Understanding brain activity is necessary to reveal the mechanisms behind thoughts, perception, and memory. Scientists monitor neural communications using techniques like Local Field Potentials (LFPs). LFPs offer a window into how large communities of neurons coordinate their activity, providing insight into the collective electrical “hum” generated by many cells working together.
What Exactly Are Local Field Potentials?
Local Field Potentials are voltage fluctuations recorded from the brain’s extracellular space. This measurement reflects the summed electrical activity within a small volume of brain tissue, typically encompassing a few cubic millimeters. LFPs represent the input and processing activity of a neural circuit, rather than the final output signal.
The LFP signal is fundamentally different from the action potential, often called a “spike,” which is the brief, high-speed electrical impulse used by a single neuron to transmit information. The spike is a fast, high-frequency event. In contrast, the LFP is a slow, low-frequency signal, analogous to the collective murmur of a large crowd. This collective electrical activity is typically filtered to include frequencies below about 300 Hertz.
LFPs are comprised of the synchronous electrical effects of thousands of neurons in the vicinity of the recording electrode. Because they capture the net activity of a neural population, LFPs are considered a measure of synaptic processing rather than the final output of the cells. The amplitude of the LFP provides information about the level of synchronization among the local neural network.
How Synaptic Activity Creates the Signal
The Local Field Potential is generated by ionic current flows across the membranes of many neurons, primarily driven by synaptic communication. When a neuron receives a chemical signal, ion channels open or close at the synapse, causing ions to move across the cell membrane. This movement creates tiny electrical currents that flow into or out of the cell.
These transmembrane currents do not cancel out completely and instead create subtle voltage gradients in the fluid surrounding the neurons. Specifically, the region where ions flow into the cell, such as at an excitatory synapse on a dendrite, is called a “current sink.” This inward current is balanced by a passive outward current that flows out of the cell’s membrane elsewhere, typically near the cell body, which is called a “current source.”
This separation of current sinks and sources along the neuron’s structure, particularly the long dendrites of pyramidal cells in the cortex, establishes an electrical dipole. The electrical fields created by thousands of these dipoles sum together in the extracellular space. The LFP electrode measures this summed potential difference between its tip and a distant reference point.
Both excitatory and inhibitory synaptic inputs contribute to the measured potential. Neurons that are geometrically aligned, such as those forming the layered structure of the hippocampus and cortex, generate larger, more easily measured LFP signals because their electrical fields sum up more effectively.
Capturing Brain Rhythms
Scientists use specialized tools, primarily small microelectrodes, to capture and isolate the LFP signal from the brain’s overall electrical activity. These electrodes are placed directly into the brain tissue to detect voltage fluctuations in the extracellular fluid within a specific region.
The raw signal picked up by the electrode is complex, containing both rapid-fire action potentials and slower LFP fluctuations. To isolate the LFP, researchers employ frequency filtering, separating the different frequency components of the signal.
The LFP is defined by its low-frequency components, typically falling within the 0.5 to 300 Hertz range. By applying a low-pass filter, the high-frequency components are removed. This leaves the slower, summated synaptic potentials that characterize the LFP. The resulting signal can then be analyzed for its rhythmic patterns, known as brain oscillations.
What the Patterns Reveal About Brain Function
The rhythmic oscillations measured in the LFP are categorized into distinct frequency bands. Analyzing the power and synchronization of these bands allows researchers to link population-level neural activity to specific functions. The lowest frequency band, Delta (0.5–3.5 Hz), is associated with deep, restorative sleep. It is also hypothesized to reflect a mechanism of cortical inhibition that blocks out sensory interference during intense internal concentration.
Slightly faster is the Theta band (4–8 Hz), which is linked to memory formation, spatial navigation, and learning. Theta activity often increases during explorative behavior. The Alpha band (8–12 Hz) is often observed when a person is in a relaxed, non-aroused state with their eyes closed. Alpha is thought to play a role in suppressing or actively inhibiting irrelevant brain regions, helping to focus attention.
The Beta band (13–30 Hz) is typically associated with active thinking, problem-solving, and motor control. Beta power is maintained during the sustained posture of a limb but is suppressed or “desynchronized” just before and during active movement. The high-frequency Gamma band (>30 Hz, extending up to 150 Hz or more) is linked to higher-order cognitive processes. Gamma activity plays a role in conscious perception, attention, and the integration of information across different brain areas, often correlating with working memory and active processing.