<p>EEG and MEG measure two different but related signals. While EEG records the electric activity from the scalp, MEG records changes in the magnetic field induced by electric activity in the brain. Because of this, MEG is largely insensitive to sources oriented radially, but can pick up sources oriented tangentially to the cortex. Moreover, EEG is sensitive to volume currents (i.e., the spread of currents across the brain), while MEG less so [1].</p>

<p>Because of this distinction, while two components can have the same latency — let's say the face-specific N170 and the M170 — the sources originating them might be different. Naming the components differently stresses this important distinction.</p>

<p>For reference, I recommend the following articles and books</p>

<ol>
<li>Baillet, S., Mosher, J. C., &amp; Leahy, R. M. (2001). Electromagnetic brain mapping. Signal Processing Magazine, IEEE, 18(6), 14–30.</li>
<li>Hansen, P., Kringelbach, M., &amp; Salmelin, R. (2010). MEG:An Introduction to Methods. Oxford University Press, USA.</li>
</ol>

CC BY-SA 3.0
2016-02-25T23:22:54.130Z
Still doesn't answer why the prefix of MEG components doesn't indicate polarity. This will remain one of the deep mysteries of neuroscience
K A
CC BY-SA 3.0
2016-02-27T13:38:23.680Z
I guess that would be because there is no negative or positive magnetic field
Joe Bathelt
CC BY-SA 3.0
2016-02-25T13:42:33.957Z
2016-02-25T13:42:33.957Z
mvdoc
<p>Apart from making the distinction between recording techniques (talking about N170 and you refer to ERP), here is an explanation:</p>

<p><strong>Naming convention refers more to neurophysiology than waveform (component) directions.</strong></p>

<p>The standard tool modeling the EEG/MEG activity is a <strong>dipole</strong>. A dipole consists of a pair of a negative and positive charges and it <strong>generates both an electric and a magnetic field</strong> (see figure). You can see in the electric field panel that the positive activity is ahead of the positive charge, here the head of the arrow (on the contrary, the magnetic field is oriented on the side of the dipole). Therefore, a positivity in an ERP signal (P300 for example) corresponds to an increase of the dipole activity measured ahead of the positive charge (resp. negative for N400). </p>

<p>Here, we can say the electrical positive P300 component (or waveform) is neurophysiologicaly a positive electrical activity. </p>

<p>On the contrary, the magnetic main activity does not follow the main direction of the dipole but its perpendicular one. Therefore, we can't relate an increase in a magnetic field to the positive/negative direction of the dipole. </p>

<p>Here, a magnetic N170 would not be grounded on neurophysiological basis, but just in the fact that N only refers to a negative on-going measure.</p>

<p><a href="https://i.sstatic.net/xH8v5.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/xH8v5.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2016-02-26T01:36:16.860Z
2016-02-26T08:13:58.810Z
2016-02-26T08:13:58.810Z
marsei
<p>My question is why we use P and N for naming positive and negative EEG component(for instance N170 or P100) but for MEG data we use M in both case, I mean M170 and M100 in this example?</p>

<cognitive-neuroscience><measurement><eeg><meg>
Component naming of MEG vs. EEG
Sona
# Component naming of MEG vs. EEG

(Asked by: username_0 on 2016-02-22)

My question is why we use P and N for naming positive and negative EEG component(for instance N170 or P100) but for MEG data we use M in both case, I mean M170 and M100 in this example?

**Tags:** cognitive-neuroscience, measurement, eeg, meg


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## 2 answers

### Answer by username_1 on 2016-02-25. Score: 4

EEG and MEG measure two different but related signals. While EEG records the electric activity from the scalp, MEG records changes in the magnetic field induced by electric activity in the brain. Because of this, MEG is largely insensitive to sources oriented radially, but can pick up sources oriented tangentially to the cortex. Moreover, EEG is sensitive to volume currents (i.e., the spread of currents across the brain), while MEG less so [1].

Because of this distinction, while two components can have the same latency — let's say the face-specific N170 and the M170 — the sources originating them might be different. Naming the components differently stresses this important distinction.

For reference, I recommend the following articles and books

1. Baillet, S., Mosher, J. C., & Leahy, R. M. (2001). Electromagnetic brain mapping. Signal Processing Magazine, IEEE, 18(6), 14–30.
2. Hansen, P., Kringelbach, M., & Salmelin, R. (2010). MEG:An Introduction to Methods. Oxford University Press, USA.

> **Comments:**
> **username_2**: Still doesn't answer why the prefix of MEG components doesn't indicate polarity. This will remain one of the deep mysteries of neuroscience  
> **username_3**: I guess that would be because there is no negative or positive magnetic field  

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### Answer by username_4 on 2016-02-26. Score: 2

Apart from making the distinction between recording techniques (talking about N170 and you refer to ERP), here is an explanation:

**Naming convention refers more to neurophysiology than waveform (component) directions.**

The standard tool modeling the EEG/MEG activity is a **dipole**. A dipole consists of a pair of a negative and positive charges and it **generates both an electric and a magnetic field** (see figure). You can see in the electric field panel that the positive activity is ahead of the positive charge, here the head of the arrow (on the contrary, the magnetic field is oriented on the side of the dipole). Therefore, a positivity in an ERP signal (P300 for example) corresponds to an increase of the dipole activity measured ahead of the positive charge (resp. negative for N400).

Here, we can say the electrical positive P300 component (or waveform) is neurophysiologicaly a positive electrical activity.

On the contrary, the magnetic main activity does not follow the main direction of the dipole but its perpendicular one. Therefore, we can't relate an increase in a magnetic field to the positive/negative direction of the dipole.

Here, a magnetic N170 would not be grounded on neurophysiological basis, but just in the fact that N only refers to a negative on-going measure.

[![enter image description here](https://i.sstatic.net/xH8v5.png)](https://i.sstatic.net/xH8v5.png)


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