Merge pull request #51 from neural-data-science/copilot/nesc-3505-fix-eeg-section

Fix factual error: depolarization makes cell body less negative, not more

Author: aaronjnewman

7-eeg/about_eeg.md

@@ -42,7 +42,7 @@ width: 400px
 Pyramidal neurons in the cerebral cortex. The cell bodies are the large black triangle shapes, with their axons extending upward in the image, towards the outer surface of the cerebral cortex. Drawing by Ramon y Cajal. 
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-At rest, the inside of a neuron's cell body has a negative potential. This means that, relative to the ends of the axons, the cell body is more negative, and the ends of the axons are more positive. Thus if we inserted electrodes into the brain, and placed one near the cell body and one near the end of the axon, we would measure a positive potential at the end of the axon, and a negative potential at the cell body.  When inputs arrive at the cell body of a neuron, they tend to cause it to *depolarize*, meaning that the cell body will become more negative. When we do EEG, we place the electrodes outside of the head, but since the head is a good conductor of electricity, we get essentially the same effect: an electrode directly over the patch of cortex under consideration would reflect a positive potential, relative to an electrode placed on the opposite side of the head, which would have a relatively negative potential. Another way of thinking about this is that any source of electrical activity in the brain can be modeled as an *electrical dipole*, as shown in the figure below. A dipole has positive and negative poles, and the strength of activation of the brain area is the difference in electrical potential between the poles. Electrodes placed above the dipole in the figure would measure a positive potential (the arrowhead represents the positive pole), and electrodes placed below the dipole would measure a negative potential.
+At rest, the inside of a neuron's cell body has a negative potential. This means that, relative to the ends of the axons, the cell body is more negative, and the ends of the axons are more positive. Thus if we inserted electrodes into the brain, and placed one near the cell body and one near the end of the axon, we would measure a positive potential at the end of the axon, and a negative potential at the cell body.  When inputs arrive at the cell body of a neuron, they tend to cause it to *depolarize*, meaning that the cell body will become less negative. When we do EEG, we place the electrodes outside of the head, but since the head is a good conductor of electricity, we get essentially the same effect: an electrode directly over the patch of cortex under consideration would reflect a positive potential, relative to an electrode placed on the opposite side of the head, which would have a relatively negative potential. Another way of thinking about this is that any source of electrical activity in the brain can be modeled as an *electrical dipole*, as shown in the figure below. A dipole has positive and negative poles, and the strength of activation of the brain area is the difference in electrical potential between the poles. Electrodes placed above the dipole in the figure would measure a positive potential (the arrowhead represents the positive pole), and electrodes placed below the dipole would measure a negative potential.
 
 ```{figure} images/dipole_head.svg
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