Single-unit recording
In neuroscience, single-unit recordings provide a method of measuring the electro-physiological responses of a single neuron using a microelectrode system. When a neuron generates an action potential, the signal propagates down the neuron as a current which flows in and out of the cell through excitable membrane regions in the soma and axon. A microelectrode is inserted into the brain, where it can record the rate of change in voltage with respect to time. These microelectrodes must be fine-tipped, impedance matching; they are primarily glass micro-pipettes, metal microelectrodes made of platinum, tungsten, iridium or even iridium oxide. Microelectrodes can be carefully placed close to the cell membrane, allowing the ability to record extracellularly.
Single-unit recordings are widely used in cognitive science, where it permits the analysis of human cognition and cortical mapping. This information can then be applied to brain–machine interface technologies for brain control of external devices.
Overview
There are many techniques available to record brain activity—including electroencephalography, magnetoencephalography, and functional magnetic resonance imaging —but these do not allow for single-neuron resolution. Neurons are the basic functional units in the brain; they transmit information through the body using electrical signals called action potentials. Currently, single-unit recordings provide the most precise recordings from a single neuron. A single unit is defined as a single, firing neuron whose spike potentials are distinctly isolated by a recording microelectrode.The ability to record signals from neurons is centered around the electric current flow through the neuron. As an action potential propagates through the cell, the electric current flows in and out of the soma and axons at excitable membrane regions. This current creates a measurable, changing voltage potential within the cell. This allows for two basic types of single-unit recordings. Intracellular single-unit recordings occur within the neuron and measure the voltage change across the membrane during action potentials. This outputs as a trace with information on membrane resting potential, postsynaptic potentials and spikes through the soma. Alternatively, when the microelectrode is close to the cell surface extracellular recordings measure the voltage change outside the cell, giving only spike information. Different types of microelectrodes can be used for single-unit recordings; they are typically high-impedance, fine-tipped and conductive. Fine tips allow for easy penetration without extensive damage to the cell, but they also correlate with high impedance. Additionally, electrical and/or ionic conductivity allow for recordings from both non-polarizable and polarizable electrodes. The two primary classes of electrodes are glass micropipettes and metal electrodes. Electrolyte-filled glass micropipettes are mainly used for intracellular single-unit recordings; metal electrodes and used for both types of recordings.
Single-unit recordings have provided tools to explore the brain and apply this knowledge to current technologies. Cognitive scientists have used single-unit recordings in the brains of animals and humans to study behaviors and functions. Electrodes can also be inserted into the brain of epileptic patients to determine the position of epileptic foci. More recently, single-unit recordings have been used in brain machine interfaces. BMIs record brain signals and decode an intended response, which then controls the movement of an external device.
History
The ability to record from single units started with the discovery that the nervous system has electrical properties. Since then, single unit recordings have become an important method for understanding mechanisms and functions of the nervous system. Over the years, single unit recording continued to provide insight on topographical mapping of the cortex. Eventual development of microelectrode arrays allowed recording from multiple units at a time.- 1790s: The first evidence of electrical activity in the nervous system was observed by Luigi Galvani in the 1790s with his studies on dissected frogs. He discovered that you can induce a dead frog leg to twitch with a spark.
- 1888: Santiago Ramón y Cajal, a Spanish neuroscientist, revolutionized neuroscience with his neuron theory, describing the structure of the nervous system and presence of basic functional units— neurons. He won the Nobel Prize in Physiology or Medicine for this work in 1906.
- 1928: One of the earliest accounts of being able to record from the nervous system was by Edgar Adrian in his 1928 publication "The Basis of Sensation". In this, he describes his recordings of electrical discharges in single nerve fibers using a Lippmann electrometer. He won the Nobel Prize in 1932 for his work revealing the function of neurons.
- 1940: Renshaw, Forbes & Morrison performed original studies recording discharge of pyramidal cells in the hippocampus using glass microelectrodes in cats.
- 1950: Woldring and Dirken report the ability to obtain spike activity from the surface of the cerebral cortex with platinum wires.
- 1952: Li and Jasper applied the Renshaw, Forbes, & Morrison method to study electrical activity in the cerebral cortex of a cat. Hodgkin–Huxley model was revealed, where they used a squid giant axon to determine the exact mechanism of action potentials.
