Behind the scenes of functional brain imaging: A historical and physiological perspective
Abstract
At the forefront of cognitive neuroscience research in normal humans are the new techniques of functional brain imaging: positron emission tomography and magnetic resonance imaging. The signal used by positron emission tomography is based on the fact that changes in the cellular activity of the brain of normal, awake humans and laboratory animals are accompanied almost invariably by changes in local blood flow. This robust, empirical relationship has fascinated scientists for well over a hundred years. Because the changes in blood flow are accompanied by lesser changes in oxygen consumption, local changes in brain oxygen content occur at the sites of activation and provide the basis for the signal used by magnetic resonance imaging. The biological basis for these signals is now an area of intense research stimulated by the interest in these tools for cognitive neuroscience research.
Footnotes
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↵ † e-mail: marc{at}npg.wustl.edu.
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This paper was presented at a colloquium entitled “Neuroimaging of Human Brain Function,” organized by Michael Posner and Marcus E. Raichle, held May 29–31, 1997, sponsored by the National Academy of Sciences at the Arnold and Mabel Beckman Center in Irvine, CA.
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↵ ‡ Some have wondered whether these reductions in blood flow are merely the hemodynamic consequence of increases elsewhere (i.e., an intracerebral steal phenomenon). Such a hypothesis is very unlikely to be correct because of the tremendous hemodynamic reserve of the brain (51) and also because there is no one to one spatial or temporal correlation between increases and decreases (e.g., see Fig. 1 and 2).
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↵ § Wilder Penfield is frequently given credit for the observation that venous oxygenation increases during a seizure discharge (i.e., so-called “red veins on the cortex”). Careful reading of his many descriptions of the cortical surface of the human brain during a seizure fail to disclose such a description. Rather, he describes quite clearly the infrequent appearance of arterial blood locally in pial veins after a focal cortical seizure (55): “… the almost invariable objective alteration in the exposed hemisphere coincident with the onset of the fit is a cessation of pulsation in the brain” (page 607).
- ABBREVIATIONS:
- PET,
- positron emission tomography;
- MRI,
- magnetic resonance imaging;
- MRS,
- magnetic resonance spectroscopy;
- fMRI,
- functional MRI;
- BOLD,
- blood-oxygen-level-dependent
- Copyright © 1998, The National Academy of Sciences
