【正文】 in regional blood flow and metabolism within the brain. The principle is simple. Neurons that are active demand more glucose and oxygen. The brain vasculature (脈管系統(tǒng) ) responds to neural activity by directing more blood to the active regions. Thus by detecting changes in blood flow, PET and fMRI reveal the regions of brain that are most active under various circumstance. Until recently,”mind reading” has been beyond the reach of science. With the introduction of PET and f MRI, it is possible to observe and measure changes in brain activity associated with the planning and execute of specific tasks. PET imaging was developed in the 1970s, by two groups of physicists, one at at Washington University, led by M. Terpogossian, and the second at UCLA, led by Z. H. Cho. The basic procedure is simple. A radioactive solution containing atoms that emit positrons (positively charged electron) is introduced into the bloodstream. Positrons, emitted wherever the blood goes, interact with the electrons produce photons (光子 ) of electromagic radiation. The location of the positronemitting atoms are found by detector that pick up the photons. One powerful application of PET is the measurement of metabolic activity in the brain. In a technique developed by Louis Sokoloff and his colleagues at the National Institute of Mental Health, a positronemitting isotope of fluorine or oxygen is attached to 2deoxyglucose (2DG). This radioactive 2DG is injected into the blood stream, and it travels to the brain. Metabolically active neurons, which normally use glucose, also take up the 2DG. The 2DG is phosphorylated by enzymes inside the neuron, and this modification prevents the 2DG from leaving. Thus, the amount of radioactive 2DG accumulated in a neuron and the number of positron emissions indicated the level of neuronal metabolic activity. In a typical PET application, a person’s head is placed in an apparatus surrounded by detectors. By use of puter algorithms, the photons resulting from positron emissions reaching each of the detectors are recorded. With this information, levels of activity for populations of neurons at various site in the brain can be calculated. Compiling these measurements produce an image of the brain activity pattern. The researcher monitors brain activity while the subject perform a task, such as moving a finger or reading aloud. Different task “l(fā)ight up” different brain areas. To obtain a picture of the activity induced by a particular behavioral or thought task, a subtraction technique is used. Even in the absence of any sensory stimulation, the PET image will contain a great deal of brain activity. To create an image of the brain activity resulting from a specific task, such as a person looking at a picture, this background activity is subtracted out. Although PET imaging has proven to be a valuable technique, it has significant limitations. Because the spatial resolution is only 510 mm3, the images show the activity of many thousands of cells. Also, obtaining a single PET brain scan may take one to many minutes. This along with concerns about radiation exposure, limit the number of obtainable scans from one person ina reasonable time. Thus, the work of S. Ogawa at Bell Labs, showing that MRI technique could be used to measure local changes in blood oxygen levels that result from brain activity, was an important advance. The fMRI method takes advantage of the fact that oxyhemoglobin (the oxygenated form of hemoglobin in the blood) has a different magic resonance from that of deoxyhemoglobin. More active regions of the brain receive more blood, and this blood donates more of tis oxygen. Functional MRI detects the locations of increased neural activity by measuring the ratio of oxyhemoglobin to deoxyhe