Singleunit recording

Single-unit recording is a fine-grain technique developed over 40 years ago to permit the study of single neurons. A micro-electrode about one 10,000th of a millimetre in diameter is inserted into the brain of an animal to obtain a record of extracellular potentials. A stereotaxic apparatus is used to fix the animal's position, and to provide the researcher with precise information about the location of the electrode in three-dimensional space. Single-unit recording is a very sensitive technique, as electrical charges of as little as one-millionth of a volt can be detected.

The best known application of this technique was by Hubel and Wiesel (1962, 1979). They used it with cats and monkeys to study the neurophysiology of basic visual processes. Hubel and Wiesel found there were simple and complex cells in the primary visual cortex, but there were many more complex cells. These two types of cells both respond maximally to straight-line stimuli in a particular orientation (see Chapter 4). The findings of Hubel and Wiesel were so clear-cut that they constrained several subsequent theories of visual perception, including that of Marr (1982; see Chapter 2).

Evaluation

The single-unit recording technique has the great value that it provides detailed information about brain functioning at the neuronal level, and is thus more fine-grain than other techniques (see Figure 1.8). Another advantage is that information about neuronal activity can be obtained over a very wide range of time periods from small fractions of a second up to several hours or days. A major limitation is that it is an invasive technique, and so would be unpleasant to use with humans. Another limitation is that it can only provide information about activity at the neuronal level, and so other techniques are needed to assess the functioning of larger areas of the cortex.

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