In humans, by far the largest part of the brain is the cerebral cortex, the outer layer of the forebrain. The cerebral cortex and underlying parts of the forebrain are divided into two halves, called cerebral hemispheres that communicate with each other by means of large band of axons, the corpus callosum. The cerebral cortex is the most sophisticated in formation processing center known, and it is also the are of the brain that scientists know the least about. Roughly 50 to 100 billion neurons are  packed into this thin surface layer. To accommodate this profusion of cells, the cortex is thrown into folds, called convolutions that greatly increase its area. In the cortex, cell bodies of neurons predominate giving this outer layer of the brain a gray appearance. These neurons receive sensory information, process it, store some in memory for future use, and direct voluntary movements.

The cerebral cortex is divided into four regions base temporal lobes. Functionally, the cortex contains primary sensory areas where signals originating in sensory organs such as the eyes and ears are received and converted into subjective impressions, for example, light and sound. Nearby association areas interpret the sounds, as speech, for example, and the visual stimuli as recognizable objects. Association areas also link the stimuli with previous memories stored in the cortex and generate commands to produce speech. Primary sensory areas in the parietal lobe interpret sensations of touch originating in all parts of the body, which is "mapped" in an orderly sequence. In an adjacent region of the frontal lobe, primary motor areas generate commands for movements in corresponding areas of the body. The association area of the frontal lobe protected by bones of the forehead seems to be involved in complex reasoning such as decision making, predicting the consequences of actions, controlling aggression, and planning for the future. Damage to the cortex due to trauma, stroke, or a tumor result in specific deficits, such as problems with speech, difficulty reading, or the inability to sense or move specific parts of the body. Because brain cells cannot reproduce, once a brain region is destroyed it cannot be repaired or replaced, so these deficits are often
permanent. Fortunately, however, in some cases diligent training can cause undamaged regions of the cortex to take control over and restore some of the lost functions.