What is the function of the cerebral cortex?

Each of the two hemispheres of the brain's cerebral cortex consists of four lobes. Although there is some overlap among these sections, each has distinct functions. The front portion is the frontal lobe, which controls voluntary movement, planning, organizational skills, and abstract thinking. Speech -- the expression of language -- is controlled in the lower part of the frontal lobe, called Broca's area. Behind it lies the parietal lobe, which controls sensory information; the left side of it enables you to do mathematical calculations and carry out some language functions, and the right side controls tasks that involve spatial orientation. Farther back is the occipital lobe, which perceives and interprets vision. Wrapped around the sides is the temporal lobe, which is involved in hearing, language comprehension, long-term memory, and behavior.

The visual cortex, where vision is processed, is in the occipital lobe, in the back of the cerebral cortex. The right visual cortex controls the left visual field for both eyes, and the left visual cortex controls the right visual field. That's why defective vision or blindness in half the visual field of one or both eyes -- a condition called hemianopia -- is a common consequence of stroke.