Local field potentials (LFP) are transient electrical signals generated in
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...
s and other tissues by the summed and synchronous electrical activity of the individual cells (e.g. neurons) in that tissue. LFP are "extracellular" signals, meaning that they are generated by transient imbalances in ion concentrations in the spaces outside the cells, that result from cellular electrical activity. LFP are 'local' because they are recorded by an electrode placed nearby the generating cells. As a result of the
Inverse-square law
In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental ca ...
, such electrodes can only 'see' potentials in a spatially limited radius. They are 'potentials' because they are generated by the voltage that results from charge separation in the extracellular space. They are 'field' because those extracellular charge separations essentially create a local electric field. LFP are typically recorded with a high-impedance
microelectrode placed in the midst of the population of cells generating it. They can be recorded, for example, via a microelectrode placed in the
brain
The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
of a human or animal subject, or in an
in vitro
''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
brain
thin slice.
Background
During local field potential recordings, a signal is recorded using an
extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
microelectrode placed sufficiently far from individual local
neurons
A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
to prevent any particular
cell from dominating the electrophysiological signal. This signal is then
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
ed, cut off at ~300
Hz, to obtain the local field potential (LFP) that can be recorded electronically or displayed on an
oscilloscope
An oscilloscope (formerly known as an oscillograph, informally scope or O-scope) is a type of electronic test instrument that graphically displays varying voltages of one or more signals as a function of time. Their main purpose is capturing i ...
for analysis. The low impedance and positioning of the
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
allows the activity of a large number of neurons to contribute to the signal. The unfiltered signal reflects the sum of action potentials from cells within approximately 50-350 μm from the tip of the electrode
and slower ionic events from within 0.5–3 mm from the tip of the electrode.
The low-pass filter removes the
spike component of the signal and passes the lower
frequency
Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
signal, the LFP.
The voltmeter or analog-to-digital converter to which the microelectrode is connected measures the
electrical potential difference (measured in
volts
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI).
Definition
One volt is defined as the electric potential between two point ...
) between the microelectrode and a reference electrode. One end of the reference electrode is also connected to the voltmeter while the other end is placed in a medium which is continuous with, and compositionally identical to the extracellular medium. In a simple
fluid
In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
, with no
biological component present, there would be slight fluctuations in the measured potential difference around an
equilibrium point
In mathematics, specifically in differential equations, an equilibrium point is a constant solution to a differential equation.
Formal definition
The point \tilde\in \mathbb^n is an equilibrium point for the differential equation
:\frac = ...
, this is known as the
thermal noise
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
. This is due to the random movement of ions in the medium and electrons in the electrode. However, when placed in
neural tissue the opening of an ion channel results in the net flow of ions into the cell from the extracellular medium, or out of the cell into the extracellular medium. These local currents result in larger changes in the electrical potential between the local extracellular medium and the interior of the recording electrode. The overall recorded signal thus represents the potential caused by the sum of all local currents on the surface of the electrode.
Synchronised input

The local field potential is believed to represent the sum of synaptic inputs into the observed area, as opposed to the
spikes
The SPIKES protocol is a method used in clinical medicine to break bad news to patients and families. As receiving bad news can cause distress and anxiety, clinicians need to deliver the news carefully. Using the SPIKES method for introducing and ...
, which represents the output from the area. The fast fluctuations are mostly caused by the short inward and outward currents of action potentials, while the LFP is composed of the more sustained currents in the tissue that are generated by synaptic activity (
EPSCs and
IPSCs).
Data-driven models have shown a predictive relationship between the LFPs and spike activity.
A common method to investigate LFP oscillations that lead to spikes is to calculate spike-triggered averages (see figure). This is done after the recording (off line) by detecting the spikes as fast downward deflections, cutting out the temporal sections around the spike (+/- 250 ms) and averaging the spike-aligned traces for each recording site.
Alternatively, spikes can be removed from the extracellular recording traces by low-pass filtering, revealing the LFP.
Geometrical arrangement
Which cells contribute to the slow field variations is determined by the geometric configuration of the cells themselves. In some cells, the dendrites face one direction and the
soma another, such as the
pyramidal cells
Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal cells are the primary excitation units of the mammalian prefrontal cort ...
. This is known as an open field geometrical arrangement. When there is simultaneous activation of the dendrites a strong
dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
* An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
is produced. In cells where the
dendrites
A dendrite (from Greek δένδρον ''déndron'', "tree") or dendron is a branched cytoplasmic process that extends from a nerve cell that propagates the electrochemical stimulation received from other neural cells to the cell body, or soma ...
are arranged more
radial
Radial is a geometric term of location which may refer to:
Mathematics and Direction
* Vector (geometric), a line
* Radius, adjective form of
* Radial distance (geometry), a directional coordinate in a polar coordinate system
* Radial set
* A ...
ly, the potential difference between individual dendrites and the soma tend to cancel out with diametrically opposite dendrites, this configuration is called a closed field geometrical arrangement. As a result the net potential difference over the whole cell when the dendrites are simultaneously activated tends to be very small. Thus changes in the local field potential represent simultaneous dendritic events in cells in the open field configuration.
Simple interpretation of LFP
Interpreting LFP through the characteristics of neuronal activity remains a challenge. At the very least, it is clear that electrically compact neurons do not contribute to LFP. Consequently, the minimal model for calculating LFP is a two-compartment model. According to this model, the LFP is determined by the current flowing between the dendritic and somatic compartments. The synaptic component of this current is approximately proportional to the difference between the dendritic and somatic membrane potentials and is combined with the spiking component.
Low-pass filtering of extracellular space
Part of the
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
ing giving rise to local field potentials is due to complex electrical properties of extracellular space.
The fact that the extracellular space is not homogeneous, and is composed of a complex aggregate of highly
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...
fluids and low-conductive and
capacitive membranes, can exert strong low-pass filtering properties. Ionic
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
, which plays an important role in membrane potential variations, can also act as a low-pass filter.
References
{{Reflist
External links
Mechanisms of local field potentials (Scholarpedia)
Electrophysiology
Action potentials