Blood Flow and Activation. According to Legrenzi and Umilta: The idea that ''the brains can be subdivided into a large number of portions (areas) with different functions, which are independent of each other, and that cerebral blood flow can be used to obtain information about mental functions'' may seem a risky business. I.e. According to the authors: Indeed, using blood flows to pinpoint mental activity is both difficult and leads to problems. One problem, that remains to be solved (according to Legrenzi and Umilta), is the question of variations in blood flow, which have a latency of 5 seconds or more (i.e. it takes at least 5 seconds to get started). Whereas, human thought on the other hand, has a latency of just a few tens of milliseconds. So, how (can we be sure that) is it possible that very fast variations in thought are signalled by much slower variations in cerebral blood? Difficult: The objective of neuroimaging is to identify the areas, which selectively become active during any given task that requires the intervention of known mental functions. The cerebral areas are composed of a multitude of nerve cells and neurons, whose need for oxygen and glucose depends on the level of activation. Therefore, knowing the quantity of blood feeding the various areas at any given time, makes it possible to gauge the level of activation of these areas. In one method a radioactive isotope is injected into the blood stream. Then the level of the isotope in various areas of the brain is measured by using sensors placed around the patients head. (In PET scans the radioactive isotope emits positrons which collide with electrons, producing gamma rays, which are recorded by sensors around the head of the patient). In fMRI, the volunteers are placed in a magnetic field, which aligns the hydrogen atoms present in water molecules circulating in the blood (in the brain). When the hydrogen atoms come into contact with radio-waves they resonate [1] - they emit a quantity of radio-waves in proportion to their number - The presence of large number of hydrogen atoms indicate a high incidence of water, which again indicates that the neurons are using large quantities of oxygen, indicating a high activity. |
Cognitive subtraction. Is another problem area for neuro-imaging, according to the authors. First a control task must be identified. That is a task which involves all the same mental functions of the experimental task with the exception of the function whose neural bases you want to identify. While each task is being executed, the activation of tiny regions (voxels) of the brain is measured. Then the level of activation in the control task is subtracted from the activation in the experimental task. If the result is greater than zero the neurons in the particular voxel is more active in the experimental task than in the control task. (It follows that an incorrect choice of control task leads to an inaccurate conclusion). However the level of activity in a voxel can also be due to chance factors. So, normally a result would not be accepted unless it was very unlikely to come about as a result of chance fluctuations. |
-Simon
Simon Laub
www.simonlaub.net
-Simon
Simon Laub
www.simonlaub.net
-Simon
Simon Laub
www.simonlaub.net
-Simon
Simon Laub
www.simonlaub.net
-Simon
Simon Laub
www.simonlaub.net