|MadSci Network: Neuroscience|
When someone has a coma, there is a major problem with the brain. This can be caused by toxins, like drugs like morphine and alcohol, shock, severe blood loss, a blockage of blood flow to the brain, brain swelling, blood leaking into the brain, or a severe head injury, like the injury one would get in a car crash. With a coma, either major parts of the brain aren't working or parts of the brain that are responsible for activating the rest of the brain aren't working. A coma can be permanent or temporary.
In the case of a coma, the problem is really with processing of information, rather than with the information that is entering the brain. This is because the damage is to the brain itself, rather than the sensory nerves coming into the brain.
There are different ways to measure the signals entering the brain. One of them is called BAER (for brainstem auditory evoked response). The BAER is done by putting electrodes on the head and then seeing the electrical activity in the branstem where auditory signals first enter the brain. In a comatose patient the input going into the brain would be the same, but once the signals get into the brain, they will be processed differently or not all, depending on where the brain damage is. For example, if the brain damage includes the area of the brainstem that is involved with hearing, there may not be any signals at all. However, if the damage is elsewhere, there may be more or less normal signals in the brainstem. The signals probably will be different in other parts of the brain, like in the cortex, which is where do most of our thinking.
So, the answer to your question is that the signals entering the brain are more or less normal in a coma, but, because the brain itself is damaged, the processing of information in the brain is different.
You can read more about comas here: http://www.mamashealth.com/coma.asp
and you can read about BAER here: http://www.ccent.com/PHS/brain.html
Thank you for your interesting question.
Try the links in the MadSci Library for more information on Neuroscience.