Human brain: imaging and experiences
Identifying blood flow changes or increased metabolism in certain parts of the human brain, during an experience, do not allow us to determine:
- how a physical collection of cells gives rise to conscious experiences
- whether an experience is real or not (see also Spikes in EEG at time of death - what do they tell us about near death experiences?) since the reality of an experience is determined socially and is not based upon a pattern of changes in brain cells.
Let’s look at how imaging of the human brain works. Modern methods of analyzing human brain function by involving special brain scanners called:
- functional MRI (magnetic resonance imaging) scanners.
- and PET (positron emission tomography) scanners.
These work upon the principle that brain cells have a constant need for blood, which carries with it all the vital nutritional substances needed to work, including oxygen and glucose. These scanners essentially detect and follow the movement of blood to various parts of the brain.
Watching one of these scanners can be analogized to using satellite technology, to observe the changes in the flow of water through water canals from space. A scientist would be able to tell which parts of the land were being worked on by agricultural workers at any one time simply by observing the changes in the flow of water in that region. In a similar way, brain scanners detect changes in the flow of blood to various regions of the brain, which in turn indicate which brain cells are working.
In addition to detecting changes in blood flow, specialized scanners can also detect the areas of the brain that have increased their consumption of oxygen and glucose. Ascertaining the changes in blood flow and oxygen and glucose consumption in various parts of the brain, scientists can determine which areas of the brain are involved with specific thought processes. This is called ‘mapping’ the brain. To do this, scientists will place someone into a scanner and scan the brain while they are having certain thoughts.
Whatever we do, think or feel, is accompanied by a reciprocal change in the pattern of blood flow to the part of the brain involved with this sensation. For example, if we really enjoy listening to music and stop paying attention to other things, the areas that had been receiving more blood will now receive less blood, but other areas will then start to receive more blood. Interestingly, brain scans have shown that for any thought many areas of the brain become active suggesting that multiple areas of the brain mediate thought processes.