Hi Kevin,

you need to keep in mind, that in ERPs, a negative peak does not indicate inhibition or reduced activity. You can think of the generators of ERPs as dipoles with a positive and a negative side, and it's the amplitude of the activity, not the sign that indicates strength of activation.

I hope this helps - if not, just let me know.

Best,

Julian

 Julian, could you clarify your comment?

The "ERP generators" are populations of neurons, no? If there is a negativity in recording these neurons doesn't that mean that the neurons are receiving inhibitory inputs? The amplitude of which would seem to indicate the strength of that inhibition. 

Hi Kevin,

you are right, the ERP generators are populations of (presumably pyramidal) neurons. The signal you record in the EEG is the electrical potential generated along the axons of these neurons. As in any electrical potenial, you have a positive pole and a negative pole (hence dipole). Now, what you have to keep in mind is that the negative pole does not mean that this part of the dipole is inhibited, it's rather one side of the activated dipole. Thus, the sign (positivity or negativity) in EEG does not convey a lot of information. The absolute amplitude of signal change is much more relevant.

I assume your questions comes from the findings of single cell recordings: Here a negative membrane potential can indicate inhibition of this neuron. However, this does not transfer to EEG.

So, in short: No, negativity in EEG recordings does not indicate inhibitory inputs.

Best,

Julian

P.S.: I highly recommend the ERP resources by Steven J. Luck (http://erpinfo.org/Resources) who also wrote an excellent boon on the basics of ERPs