Recent exciting discoveries have revolutionized our ideas of the relationship between thought and blood flow. Previously it was considered that thinking did not use any significant amount of energy and the blood flow to the brain was constant. It is now known, however, that when a group of brain cells is activated it uses up oxygen, produces carbon dioxide, and causes a local increase in blood flow to that area by enlarging the blood vessels.
Using radioactive tracers it is now possible to follow these changes and remarkable pictures have been obtained. For instance, the resting pattern of blood flow shows a marked increase in the frontal areas; talking to the subject causes a decrease of flow to these areas whilst part of the temporal lobe of the brain shows a marked increase of activity – dynamic proof of the long-held hypothesis that this area is one of the speech centers. Similar things happen when other intellectual activities are undertaken so that these techniques confirm localization of various functions in specific areas of the brain. This close relationship between blood vessels and brain cells leads to interesting possibilities. For instance, if the control of blood vessels were to be deranged, subtle changes in brain function could be expected; this has been confirmed in certain forms of mental disease.
Pain has been shown to increase metabolism in the brain and to dilate vessels generally. The increase in vessel diameter makes small local controlling changes more difficult and this could explain why thinking becomes more difficult during severe pain. The paradox is that pain, like stress, produces increased sympathetic activity, yet it increases blood flow to the brain. The resulting vasoconstriction would be expected to decrease it. The answer is probably that the sympathetic controls the larger, so-called resistance vessels, while pain in this context activates local capillaries.
The system which controls blood flow to the brain is highly complex as it is affected by nervous factors, circulating chemicals (amines), and outside factors such as pain, as well as by mental processes. An interesting experiment highlighted the importance of the latter: subjects had to solve problems of mental arithmetic while their brain blood flow was being measured. There was a slight increase to appropriate areas but this became greatly magnified when they were offered money for the successful solution to the problems. This shows that motivation and concentration have a direct effect on the blood flow to the brain and the experiment clearly confirms yet another psychosomatic link.
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