The brain is divided into two hemispheres, one on the left and one on the right, connected by a thick band of tissue called the corpus callosum. Oddly enough, the right hemisphere controls the left side of the body, and the left hemisphere controls the right side of the body. Activity in the left hemisphere is correlated with speech, writing, language, and calculation. Activity in the right hemisphere is correlated with spatial abilities, face recognition, and some musical abilities.
The outside of the brain is a grey layer, folded into strange ridges, and called the cerebral cortex, or the “gray matter”. The gray color comes from billions of nerve cells, and connects to bundles of of white, insulated fibers – “white matter”. You can think of it like an outer “rind” – in humans, it makes up 80% of the brain. In monkeys, only about 66%. The amount of “gray matter” appears to contribute significantly to how advanced a species is, and humans have the most of it.
The brain itself can be divided roughly into four lobes.
|The frontal lobes help make us human. The association cortex in this region helps us generate goals that are personally desirable, to determine how socially appropriate they are, and then to decide which behaviors will result in the best future outcome. Many higher order executive functions arise in the frontal lobes, including those that direct speech and other precisely managed movements. The frontal lobe contains most of the dopamine-sensitive neurons in the cerebral cortex.|
|Behind lie the parietal lobes. These lobes not only receive sensation but allow fine discrimination between them. When we fumble in our pocket, which coin is a nickel and which coin is a quarter? Our fingers tells us “this one”, as the parietal lobes make abstract representations of the sensations from our fingers, relating these representations to the type of coin.|
|Below each parietal lobe, deep to the temple on either side, are the temporal lobes. They decode and interpret what we hear and see, and process other more elaborate, patterned sensory messages. For instance, the left temporal lobe is important in understanding language-related concepts.|
|The occipital lobes at the back of the brain register impulses concerned with vision. They then pattern them into streams of visual messages that are relayed forward to both the temporal and parietal lobes. Note that they do not relay patterns of photons hitting your eye, but more abstract and compressed representations. These mental images are generated and regenerated, and could be thought of as our “mind’s eye”, the mental templates that allow us to recognize and insert meaning into what we see. Strong increases in occipital activity is noted under the influence of certain psychedelic drugs.|