[5] There are many techniques available to record brain activity—including electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI)—but these do not allow for single-neuron resolution.
[6] Neurons are the basic functional units in the brain; they transmit information through the body using electrical signals called action potentials.
As an action potential propagates through the cell, the electric current flows in and out of the soma and axons at excitable membrane regions.
Intracellular single-unit recordings occur within the neuron and measure the voltage change (with respect to time) across the membrane during action potentials.
Fine tips allow for easy penetration without extensive damage to the cell, but they also correlate with high impedance.
[3] Single-unit recordings have provided tools to explore the brain and apply this knowledge to current technologies.
BMIs record brain signals and decode an intended response, which then controls the movement of an external device (such as a computer cursor or prosthetic limb).
Since then, single unit recordings have become an important method for understanding mechanisms and functions of the nervous system.
When a microelectrode is inserted into an aqueous ionic solution, there is a tendency for cations and anions to react with the electrode creating an electrode-electrolyte interface.
The charges orient at the interface to create an electric double layer; the metal then acts like a capacitor.
Single unit recordings from the cortical regions of rodent models have been shown to dependent on the depth at which the microelectrode sites were located.
[28] When comparing anestheized vs. awake states, single unit activity in rodent models under 2% isoflurane has shown to lower the noise level in the neurological recordings; even though the awake state recordings showed a 14% increase in peak-to-peak voltage magnitude.
With Ag-AgCl electrodes, ions react with it to produce electrical gradients at the interface, creating a voltage change with respect to time.
The small tips make it easy to penetrate the cell membrane with minimal damage for intracellular recordings.
Metal electrodes are beneficial in some cases because they have high signal-to-noise due to lower impedance for the frequency range of spike signals.
"They normally give stable recordings, a high signal-to-noise ratio, good isolation, and they are quite rugged in the usual tip sizes".
Silicon technology provides better mechanical stiffness and is a good supporting carrier to allow for multiple recording sites on a single electrode.
Single unit recording methods give high spatial and temporal resolution to allow for information assessing the relationship between brain structure, function, and behavior.
For example, Boraud et al. report the use of single unit recordings to determine the structural organization of the basal ganglia in patients with Parkinson's disease.
This technology has potential to reach a wide variety of patients but is not yet available clinically due to lack of reliability in recording signals over time.
The primary hypothesis regarding this failure is that the chronic inflammatory response around the electrode causes neurodegeneration that reduces the number of neurons it is able to record from (Nicolelis, 2001).
[33] In 2004, the BrainGate pilot clinical trial was initiated to "test the safety and feasibility of a neural interface system based on an intracortical 100-electrode silicon recording array".
This initiative has been successful in advancement of BCIs and in 2011, published data showing long term computer control in a patient with tetraplegia (Simeral, 2011).