brain activation Fast, transient biophysical and biochemical variations in neurons and, to some extent, in the glial cells that support them.

cerebral cortex Continuous sheet of tissue covering each hemisphere of the brain composed of billions of neurons and glial cells.

Gauss Unit of magnetic field strength. The preferred International System unit of field strength is the tesla (1 T = 10,000 G).

gray matter The gray tissue of the nervous system that contains a high proportion of nerve cell bodies.

hemodynamic response Variation of a vascular property, such as regional cerebral blood flow, in response to a change in neural activity.

sampling rate Rate at which physiological signals are sampled for subsequent analysis.

spatial resolution Precision with which physiological variables can be localized in space by a given neuroimaging technique.

voxel "Volume element'' in a three-dimensional image.

white matter The white tissue of the nervous system consisting mainly of axons connecting nerve cells.

The ultimate goal of human neuroimaging is to monitor the living brain at all levels of structure and function, from neurotransmitter and receptor molecules to large networks of brain cells, with time scales ranging from milliseconds to minutes. Achievement of this goal would provide psychologists and neuroscientists with biological building blocks roughly similar to those that the human genome project provides geneticists. However, this analogy is misleading because the task of the neuroimager is probably more complicated than that of the geneticist. For instance, whereas the human genome contains an estimated 3 x 109 base pairs within at most approximately 105 genes, the human brain contains perhaps 1014 synapses on about 1011 neurons. In both the brain and genetic realms, it is important to realize that even perfect knowledge of biological building blocks would not automatically lead to knowledge of how the mechanisms function. Although knowledge of the structure of the brain tells us something about how it operates (just as knowledge about the structure of a lens tells one something about why people sometimes wear them in front of their eyes), further research will be required that specifically seeks to characterize how mental processes arise from neural tissue.

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