- 1953: Iridium microelectrodes developed for recording.
- 1957: John Eccles used intracellular single-unit recording to study synaptic mechanisms in motoneurons.
- 1958: Stainless steel microelectrodes developed for recording.
- 1959: Studies by David H. Hubel and Torsten Wiesel. They used single neuron recordings to map the visual cortex in unanesthesized, unrestrained cats using tungsten electrodes. This work won them the Nobel Prize in 1981 for information processing in the visual system.
- 1960: Glass-insulated platinum microelectrodes developed for recording.
- 1967: The first record of multi-electrode arrays for recording was published by Marg and Adams. They applied this method to record many units at a single time in a single patient for diagnostic and therapeutic brain surgery.
- 1978: Schmidt et al. implanted chronic recording micro-cortical electrodes into the cortex of monkeys and showed that they could teach them to control neuronal firing rates, a key step to the possibility of recording neuronal signals and using them for BMIs.
- 1981: Kruger and Bach assemble 30 individual microelectrodes in a 5x6 configuration and implant the electrodes for simultaneous recording of multiple units.
- 1992: Development of the "Utah Intracortical Electrode Array, a multiple-electrode array which can access the columnar structure of the cerebral cortex for neurophysiological or neuroprosthetic applications".
- 1994: The Michigan array, a silicon planar electrode with multiple recording sites, was developed. NeuroNexus, a private neurotechnology company, is formed based on this technology.
- 1998: A key breakthrough for BMIs was achieved by Kennedy and Bakay with development of neurotrophic electrodes. In patients with amyotrophic lateral sclerosis, a neurological condition affecting the ability to control voluntary movement, they were able to successfully record action potentials using microelectrode arrays to control a computer cursor.
- 2016: Elon Musk co-founded and invested $100 million for Neuralink, which aims to develop ultra-high bandwidth BMIs. In 2019, he and Neuralink published their work followed by a live-stream press conference.
Electrophysiology
The basis of single-unit recordings relies on the ability to record electrical signals from neurons.Neuronal potentials and electrodes
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 forming of this layer has been termed the Helmholtz layer. A charge distribution occurs across the electrode, which creates a potential which can be measured against a reference electrode. The method of neuronal potential recording is dependent on the [|type] of electrode used. Non-polarizable electrodes are reversible. This creates a current flowing through the electrode, allowing for voltage measurement through the electrode with respect to time. Typically, non-polarizable electrodes are glass micropipettes filled with an ionic solution or metal. Alternatively, ideal polarized electrodes do not have the transformation of ions; these are typically metal electrodes. Instead, the ions and electrons at the surface of the metal become polarized with respect to the potential of the solution. The charges orient at the interface to create an electric double layer; the metal then acts like a capacitor. The change in capacitance with respect to time can be measured and converted to voltage using a bridge circuit. Using this technique, when neurons fire an action potential they create changes in potential fields that can be recorded using microelectrodes. 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. When comparing anesthetized 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.Intracellularly, the electrodes directly record the firing of action, resting and postsynaptic potentials. When a neuron fires, current flows in and out through excitable regions in the axons and cell body of the neuron. This creates potential fields around the neuron. An electrode near a neuron can detect these extracellular potential fields, creating a spike.
Experimental setup
The basic equipment needed to record single units is microelectrodes, amplifiers, micromanipulators and recording devices. The type of microelectrode used will depend on the application. The high resistance of these electrodes creates a problem during signal amplification. If it were connected to a conventional amplifier with low input resistance, there would be a large potential drop across the microelectrode and the amplifier would only measure a small portion of the true potential. To solve this problem, a cathode follower amplifier must be used as an impedance matching device to collect the voltage and feed it to a conventional amplifier. To record from a single neuron, micromanipulators must be used to precisely insert an electrode into the brain. This is especially important for intracellular single-unit recording.Finally, the signals must be exported to a recording device. After amplification, signals are filtered with various techniques. They can be recorded by an oscilloscope and camera, but more modern techniques convert the signal with an analog-to-digital converter and output to a computer to be saved. Data-processing techniques can allow for separation and analysis of single units.