Discovering the Brain (1992) / Chapter Skim
Currently Skimming:
3 Glimpses of the Living Brain
3 Glimpses of the Living Brain
Pages 34-45
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From page 34... ...
In all these instances, of course, the brain was very far from its usual level of functioning, and so it was never clear how much these studies could contribute to an understanding of the "normal" brain. Surgeries, autopsies, and neurological examinations of people who had survived brain injury or stroke offered rich details, but the full view of an awake, behaving human brain has continued to tantalize and elude researchers until very recently.
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From page 35... ...
As an even more flexible measurement of function in the normal brain, PET can detect changes in local blood flow. Small blood vessels respond very rapidly to the needs of nerve cells, axons, and dendrites, so that a full scan's worth of information can be gathered in 40 seconds, as compared with the 45 minutes required to measure glucose metabolism.
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From page 36... ...
There is also the need to distinguish between slight but meaningful shifts in activity and random irregularities, or background "noise." To take the primary visual cortex as an example, a bright light flashed in the subject's eyes evokes an unmistakable peak of activity visible in a PET image; but a less drastic stimulus, such as a word or two appearing on a screen, will act on nerve cells in the brain much less vigorously, and the corresponding PET image will be harder to read. Moreover, the response may be dispersed among several areas, perhaps those having to do with learning and memory, language, and emotion, as well as the areas containing the visual receptors that must take in the stimulus to begin with.
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From page 37... ...
As with any inviting area of research, the first task was to define specifically what would be studied. From the new perspective of the PET scanner, many appearing interrelated aspects of human language abilities called for investigation.
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From page 38... ...
. The findings were dramatic: an area known as the medial extrastriate cortex on the left side of the brain came smartly into action as the tasks reached the third and fourth levels (those of pronounceable nonwords and of real words)
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From page 39... ...
And still further areas became active for tasks that required subjects to turn their attention to the meaning of the words: not only a few sites in the left frontal lobe, in or near the so-called association areas that process many modes of information, but also and this was a surprise several sites in the cerebellum. This finding was unexpected because the cerebellum is best known for many functions that are far removed from language: the coordination of movement, fine manual skills, repetitive physical tasks, and so on.
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From page 40... ...
The advantages of laboratory observation are several: the scanning equipment can be set up in advance and timed so as to obtain the maximum information from the panicking subjects in the shortest possible time; and many extraneous factors that might otherwise appear to be associated with a panic attack can, in this neutral setting, be ruled out. For the Washington University group, eager to see what PET could reveal about panic disorder, the first question was: How does "normal" anxiety (that which would be felt by most individuals, to varying degrees, in a stressful situation)
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From page 41... ...
This time the resulting PET scans resemblecT those of the depressed patients: they showed the same site of increased activity in the left frontal cortex. If the same local pattern of reduced activity in the brain holds true both for long-term clinical depression and for transient feelings of sadness, this observation may point toward new ways of exploring both the disabling ailment of depression and the physiological basis for healthy feelings of sadness.
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From page 42... ...
X-ray technology, for example, produces highly readable images with fine resolution of detail down to 0.1 millimeter, but these images are static and show only existing structures rather than ongoing processes. On the other hand, electroencephalography, or LEG, which detects small shifts in electric potential at the surface of the skull, measures overall activity of the brain in real time but indicates only roughly where in the brain the information is coming from.
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From page 43... ...
Another limitation is that x-ray tomograms can show only the comparative densities of various structures; thus they may be very helpful in disease states that affect the overall size or shape of body tissues, but they offer little information on maladies that leave the boundaries of a particular structure unchanged. Within these limits, computed tomography will continue to be used widely for imaging the brain, particularly for possible diagnoses of brain hemorrhage, stroke, or disorders involving the cerebral ventricles.
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From page 44... ...
This technique locates the sources of the very weak fields that accompany the electrical firing of nerve cells in the brain. The extremely sensitive detectors of magnetic source imaging are arranged around the patient's head to track varying levels of activity in many locations of the brain.
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From page 45... ...
Ultrasound can also produce useful images of blood flow and of structures that are moving, with a fairly high resolution of about 0.5 to ~ millimeter. Sound waves penetrate very poorly through bone, and this fact might appear to make ultrasound imaging of the brain impossible.
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