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Medical Difference between EEG, EMG, EOG and EKG?

  1. Jul 9, 2010 #1
    This question is pretty straight forward:

    What is the real difference (if any) between EEG, EMG, EOG and EKG? All of them registers electrical activity, measured by placing electrodes on the skin, right?

    I understand that they indirectly measure different things (that is, neuronal activity, muscle activity et cetera) with the use of different setups (electrode type, size et cetera), but is there really any difference between them, in what and how they really measure what they are trying to measure?
  2. jcsd
  3. Jul 9, 2010 #2
    In terms of the physics, they all measure changes in electrical activity (depolarization and repolarization potentials) over time. They differ in their technical application and interpretation insofar as they are measuring this activity in the retina, cerebral cortex, the heart or selected muscle tissue. There are certain levels of normal or resting activity in all four which serve as a baseline for evaluating various known physiological or pathological patterns. These are fairly specific to the target organ or tissue. Becoming expert in any of the four applications requires considerable training and experience. There's material online of a fairly basic nature, but detailed information is mostly available from textbooks usually sold in college/university bookstores or at professional meetings.
    Last edited: Jul 9, 2010
  4. Jul 10, 2010 #3
    I've been confused by the terminology mostly because of the complete change of the name for the whole method when you move an electrode from registering on the face to registering over the heart, which to me suggest that the whole basic setup has changed. If you drive your car on gravel, you're driving your car. However, if you drive it on dirt, you're not suddenly driving your dar or anything. You're still driving your car, only in a different terrain.

    Oh well. I guess I have to get used to other people applying different logic to the structure of the language than me :) Thank you for your answer!
  5. Jul 10, 2010 #4
    well, keep in mind that what you are dealing with is a big electrical circuit with lots of little voltage sources. where you put your reference electrode, and your measurement electrode, determine which signals you measure. if i put two electrodes across my biceps, i'm not measuring input from the heart or brain at all, because there is no return path for electricity to flow. do some study of electrical circuits and it should make a little more sense. note that the "Einthoven's Law" is actually Kirchhoff's voltage law (a sum of voltages along any path sum to zero when you return to the origin).

  6. Jul 12, 2010 #5


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    Staff: Mentor

    You would probably really enjoy taking an intro class in Medical Electronics or Medical Instrumentation. See if such a class is offered at your school or a university (or extension) near you.
  7. Jul 13, 2010 #6
    A large part of the difference lies in the instrument which is interpreting the data. If you place standard EKG electrodes in their proper locations and hook that up to an EEG machine, you'll get some readout, but it will be nonsense. Think of the antenna in a radio, which is just part of what is going on; namely it is the receptor for a signal. In the case of EEG, it may be that the particular setup is more sensitive, but that goes to reasons already discussed in the thread.
  8. Jul 14, 2010 #7
    Thank you for your answers everybody.

    I guess this is more a linguistic question than a technical. What would happen if I stuck some electrodes to my foot. What would I be doing then? Skin conductance measures? Or would I have to invent a new name? EFG (Electrofootography)?
  9. Jul 14, 2010 #8
    you might get an electromyogram of the muscles in your foot if you simply observed the voltage. to measure skin conductance, you'd have to force current through the skin and measure the voltage drop (and probably use a different kind of electrode).

    there is also a noise-cancelling EKG circuit that uses a driven right leg electrode (in addition to others). http://en.wikipedia.org/wiki/Driven_right_leg_circuit
  10. Jul 14, 2010 #9
    You can measure any electrical activity, the issue is just calibration, the correct interpretation by the machine and operator, and the correct electrodes. An EEG is an EEG because... it's an EEG. You could place electrodes at different locations, but you wouldn't get the useful information you wanted. It's not as though there are key regions which must be "tapped", just advantageous ones that require less calculation and adjustment, and less sensitive electrodes. An EEG can be performed with very few electrodes, say, for the purposes of a sleep study for apnea, but that would not be enough for an assay to determine occult seizure activity for a neurologist.

    You can in fact, measure muscular and nerve responses with a rather unpleasant process, but one that is often used to gauge functionality and involvement before spinal surgery. This is your EMG. You have to say what kind of electrode, where on the foot, etc. If you place a profoundly sensitive electrode anywhere on your body you'll measure the electrical activity in that region, plus background noise. Without qualifying your question, there can be no specific answer.
  11. Jul 14, 2010 #10
    sorry, but you are not going to get EEG data by placing electrodes on the feet
  12. Jul 14, 2010 #11
    You will get a READOUT, but as I said it will be meaningless for the purposes of determining what is going on in the brain. There will be electrical activity which will be recorded, and be utterly without meaning in the context of an EEG. Maybe I said that badly.
  13. Jul 14, 2010 #12
    so you're referring to regions on the head ?
  14. Jul 15, 2010 #13
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