No two brains are the same. One difference is size. A normally functioning human brain weighs between one and two kilograms. If we look at brain structure, we also see individual differences such as in the thickness of the cortex (the outer layer of the brain). The relation between high-speed connections (white matter—myelinated tracts) and neurons varies, as can be seen through brain scans. Some brain regions have more white matter at the expense of neurons. Within the confines of the skull, it is spatially impossible to maximize both cell count and white matter density. The ratio between glia and neurons also varies. Einstein had a high glia to neuron ratio in parts of his brain, and some speculate that this put his neurons there into overdrive, flushing them with metabolic agents.
Brains vary in functional organization. For example, speech is typically located in the dominant hemisphere; it can be the right or left. If you are right handed, then you probably have a dominant left hemisphere, but if you are left handed, you likely have a dominant right hemisphere. In some, often left-handed people, the language areas do not localize to either hemisphere but are found in both. We have statistical data on where areas associated with higher-level capacities are situated, but there are plenty of individual differences.
If you look at an exposed brain, you see the telencephalon (also known as the cerebrum). It contains higher-level brain structures and areas mapped to cognitive functions. These surface structures evolved later than deeper ones. The columnar cytoarchitecture varies across the cerebral cortex, as Brodmann noted. He found and described 52 distinct so-called Brodmann areas. As noted in the section “Functional Neuroanatomy and Biochemistry” earlier in this chapter, Brodmann areas have been used to map cognitive functions, and his physiological framework has proven useful.
Canadian neurosurgeon Wilder Penfield (1891-1976) also helped us to understand how the cerebral cortex divides into functional areas. He surgically treated epileptic patients. As they remained conscious, Penfield probed the cerebral cortex to diagnose where seizures originated. He stimulated cells planned for destruction with an electrode to assess their function through patient responses. In this manner, he could avoid cutting into critical areas. Penfield routinely explored the cortex during his operations and learned about areas that could trigger motor movements; audio, visual, and tactile sensations; and automatic vocalizations (Penfield and Jasper 1954).