Maps and their limits
Neuroscientists often describe the brain in terms of regions, each linked to particular functions. This is a useful way of organizing information, much as a map of a country with labeled provinces is useful. But like any map, it simplifies. Regions have blurry boundaries, individual brains vary, and the same region may contribute to many functions. The physical layout of the brain, with its lobes, cerebellum and brainstem, is described in the article on brain anatomy. This piece is about the functional side: what areas are generally associated with, and why those associations need hedging.
Functions with fairly strong regional links
Some links between regions and functions are well supported by many decades of evidence from injury studies, recordings and imaging.
Primary visual cortex. Located at the back of the brain, in the occipital lobe, it receives visual information relayed from the eyes and handles early stages of processing. Damage can cause blind areas in the field of vision even when the eyes are healthy.
Primary auditory cortex. Found in the temporal lobe, it takes part in processing sound. Higher areas nearby contribute to recognizing speech and music.
Motor cortex. A strip toward the back of the frontal lobe is involved in controlling voluntary movement. A neighboring strip in the parietal lobe, the somatosensory cortex, receives information on touch. Both are arranged in a rough map of the body, with more space devoted to sensitive or finely controlled parts such as the hands and lips.
Language areas. In most people, language depends heavily on regions in the left hemisphere. Broca’s area, in the frontal lobe, was linked in the nineteenth century to the production of speech, and Wernicke’s area, in the temporal lobe, to comprehension. Modern research suggests language involves a wider network, and the classical picture is considered too simple.
Hippocampus. Deep in the temporal lobe, it is important for forming new explicit memories, as the case of H.M. famously showed.
Cerebellum. At the back and underneath, it is associated with coordination, balance and the smoothness of movement.
Brainstem. It regulates vital functions such as breathing and arousal.
Functions that are more distributed
For more complex abilities, the connection to specific regions is looser.
Prefrontal cortex. The front part of the frontal lobe is associated with planning, working memory, flexible thinking and regulating impulses. It matures late, into the twenties, according to many imaging studies, which is sometimes used to explain adolescent behavior, but that story is oversimplified. Prefrontal areas work with many other regions.
Amygdala. Often called the fear center, this almond-shaped structure contributes to detecting emotionally important events and shaping responses, but it is involved in many types of emotion and learning, and fear responses depend on multiple regions. See the discussion of emotion circuits for a fuller picture.
Reward-related regions. Deep structures that use dopamine, along with parts of the frontal cortex, are involved in motivation and learning from rewards, as described under the brain reward system. Calling dopamine the pleasure chemical is a simplification.
Attention and awareness. These involve networks across the frontal and parietal lobes and deeper structures, rather than any one center.
Default mode network. Researchers have identified a set of regions that tend to be active when a person is not focused on an outside task, such as during daydreaming or recalling the past. Its precise functions are still debated.
Why strict localization fails
Several observations argue for treating regions as parts of networks.
- Imaging typically shows many areas active during even simple tasks.
- A single area often takes part in different tasks, depending on which other areas it is working with.
- After injury, other areas can sometimes take over part of a lost function, which would be difficult if functions were rigidly tied to one place.
- People differ in the exact layout of their brain areas, so group averages do not describe everyone.
For this reason, researchers increasingly speak of large-scale networks and the connections between regions, topics also discussed under neural circuits.
Common myths
The idea that the left brain is logical and the right brain creative is not supported by research. The hemispheres differ in some specializations, but healthy people use both together. Similarly, labeling a region as the seat of a complex trait like intelligence, love or morality is usually too simple, and findings like that should be treated cautiously, as in the discussion of the brain-behavior link.
Using the map wisely
A sensible way to use regional knowledge is as shorthand. Saying the visual cortex processes sight is a decent starting point, and saying the hippocampus supports memory formation is also reasonable. But behind each phrase is a network of cells, connections and interactions with other regions. Treat names as handles for further exploration, not as final explanations, and expect labels to be refined as methods improve.