The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

FIGURE INT.1 Drawing of a neuron showing the cell body, dendrites, and axon. The axon transmits information (in the form of nerve impulses) to the next neuron (or set of neurons) in the chain. The axon is quite long, and only part of it is shown here. The dendrites receive information from the axons of other neurons. The flow of information is thus always unidirectional.

The information processing that occurs both within and between brain structures can get quite complicated—this is, after all, the information-processing engine that generates the human mind—but there is plenty that can be understood and appreciated by nonspecialists. We will revisit many of these areas in greater depth in the chapters ahead, but a basic acquaintance now with each region will help you to appreciate how these specialized areas work together to determine mind, personality, and behavior.

The human brain looks like a walnut made of two mirror-image halves (Figure Int.2). These shell-like halves are the cerebral cortex. The cortex is split down the middle into two hemispheres: one on the left, one on the right. In humans the cortex has grown so large that it has been forced to become convoluted (folded), giving it its famous cauliflower-like appearance. (In contrast, the cortex of most other mammals is smooth and flat for the most part, with few if any folds in the surface.) The cortex is essentially the seat of higher thought, the tabula (far from) rasa where all of our highest mental functions are carried out. Not surprisingly, it is especially well developed in two groups of mammals: dolphins and primates. We’ll return to the cortex later in the chapter. For now let’s look at the other parts of the brain.

FIGURE INT.2 The human brain viewed from the top and from the left side. The top view shows the two mirror-symmetric cerebral hemispheres, each of which controls the movements of—and receives signals from—the opposite side of the body (though there are some exceptions to this rule). Abbreviations: DLF, dorsolateral prefrontal cortex; OFC, orbitofrontal cortex; IPL, inferior parietal lobule; I, insula, which is tucked away deep beneath the Sylvian fissure below the frontal lobe. The ventromedial prefrontal cortex (VMF, not labeled) is tucked away in the inner lower part of the frontal lobe, and the OFC is part of it.

FIGURE INT.3 A schematic drawing of the human brain showing internal structures such as the amygdala, hippocampus, basal ganglia, and hypothalamus.

Running up and down the core of the spinal column is a thick bundle of nerve fibers—the spinal cord—that conducts a steady stream of messages between brain and body. These messages include things like touch and pain flowing up from the skin, and motor commands rat-a-tat-tatting down to the muscles. At its uppermost extent the spinal cord pokes up out of its bony sheath of vertebrae, enters the skull, and grows thick and bulbous (Figure Int.3). This thickening is called the brainstem, and it is divided into three lobes: medulla, pons, and midbrain. The medulla and nuclei (neural clusters) on the floor of the pons control important vital functions like breathing, blood pressure, and body temperature. A hemorrhage from even a tiny artery supplying this region can spell instant death. (Paradoxically, the higher areas of the brain can sustain comparatively massive damage and leave the patient alive and even fit. For example, a large tumor in the frontal lobe might produce barely detectable neurological symptoms.)