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An electrode introduced into the brain of a living animal will detect electrical activity that is generated by the neurons adjacent to the electrode tip. If the electrode is a microelectrode, with a tip size of 3 to 10 micrometres, the electrode will often isolate the activity of a single neuron. The activity consists of the voltages generated in the extracellular matrix by the current fields outside the cell when it generates an action potential. Recording in this way is generally called "single unit" recording. The recorded action potentials look very much like the action potentials that are recorded intracellularly, but the signals are very much smaller (typically about 0.1 mV). Most recordings of the activity of single neurons in animals are made in this way. Recordings of single neurons in living animals have provided important insights into how the brain processes information, following the hypothesis put forth by Edgar Adrian that unitary action potential events are the fundamental means of communication in the brain. For example, David Hubel and Torsten Wiesel recorded the activity of single neurons in the primary visual cortex of the anesthetized cat, and showed how single neurons in this area respond to very specific features of a visual stimulus. Hubel and Wiesel were awarded the Nobel Prize in Physiology or Medicine in 1981.
Isolated neural activity is also recorded from awake, behaving animals. This is not painful because the brain has no internal pain receptors. Animals are also maintained at highest possible physical and mental health for best performance during the lengthy training and data collection process spanning months to years. The goal is to observe how the activity of single neurons relates both to the applied stimulus and the animal's behavioural outcome. Causal relationships can be established by passing current through the microelectrode, known as microstimulation. Electrical current manipulates the activity of local neural populations which biases the animal's behaviour.
An example procedure for finding single neurons in the Medial Temporal (MT) area is described. This region processes motion of visual stimuli and its constituent cells are tuned to a visual object's location, speed, size, and disparity; the tuning of cells is critical for identifying which region of brain the microelectrode tip (see below) is passing through. Signal coming from the electrode is played over a speaker and the electrode is advanced while the animal performs the experimental task. When the tip is in grey matter, speaker output sounds like white noise except for the occasional clipping, screetching, and clapping sounds of nearby neurons. This fades away to silence when the tip enters white matter. A series of visual stimuli are shown once the electrode is suspected of being in MT. For example, a manually controlled bar of light will be moved back and forth to find the region of visual field, preferred direction, and speed of the local neural population. If the tip is in MT then dragging a bar of light in the right way causes the action potential firing rate of cells within earshot to increase dramatically and stop abruptly with cessation of motion in the preferred direction. Subsequently, advancing the electrode at bursts of 5 to 15 micrometres every 20 to 30 seconds helps the electrode to push through brain matter and close to single neurons. It is important not to advance to quickly because it takes time for the brain to settle around the electrode. Preferred stimulus characteristics are established again once a single neuron is found after which the main experimental task may begin.
Microelectrodes used for extracellular single unit recordings are usually very fine wires made from tungsten or platinum-irridium alloys that are insulated except at their extreme tip and are less often glass micropipettes filled with a weak electrolyte solution similar in composition to extracellular fluid. Subtle differences in microelectrode shape and composition benefit different tasks. For instance, the use of low impedance electrodes (< 1MOhm) is favourable for mapping gross regions of grey and white matter because they pick up more signal from surrounding neurons, albeit, at the expense of focusing on the activity of individual cells for which higher impedance microelectrodes are used.