The third meeting took place at ELSC, Edmond J Safra campus, on 13.02.2020. Vitaly (one of the group's participants) lead the discussion about Local Field Potentials (LFPs), focusing on the issues of interpreting the meaning of these potentials.

To introduce the topic, we discussed "Action Currents" as measured from frog-leg nerve bundles in the 1920's. In such cases, where the measured potential is entirely due to a bundle of axons being active, the interpretation of the field is relatively simple, as it is solely related to Action Potentials (APs) traveling down the nerve bundle.

Another system in which LFPs have been extensively studied is the hippocampus: studies performed on this region take advantage of the relatively simple anatomical organization (pyramidal cells are arranged in a layer, with dendritic trees pointing upwards) and the possibility to perform pharmacological experiments. Such experiments for example showed that Na-currents are not the only ones involved in generating the LFP response, as the potential disappeared only partially after applying TTX (a blocker of Na channels).

We next discussed the concept of "sinks" and "sources" in the context of the currents that generate LFPs. If we focus on a small unit of space next to the axon(s) that are active, this unit of space is a source (of current) if ions are flowing out of it, or a sink if ions are flowing into it. Notably, 'ions' in this case can be either positively or negatively charged (for example, Na+ or Cl-). However, negative current flowing out or positive current flowing in are going to have very similar effects on the local field. This results in what is known as the "inverse problem" – that is: while it's straightforward to deduce what voltage changes will be caused by known current flows, it's usually impossible to do the opposite (determining what current flows caused a measured change in voltage). We discussed several examples of this issue (e.g. Sharp Wave Ripples in the hippocampus).

Finally, we discussed work from the lab of Ilan Lampl (Weizmann Institute), which showed that volume conductance in the brain can underlie the recorded signals from very distant areas. They showed that in anesthetized mice, the field response to a stimulation applied exclusively to the whiskers travels from the barrel (somatosensory) cortex all the way to the olfactory bulb via the volume of the brain in between those areas. The takeaway: the "local" field potential is not really all that local.

For the next meeting we decided to learn about some of the cognitive sides of oscillations.

 

Participants:

Ruoyi Cao
Inbar Kotzer
Nora Vrieler
Noam Shwartz
Moran Aharoni
Noa Rahamim
Gal Vishne
Lena Gorina
Lorenzo Guarnieri
Vitaly Lerner