The classical and most direct method of measuring neuronal activity in individual cells in vivo is by means of invasive electrical recordings in animals. In this method, tiny electrodes are placed into or near neurons, and the effects of different activities on the firing rate of the neurons are recorded. Although this method and its variants have provided a wealth of knowledge about how neurons operate, the invasive nature of intracranial electrical recording methods strictly limits their application to only special cases in humans (e.g., patients undergoing brain surgery). A host of noninvasive neuroimaging methods for

Encyclopedia of the Human Brain Volume 3

Copyright 2002, Elsevier Science (USA).

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measuring brain activity in human beings has evolved. Herein, a brief overview is offered of the major human neuroimaging methods currently in use, emphasizing the strengths and limitations of each. Currently, there are two broad classes of noninvasive neuroimaging techniques to measure brain activity, which are the focus of this article: (1) direct methods that monitor electrical or magnetic fields directly linked to neural activity and (2) indirect methods that monitor metabolic and vascular changes associated with neural activity.

Note that in the literature the first category of methods often is not referred to as "neuroimaging." This is misleading not only because images can be produced on the basis of data obtained with electromagnetic measurements but also because research often integrates multiple methods, resulting in combined maps with high spatial and temporal resolution. In addition to neuroimaging techniques to monitor human brain activity, there are also techniques that provide structural information about the brain; these will be addressed only very briefly. Finally, techniques based on brain stimulation, such as transcranial magnetic stimulation, although important for cognitive neuroscientists, will not be discussed here because they are not neuroimaging techniques.



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