CHAPTER 1 7Imaging the Brain
Over the last couple centuries, much has been learned about the brain
by observing how damage to different parts of the brain relates to
specific changes in behavior. Lesion is the general term used to denote
an injury to, or other abnormality in, the body. Among the causes of
brain lesions are stroke, tumor, physical trauma, and certain brain
diseases.
A stroke occurs when there is a disturbance in blood flow toa
region of the brain sufficient to produce a loss of function. If the dis-
turbance lasts for more than a very brief time (a few minutes), then
there is likely to be cell death and loss of function that may be perma-
nent. The two immediate causes of stroke are blockage of blood flow
and hemorrhage. Blockage occurs when particulate matter (usually
a blood clot or piece of atherosclerotic plaque) becomes lodged ina
blood vessel. Hemorrhage occurs when a blood vessel breaks and leaks
blood into the surrounding tissue, rather than delivering blood in
the normal way. A hemorrhage often occurs at places where there is
weakness in the structure of the blood vessel. Sometimes the struc-
tural weakness bulges out from the pressure of the blood; this is called
an aneurysm. If an aneurysm ruptures, the result is a hemorrhagic