Self Comes to Mind

in the cerebellar cortex , which envelops the cerebellum. The nonlayered variety is made of nuclei , the main examples of which were listed earlier: the basal ganglia (located in the depth of each cerebral hemisphere and made up of three large nuclei, the caudate, the putamen, and the pallidum); the amygdala , a single and sizable lump located in the depth of each temporal lobe; and several aggregations of smaller nuclei that form the thalamus , the hypothalamus , and the gray sectors of the brain stem .
    The cerebral cortex is the cerebrum’s mantle, covering the surfaces of each cerebral hemisphere, including those that are located in the depth of fissures and sulci, the crevices that give the brain its unique folded appearance. The thickness of the cortex is about three millimeters, and the layers are parallel to one another and to the brain’s surface. The evolutionarily modern part of the cerebral cortex is the neocortex . The main divisions of the cerebral cortex are designated as lobes: frontal, temporal, parietal, and occipital. All other gray structures (the various nuclei mentioned earlier and the cerebellum) are subcortical.
    In the text I often refer to early sensory cortices or to association cortices or even to higher-order association cortices . The designation early has no time connotation at all; it refers to the position occupied by a region in space, along a sensory processing chain. Early sensory cortices are those located near and around the point of entry of peripheral sensory pathways into the cerebral cortex—for example, the point of entry for vision or hearing or touch signals. The early regions tend to be organized concentrically. They play a critical role in producing detailed maps using the signals brought in by the sensory pathways.
    The association cortices, as the name implies, interrelate signals arising from the early cortices. They are located everywhere in the cerebral cortex where there are no early sensory cortices or motor cortices. They are organized hierarchically, and the ones higher up in the chain are usually known as higher-order association cortices. The prefrontal cortices and the anterior temporal cortices are examples of higher-order association cortices.
    The various regions of the cerebral cortex are traditionally identified by numbers corresponding to the distinctive architectural design of its neuron arrangements, which is known as cytoarchitectonics. The best-known system for numbering the regions was proposed by Brodmann a century ago, and it remains a useful tool today. The Brodmann numbers have nothing whatsoever to do with the area’s size or functional importance.
THE IMPORTANCE OF LOCATION
    The internal anatomical structure of a brain region is an important determinant of its function. Where a given brain region is located within the three-dimensional volume of a brain is another important determinant. Placement in the global brain volume and internal anatomical structure are largely consequences of evolution, but they are also influenced by individual development. Individual experience shapes the circuitry, and although this influence is most marked at the microcircuitry level, it is inevitably felt at the macroanatomic level as well.
    The evolutionary vintage of nuclei is old, a throwback to a time in the history of life when whole brains were little more than chains of ganglia resembling beads in a rosary. A ganglion is, in essence, an individual nucleus before being evolutionarily incorporated into a brain mass. The brains of the nematodes I mentioned in Chapter 2 consist of chains of ganglia.
    The location of nuclei within the brain’s whole volume is fairly low, always below the mantle provided by the cerebral cortex. They sit in the brain stem, the hypothalamus and thalamus, the basal ganglia, and the basal forebrain (whose extension includes the collection of nuclei known as the amygdalae). Banished as they are from the prime cortical estate, they still have an evolutionary pecking order. The older they are, historically speaking, the closer they are to the brain’s midline. And because everything in the brain has two halves, left and right with a dividing median, it so happens that very old nuclei sit looking at their twin on the other side of the midline. This is the case with the brain-stem nuclei that are so vital for life regulation, and for consciousness. In the case of somewhat more modern nuclei—say, the amygdala—the left and right