Local field potential
Encyclopedia
A local field potential (LFP) is a particular class of electrophysiological signals, which is dominated by the electrical current flowing from all nearby dendritic synaptic activity
within a volume of tissue
. A voltage is produced by the summed synaptic current flowing across the resistance of the local extracellular space. In this situation, "potential" refers to electrical potential, or voltage, and particularly to voltage recorded with a very small electrode embedded within neuronal tissue, typically in the brain of an anesthetized animal or within a thin slice of brain tissue maintained in vitro
.
extracellular
microelectrode, placed sufficiently far from individual local neurons to prevent any particular cell
from dominating the electrophysiological signal. This signal is then low-pass filter
ed, cut off at ~300 Hz
, to obtain the local field potential (LFP), that can be recorded electronically or displayed on an oscilloscope
for analysis. The low impedance and positioning of the electrode
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 (Legatt 1980; Gray 1995) and slower ionic events from within 0.5-3 mm from the tip of the electrode (Juergens 1999). The low-pass filter removes the spike
component of the signal and passes the lower frequency
signal, the LFP.
The voltmeter or analog-to-digital converter to which the microelectrode is connected measures the electrical potential difference (measured in volts) 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
, with no biological component present, there would be slight fluctuations
in the measured potential difference around an equilibrium point, this is known as the thermal noise. This is due to the random movement of ions in the medium and electrons in the electrode. However, 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.
data, which represents the output from the area. In the LFP, quick fluctuations in the potential difference are filtered out, leaving only the slower fluctuations. The quick fluctuations are caused by the short inward and outward currents of the action potential. Therefore the action potential plays no part in the LFP. The LFP is thus composed of the more sustained currents in the tissue, typical of the somato
-dendritic currents. The major slow current is the postsynaptic potential
(PSP). It was originally thought that EPSP
s and IPSP
s were the exclusive constituents of LFPs, but phenomena unrelated to synaptic events were later found to contribute to the signal (Kobayashi 1997; Kamondi 1998).
another, such as the pyramidal cells. This is known as an open field geometrical arrangement. When there is simultaneous activation of the dendrites a strong dipole
is produced. In cells where the dendrites are arranged more radial
ly, the potential difference between individual dendrites and the soma tend to cancel with diametrically opposite dendrites. 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.
Chemical synapse
Chemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie...
within a volume of tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
. A voltage is produced by the summed synaptic current flowing across the resistance of the local extracellular space. In this situation, "potential" refers to electrical potential, or voltage, and particularly to voltage recorded with a very small electrode embedded within neuronal tissue, typically in the brain of an anesthetized animal or within a thin slice of brain tissue maintained in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
.
Background
A signal is recorded using a low impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
microelectrode, placed sufficiently far from individual local neurons to prevent any particular cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
from dominating the electrophysiological signal. This signal is then low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
ed, cut off at ~300 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, to obtain the local field potential (LFP), that can be recorded electronically or displayed on an oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
for analysis. The low impedance and positioning of the electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
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 (Legatt 1980; Gray 1995) and slower ionic events from within 0.5-3 mm from the tip of the electrode (Juergens 1999). The low-pass filter removes the spike
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
component of the signal and passes the lower frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
signal, the LFP.
The voltmeter or analog-to-digital converter to which the microelectrode is connected measures the electrical potential difference (measured in volts) 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
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
, with no biological component present, there would be slight fluctuations
Fluctuations
Fluctuations may refer to:* Quantum fluctuations arising from the uncertainty principle* Primordial fluctuations, density variations in the early universe* Statistical fluctuations, very important in statistics, statistical mechanics, and thermodynamics...
in the measured potential difference around an equilibrium point, this is known as the thermal noise. This is due to the random movement of ions in the medium and electrons in the electrode. However, 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 synchronised input into the observed area, as opposed to the spikeVoltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
data, which represents the output from the area. In the LFP, quick fluctuations in the potential difference are filtered out, leaving only the slower fluctuations. The quick fluctuations are caused by the short inward and outward currents of the action potential. Therefore the action potential plays no part in the LFP. The LFP is thus composed of the more sustained currents in the tissue, typical of the somato
Somatology
Somatology is defined as the study or science of the human body as a branch of anthropology. This also includes the study of material substances, as in physics, chemistry, biology, botany which are under the general heading of physicalism....
-dendritic currents. The major slow current is the postsynaptic potential
Postsynaptic potential
Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials...
(PSP). It was originally thought that EPSP
Excitatory postsynaptic potential
In neuroscience, an excitatory postsynaptic potential is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels...
s and IPSP
Inhibitory postsynaptic potential
An inhibitory postsynaptic potential is a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron or α-motoneuron...
s were the exclusive constituents of LFPs, but phenomena unrelated to synaptic events were later found to contribute to the signal (Kobayashi 1997; Kamondi 1998).
Geometrical arrangement
Cells which contribute to the slow field variations are determined by the geometric configuration of the cells themselves. In some cells the dendrites face one direction and the somaSoma
Soma , or Haoma , from Proto-Indo-Iranian *sauma-, was a ritual drink of importance among the early Indo-Iranians, and the subsequent Vedic and greater Persian cultures. It is frequently mentioned in the Rigveda, whose Soma Mandala contains 114 hymns, many praising its energizing qualities...
another, such as the pyramidal cells. This is known as an open field geometrical arrangement. When there is simultaneous activation of the dendrites a strong dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
is produced. In cells where the dendrites are arranged more radial
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
ly, the potential difference between individual dendrites and the soma tend to cancel with diametrically opposite dendrites. 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.