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

Sitting on the roof of the pons is the cerebellum (Latin for “little brain”), which controls the fine coordination of movements and is also involved in balance, gait, and posture. When your motor cortex (a higher brain region that issues voluntary movement commands) sends a signal to the muscles via the spinal cord, a copy of that signal—sort of like a CC email—gets sent to the cerebellum. The cerebellum also receives sensory feedback from muscle and joint receptors throughout the body. Thus the cerebellum is able to detect any mismatches that may occur between the intended action and the actual action, and in response can insert appropriate corrections into the outgoing motor signal. This sort of real-time, feedback-driven mechanism is called a servo-control loop. Damage to the cerebellum causes the loop to go into oscillation. For example, a patient may attempt to touch her nose, feel her hand overshooting, and attempt to compensate with an opposing motion, which causes her hand to overshoot even more wildly in the opposite direction. This is called an intention tremor.

Surrounding the top portion of the brainstem are the thalamus and the basal ganglia. The thalamus receives its major inputs from the sense organs and relays them to the sensory cortex for more sophisticated processing. Why we need a relay station is far from clear. The basal ganglia are a strangely shaped cluster of structures that are concerned with the control of automatic movements associated with complex volitional actions—for example, adjusting your shoulder when throwing a dart, or coordinating the force and tension in dozens of muscles throughout your body while you walk. Damage to cells in the basal ganglia results in disorders like Parkinson’s disease, in which the patient’s torso is stiff, his face is an expressionless mask, and he walks with a characteristic shuffling gait. (Our neurology professor in medical school used to diagnose Parkinson’s by just listening to the patient’s footsteps next door; if we couldn’t do the same, he would fail us. Those were the days before high-tech medicine and magnetic resonance imaging, or MRI.) In contrast, excessive amounts of the brain chemical dopamine in the basal ganglia can lead to disorders known a choreas, which are characterized by uncontrollable movements that bear a superficial resemblance to dancing.

Finally we come to the cerebral cortex. Each cerebral hemisphere is subdivided into four lobes (see Figure Int.2): occipital, temporal, parietal, and frontal. These lobes have distinct domains of functioning, although in practice there is a great deal of interaction between them.

Broadly speaking, the occipital lobes are mainly concerned with visual processing. In fact, they are subdivided into as many as thirty distinct processing regions, each partially specialized for a different aspect of vision such as color, motion, and form.

The temporal lobes are specialized for higher perceptual functions, such as recognizing faces and other objects and linking them to appropriate emotions. They do this latter job in close cooperation with a structure called the amygdala (“almond”), which lies in the front ties (anterior poles) of the temporal lobes. Also tucked away beneath each temporal lobe is the hippocampus (“seahorse”), which lays down new memory traces. In addition to all this, the upper part of the left temporal lobe contains a patch of cortex known as Wernicke’s area. In humans this area has ballooned to seven times the size of the same area in chimpanzees; it is one of the few brain areas that can be safely declared unique to our species. Its job is nothing less than the comprehension of meaning and the semantic aspects of language—functions that are prime differentiators between human beings and mere apes.

The parietal lobes are primarily involved in processing touch, muscle, and joint information from the body and combining it with vision, hearing, and balance to give you a rich “multimedia” understanding of your corporeal self and the world around it. Damage to the right parietal lobe commonly results in a phenomenon called hemispatial neglect: The patient loses awareness of the left half of visual space. Even more remarkable is somatoparaphrenia, the patient’s vehement denial of ownership of her own left arm and insistence that it belongs to someone else. The parietal lobes have expanded greatly in human evolution, but no part of them has grown more than the inferior parietal lobules (IPL; see Figure Int.2). So great was this expansion that at some point in our past a large portion of it split into two new processing regions called the angular gyrus and the supramarginal gyrus. These uniquely human areas house some truly quintessential human abilities.