Brief introduction of electrophysiological methods


  Why do we use electrophysiological methods to study cognition?

Because , event related potentials (ERPs) …

  1. It is a non-invasive measurement.
  2. It doesn't need the response of external behavior (e.g. action response, oral report). Therefore,  it can be used to study infants , children and the subjects who have difficult to response . We can also study some cognitive processes which without explicit response, such as reading.
  3. It can provide continuous information.
  4. It has nice time resolution.
  5. It is a kind of multi-dimensional measurement. Data can be analyzed by multiple methods.
  6. The variation of ERP signals can reflect the changes of internal cognitive process.
  7. “The quantity” changes can be distinguished from “the quality” changes.
  8. It can provide information which connecting with neurobiology.

  If you want to set up an electrophysiological laboratory, what basic equipment do you need?

1. Electrode / electrode cap

2. Amplifiers

3. Analog signal to digital conversion

4. Output device

5. Stimulation display equipment

  How to conduct an ERPs experiment?

I . preparation in advance :

A. A nice idea which can be tested

B. The program which can display stimulus. Be aware of the EEG signals must be recorded when the event you interested (which can be the emergence of a stimulus, an expectation or a reaction) is happened.

C. Prepare the consumables for sticking electrodes and electrode caps

II. Sticking electrodes / electrode caps :

A. Electrode pasted position

1. Put some electrodes around the eyes , so we can record eyeball movement. According the records, we can alter or remove the noise (such as blink or eye movement) after the experiment.

2. Put electrodes at the SAME /SIMILAR place on every subjects. Usually , we place electrodes or electrode caps between the nasion and inion. We also put some electrodes at mastoid. The method most commonly used is called “10-20 system”.

3. Remind to put the “reference electrode”. Since we measure the potential difference, some electrodes must be used as a "reference". That is, the active site needs to subtract the reference site to produce a potential difference. Choosing reference electrode is a very important thing. But as each position has more or less noise, the choice becomes difficult. The typical reference point of head is at nose or ear.

B. The preparation of scalp

1. When the scalp starts to secrete oil, the resistance between the scalp and the electrode will increase. And the records will be disturbed. To get high quality records, the scalp MUST be treated well.

2. Remind the subjects that they should wash their hair before the experiment. And do not use hair oil or hair gel at the day of the experiment.

3. Clean the skin where will stick the electrode on with alcohol or acetone. Then, rub the position with scrub to exfoliate and reduce oil .

C. Stick electrodes

1. Check :

2. Cover the electrode with EEG skin prepping gel. Then, press and stick it on the scalp or face (around the eyes) to fix it at the correct position.

3. Connect the electrode cap with headbox.

4. Inject the skin prepping gel into each electrode with injector.

5. Measure the impedance of each electrode, make sure the value is between 1kw to 5kw.

III. Determine the setting of the record.

A. How many electrodes would be used?

Typically, we use 32 or 64 electrodes.


B. How to determine which frequency of signal would be filtered?

One of the amplifier’s important function is to filter signal. This is to ensure aliasing would not be accepted. In EEG recording, we usually want to keep low-frequency signals. So those very high-frequency signals would be filtered out in the beginning, because those signals usually are not those biological signals which we want to record. The general filtering range is 0.03Hz to 30Hz, or 0.01Hz to 100Hz. We can also use the notch filter to selectively filter the 60Hz signal (non biological signal)


C. How can we decide sampling rate of analog-to-digital conversion?

The sampling rate must be able to accurately reflect the data according to Nyquist Standard. That is, the sampling rate must be at least twice as much as the maximum frequency of the signal you record. For example, if maximum frequency of signal you record is 125Hz, then you should sample by 250Hz or higher. However, we can't always such ensure the highest frequency of signal, and besides, we're not interested in very high frequency signals in usual. Therefore, if we filter the frequency above 100Hz, then use 250Hz sampling rate is very sufficient. (equivalent to sample every 4 milliseconds)

IV. What should we do after the EEG data has been collected?

We can follow the steps below:
  1. remove the artifacts (eye movement, heartbeat, muscle activity, frequency 60Hz noise, and so on)
  2. combine related signals according the design of the experiment
  3. average the brain wave of each subject
  4. normalization the data
  5. re reference the data (if necessary)
  6. filter the data (if necessary)
  7. make average of all subjects
  8. check the waveform and conduct statistical analysis

V. how to inference drawing after EEG data processing?

Since we are measuring electrical activity of the brain, most of conclusions are based on the brain activity. Let us suppose we collect ERPs in two different experimental conditions, A and B. Now, compare the average ERPs of the two conditions. If we find out that the ERPs of the two conditions…

A. Same : 

Such result is much more difficult to interpret. Because they don’t have significant different may result from the smaller number of subjects or smaller number of trials. Another must be aware is that we can’t say brain responds to two situations in the same way because of the same value of signals. Remember that we are only measuring PART of brain activity. So, we can’t ensure the activities of brain which not be recorded are same in two conditions. We can only say that we have failed to discover different responses by detectable ERPs.

B. Different:

We can be quite sure that the brain reacts differently in two situations (if we have removed artifacts carefully). And, we can discuss ...

  1. If ERPs waveform of two conditions are same, but when the peaks occur are different. At this situation, we usually conclude that the two conditions have the same neural processing, but occur at different time point.
  2. If the amplitude of one peak in two conditions is different, that may result from two possibility. One is most of the attempts in one case are more responsive than the other. Another reason is time difference of each subjects’ peaks occur. Remember that we are looking at the average wave. So, it will be larger in the average wave at the situation that all the peaks occur at the similar time point. That is, the variance of the peak time interval should be tested by whether it is caused by jitter. In usual, when we observe such result with different amplitudes, the conclusion always is that the "quality" is the same in two conditions, but the "quantity" is different. However, in neural science, there are many reasons for the amplitude reduction of a certain situation. For example, the same group of nerves but have relatively small post-synaptic membrane potential, relatively fewer amount of neural cell activity, or the time difference of activities occurred. Therefore, It is difficult to say whether the difference is due to the qualitative difference or a quantitative difference.
  3. If the brain wave activity in one situation is completely different from the other, we can quite sure that neural activities in two conditions are absolutely different (but remember the different neural activity may be the same pattern at the scalp recording). In many cases, this difference will be concluded that the two situations have qualitatively different. But this view is not 100% correct in certain situation. For example, the neural activity recorded in V1 of the right hemisphere is different from which recorded in V1 of the left hemisphere, but in this case, we perhaps not conclude that two processing have qualitatively different.


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