of relaxation and compare the records with the EEG when your eyes are open and attentive, when your eyes are closed and you are hyperventilating. You will also compare the EEG records when your eyes are open and attentive to when you are computing a mathematical problem.
Procedure: You will work in pairs. One partner will have electrodes placed on their heads according the pattern illustrated in the diagram you will be given. The other partner will be the recorder. The first task is to calibrate the equipment by selecting the calibrate button. After calibration, there will be four 30 second recording sessions divided into two 15 second components.. The first component of the recording session will always be the same. Subjects will lie quietly with their eyes closed and relaxing as much as possible. After 15 seconds of recording the recorder will instruct to subject to do the following.
Recording 1: Continue to relax.
Recording 2: Open your eyes and stare at the computer screen WITHOUT BLINKING.
Recording 3: Keep your eyes closed and continue to relax but breathe very deeply.
Recording 4: Open your eyes and stare at the computer screen WITHOUT BLINKING. Instruct the subject to take the number 2 and double it, double again, double again, divide by three, multiply by 15, divide by seven, multiply by twelve.
Instructions to the Recorder: At the end of the first 15 second interval, mark the point by the beginning of the second task component by pressing the F9 key. After recording the EEG for 30 secs, select the alpha, beta, delta and theta displays. The screen will divide up into 5 segments (EEG, alpha, beta, delta and theta). Assign the four measurement blocks to alpha, beta, delta and theta respectively. Using the drop down menu, select standard deviation as the measure parameter. Using the select tool, highlight the recording from the first 15 sec and then copy down the values that appear in each measurement block. Repeat for the last 15 seconds. Between each recording session, select the redo button.
The standard deviation is a measure that indicates the extent to which the peaks and troughs of a wave differ on average from the mean voltage. A large standard deviation is a measure of large amplitude waves, i.e., on average the peaks and troughs differ by a large extent from the mean voltage.
After each recording, if you feel the data is flawed, e.g., the subject blinked, moved or didn’t follow instruction, select redo and repeat the procedure.
Write the standard deviation values in the tables below.
When you have completed the entire four recording sessions, place the data in an class spreadsheet available on the R: drive.
|0- 15 sec||15-30 sec|
- Compute the class means (for both labs) for all the measurements taken during the lab. Construct a table (suggestion: copy and paste from Excel) of class means.
Write a Results section that summarizes our observations. Be sure to address the following in the Results.
- The manner in which the brainwaves in the second recording interval (15-30 sec) differed from the baseline recording (0-15 sec) in each of the conditions.
- The manner in which the brainwaves patterns in the second recording interval differed from each other.
- Which of the brain wave frequencies was the most sensitive to the various treatments in this exercise? Based on the basic properties of these wave frequencies (described above) were the results predictable? Speculate why.
- Using Wikipedia as a source, http://en.wikipedia.org/wiki/Electroencephalography, a) describe the source of EEG activity and b) summarize the clinical uses of the EEG.
This section of the laboratory is based upon the Virtual EEG exercise. This experiment illustrates the measurement of Event Related Potentials (ERP’s). In the previous experiment you averaged the characteristics of brain waves over an interval of time (either 0 to 15 seconds or 15 to 30 seconds). In the ERP you measured the characteristics of a brain wave at each moment after the presentation of a stimulus event and then average the response over multiple trials. The wave form produced is indicator of the sequence of processing of the event displayed. For example, the early components of the wave indicate attentional and perceptual activities. The later components are indicative of higher order processing of the information such as decision making.
The virtual EEG exercise is based on real data collected from subjects as they viewed a variety of images. The program contains a catalog of these images that you can select to create your own experiments by comparing how the brain wave patterns differ in response to particular category of images you construct. After you define the categories and select the images to be included in the categories, you “run” the experiment. The program then retrieves the store ERP responses to the displayed images and then averages the responses for the stored categories.
In Part A there are limited electrode placements, so you are recording brainwaves only in the area under the electrodes you positioned. In Part B, multiple electrodes are used enabling the monitoring of the activity of different neural areas. As you explore the data you generated you should pay attention to the distinctive wave forms that develop at varying latencies after the presentation of the event. For example, the brain wave patterns in the back of the head, over the occipital lobe, show a quicker response onset than do the brain waves measured by electrodes over the frontal lobe, the area making decisions about the events that were perceived.
Designing your projects:
You will be given a basic orientation to the Virtual EEG software in class. After the orientation you will then scan through the available images and make a decision about how to sort the images into categories. On the set-up page define two categories and on the category page assign the relevant pictures to each. Going back to the set-up page, select channel for statistics by going to drop-down menu and selecting all. Run your experiment by going to the graph page.
- Describe the categories you constructed. Report which electrode sites revealed differences in the brain wave patterns associated with the categories you selected. Describe what type of information processing is attributed to the sites in which there was a statistical difference. Also describe the latency of the response and whether the wave form was positive or negative at the point of difference.
- Which sites reveal the shortest response latencies and which sites the longest. Why?
- Go to Medline and run a search on P300. Identify with the appropriate citation five abstracts and summarize the observation made in each of those abstracts.