Neuroscientists have long considered extracellular electric fields in the brain to be too weak to affect the day to day functioning of the organ. A new study out of Caltech now shows that in reality these seemingly stray electric fields actually alter the functioning of neurons, giving scientists another puzzle to work out.

Extracellular electric fields exist throughout the living brain. Their distant echoes can be measured outside the skull as EEG waves. These fields are particularly strong and robustly repetitive in specific brain regions such as the hippocampus, which is involved in memory formation, and the neocortex, the area where long-term memories are held.
Previously, neurobiologists assumed that the fields were capable of affecting—and even controlling—neural activity only during severe pathological conditions such as epileptic seizures, which induce very strong fields. Few studies, however, had actually assessed the impact of far weaker—but very common—non-epileptic fields. "The reason is simple," Anastassiou [Costas Anastassiou, postdoc at Caltech] says. "It is very hard to conduct an in vivo experiment in the absence of extracellular fields," to observe what changes when the fields are not around.
To tease out those effects, Anastassiou and his colleagues focused on strong but slowly oscillating fields, called local field potentials (LFP), that arise from neural circuits composed of just a few rat brain cells. Measuring those fields and their effects required positioning a cluster of tiny electrodes within a volume equivalent to that of a single cell body—and at distances of less than 50 millionths of a meter from one another; this is approximately the width of a human hair.
An "unexpected and surprising finding was how already very weak extracellular fields can alter neural activity," he says. "For example, we observed that fields as weak as one volt per meter robustly alter the spiking activity [firing] of individual neurons, and increase the so-called ‘spike-field coherence’"—the synchronicity with which neurons fire. "Inside the mammalian brain, we know that extracellular fields may easily exceed two to three volts per meter. Our findings suggest that under such conditions, this effect becomes significant."
What does that mean for brain computation? At this point we can only speculate, Koch [Christof Koch, Professor of Cognitive and Behavioral Biology and professor of computation and neural systems at Caltech]says, "but such field effects increase the synchrony with which neurons become active together. This, by itself, enhances the ability of these neurons to influence their target and is probably an important communication and computation strategy used by the brain."

Abstract in Nature Neuroscience: Ephaptic coupling of cortical neurons
Link at Caltech: Neurobiologists Find that Weak Electrical Fields in the Brain Help Neurons Fire Together

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