Many people think of the brain as a single mass. When viewed directly, however, one can see that there is actually a line running down the middle, essentially splitting the brain into two halves. These halves are known as the hemispheres of the brain. In general, the right hemisphere controls the left side of the body and vice versa. Each side of the brain also has certain brain processes it is predominantly responsible for controlling. Within each hemisphere, there are also several smaller sections, called lobes, that are associated with further specialization. While the hemispheres of the brain are distinct in some ways, they are connected to each other and do share information.
In general, each of the hemispheres of the brain controls the opposite side of the body. For example, the nerve impulses needed to move the right hand typically come from the left side of the brain. Similarly, each side of the brain typically processes sensory information for the opposite side of the body. For example, if the left leg is being bitten by a dog, the pain impulses created by the nerves in the leg will be sent to the left side of the brain.
Certain brain processes are also often predominantly, though not necessarily exclusively, handled by each of the hemispheres. The left hemisphere, for example, is generally responsible for language and logical breakdown of information, while the right side is generally responsible for spatial awareness and evaluating the overall situation based on smaller pieces of information. Damage caused to one hemisphere of the brain, such as in the case of a stroke, can make these differentiations obvious. For example, a stroke in the left hemisphere can leave a person unable to communicate clearly.
The hemispheres of the brain also contain separate specialized areas called lobes. Each lobe is generally responsible for further specialized processes. For example, the frontal lobe often dominates the control of emotions, while the occipital lobe is generally responsible for vision. While each hemisphere has the same number and type of lobes, experts believe that lobes on each side may have slightly different roles to play in their dominant processes. This belief is based primarily on different patterns of symptoms that have been observed when a particular left hemisphere lobe is damaged versus when the same right hemisphere lobe is damaged.
Although the hemispheres of the brain are physically separate and tend to have predominant control over different processes, they are not completely independent from one another. They are connected in the middle through a bundle of nerve fibers called the corpus callosum. This allows the hemispheres of the brain to communicate and share information with each other, which is something they do regularly.