Following our analogy, if a single-unit spike is one person speaking clearly, and multi-unit activity (MUA) is the chaotic chatter of the 50 people closest to the microphone, then the Local Field Potential (LFP) is the slow, synchronized rumble of that entire section of the stadium.

You aren't listening to individual voices at all anymore. Instead, you are feeling the collective "mood" of the crowd as they move and react together.

Here is what LFP is, where it comes from, and how it fits into the broader picture of brain recording.

1. The Source: Inputs vs. Outputs

To understand LFP, we have to distinguish between what a neuron receives and what it sends.

• Spikes (Single/Multi-Unit): These are the outputs. They are the sharp, sudden electrical flashes (action potentials) a neuron fires down its axon to talk to other cells.
• LFP: This represents the inputs. When thousands of neighboring neurons receive chemical signals from other parts of the brain, their membranes slowly charge and discharge together. LFP is the electrical measurement of these synchronized, slow-rolling synaptic inputs.

2. How We Extract It: The Split

Just like MUA, LFP comes from the exact same microelectrode implanted in the brain. The difference lies entirely in how we filter the raw signal.

Filtering splits the raw signal into LFP (slow) and Spikes (fast). Source: ResearchGate

When the raw electrical signal hits the computer, it goes through a digital fork in the road:

• The High-Pass Filter: Blocks everything below ~300 Hz. This isolates the fast, sharp spikes for MUA or single-unit sorting.
• The Low-Pass Filter: Blocks everything above ~300 Hz. This isolates the slow, rolling waves. This is the LFP.

See how different filtering changes the data you extract from a single raw recording:

3. LFP vs. EEG (The Macro View)

You might be thinking, "Slow, rolling brain waves? Isn't that just an EEG?"

They are closely related, but the difference is scale and placement.

EEG reads from outside the skull, LFP reads from inside the tissue. Source: VectorMine / Getty Images