There are a total of about 100 billion neurons in a human brain (see page 1075), each with an average of a few thousand synapses connecting it to other cells. On a small scale the arrangement of neurons seems quite haphazard. But on a larger scale the brain seems to be organized into areas with very definite functions. This organization is sometimes revealed by explicitly following nerve fibers. More often it has been deduced by looking at what happens if parts of the brain are disabled or stimulated. In recent times it has also begun to be possible to image local electrical and metabolic activity while the brain is in normal operation. From all these methods it is known that each kind of sensory input is first processed in its own specific area of the brain. Inputs from different senses are integrated in an area that effectively maintains a map of the body; a similar area initiates output to muscles. Certain higher mental functions are known to be localized in definite areas of the brain, though within these areas there is often variability between individuals. Areas are currently known for specific aspects of language, memory (see below) and various cognitive tasks. There is some evidence that thinking about seemingly rather similar things can lead to significantly different patterns of activity.
Most of the action of the brain seems to be associated with local electrical connections between neurons. Some collective electrical activity is however revealed by EEG. In addition, levels of chemicals such as hormones, drugs and neurotransmitters can have significant global effects on the brain.