The highest resolution neural recording technique, capturing the individual action potentials (spikes) of specific neurons.
Key takeaway: Unlike EEG, ECoG, and LFP—which capture the blended, continuous analogue waveforms of thousands of neurons—SUA isolates the discrete, digital "all-or-nothing" firing events of individual neurons right at their source. Multi-Unit Activity (MUA) is recorded when an electrode is close enough to capture spikes, but not close enough to distinguish which specific cell fired which spike.
Physiological Basis
Extracellular Action PotentialsEavesdropping on neural communication.
When a neuron fires, it opens ion channels, causing a massive local influx of sodium (Na+) followed by an efflux of potassium (K+). This creates a highly localized, rapidly changing voltage field lasting about 1 millisecond.
An extracellular microelectrode placed within ~50-100 microns of the cell body can detect this sharp voltage transient as a brief "spike" atop the background noise.
Spike SortingDisentangling the chorus.
Often, a single microelectrode can "hear" 2 to 5 overlapping neurons firing.
Because each neuron has a slightly different morphology and sits at a different distance from the tip, its spike will have a unique waveform (shape and amplitude). Spike sorting algorithms use PCA (Principal Component Analysis) or advanced clustering to group these waveforms, separating the MUA signal into distinct SUA channels.
Hardware Technologies
Microelectrode Arrays (MEAs)The standard penetrators.
Utah Array: A rigid silicon grid with 100 micro-needles, each 1-1.5mm long, designed to reach the motor cortex layer V (where large Betz cells reside). Widely used in successful human BrainGate trials.
Microwire Arrays: Bundles of ultra-thin insulated metal wires (Tungsten or Platinum-Iridium) that can be splayed out into deep brain structures.
High-Density Silicon Probes (Neuropixels)Next-generation linear arrays.
New silicon probes (like Neuropixels) pack nearly 1,000 recording sites along a probe the thickness of a human hair.
They can simultaneously record thousands of individual neurons across multiple brain regions (e.g., from the cortex deep into the thalamus and hippocampus simultaneously in animal models).
Challenges & Tradeoffs
The Foreign Body Response (FBR)The enemy of chronic recording.
Inserting rigid electrodes into soft brain tissue causes acute trauma (damaging the blood-brain barrier) followed by a chronic immune response led by microglia and astrocytes.
Over months or years, these cells encapsulate the electrode tip in dense glial scar tissue, electrically insulating it and pushing neurons out of the 100-micron "recording radius," eventually causing the loss of the SUA signal.
Interactive Spike Sorting (MUA to SUA)
A single microelectrode often records the noisy chorus of several local neurons (Multi-Unit Activity). By extracting each spike's unique waveform signature (Amplitude vs Width), algorithms can cluster them into distinct single-neuron sources (Single-Unit Activity).