The Tell-Tale Brain

other neurons.

ELECTROENCEPHALOGRAPHY (EEG) A measure of the brain’s electrical activity in response to sensory stimuli. This is obtained by placing electrodes on the surface of the scalp (or, more rarely, inside the head), repeatedly administering a stimulus, and then using a computer to average the results. The result is an electroencephalogram (also abbreviated EEG).

EPISODIC MEMORY Memory for specific events from your personal experience.

EXAPTATION A structure evolved through natural selection for a particular function that becomes subsequently used—and refined through further natural selection—for a completely novel unrelated function. For example, bones of the ear that evolved for amplifying sound were exapted from reptilian jaw bones used for chewing. Computer scientists and evolutionary psychologists find the idea irritating.

EXCITATION A change in the electrical state of a neuron that is associated with an enhanced probability of action potentials (a train of electrical spikes that occurs when a neuron sends information down an axon).

FRONTAL LOBE One of the four divisions of each cerebral hemisphere. (The other three divisions are the parietal, temporal, and occipital lobes.) The frontal lobes include the motor cortex, which sends commands to muscles on the opposite side of the body; the premotor cortex, which orchestrates these commands; and the prefrontal cortex, which is the seat of morality, judgment, ethics, ambition, personality, character, and other uniquely human attributes.

FUNCTIONAL MAGNETIC RESONANCE IMAGINING (FMRI) A technique—in which the baseline activity of the brain (with the person doing nothing) is subtracted from the activity during task performance—that determines which anatomical regions of the brain are active when a person engages in a specific motor, perceptual, or cognitive task. For example, subtracting a German brain’s activity from that of an Englishman might reveal the “humor center” of the brain.

FUSIFORM GYRUS A gyrus near the bottom inner part of the temporal lobe that has subdivisions specialized for recognizing color, faces, and other objects.

GALVANIC SKIN RESPONSE (GSR) When you see or hear something exciting or significant (such as a snake, a mate, prey, or a burglar), your hypothalamus is activated; this causes you to sweat, which changes your skin’s electrical resistance. Measuring this resistance provides an objective measure of emotional arousal. Also called skin conductance response (SCR).

HEMISPHERES See Cerebral hemispheres.

HIPPOCAMPUS A seahorse-shaped structure located within the temporal lobes. It functions in memory, especially the acquisition of new memories.

HOMININS Members of the Hominini tribe, a taxonomic group recently reclassified to include chimpanzees (Pan), human and extinct protohuman species (Homo), and some ancestral species with a mix of human and apelike features (such as Australopithecus). The hominins are thought to have diverged from the gorillas (Gorillini tribe).

HORMONES Chemical messengers secreted by endocrine glands to regulate the activity of target cells. They play a role in sexual development, calcium and bone metabolism, growth, and many other activities.

“HOW” STREAM The pathway from the visual cortex to the parietal lobe that guides muscle twitch sequences that determine how you move your arm or leg in relation to your body and environment. You need this pathway to accurately reach for an object, and for grasping, pulling, pushing, and other types of object manipulation. To be distinguished from the “what” stream in the temporal lobes. Both “what” and “how” streams diverge from the new pathway, whereas the old pathway starts from the superior colliculus and projects onto the parietal lobe, converging on it with the “how” stream. Also called pathway 1.

HYPOTHALAMUS A complex brain structure composed of many cell clusters with various functions. These include emotions, regulating the activities of internal organs, monitoring information from the autonomic nervous system, and controlling the pituitary gland.

INFERIOR PARIETAL LOBULE (IPL) A cortical region in the middle part of the parietal lobe, just below the superior parietal lobule. It became several times bigger in humans compared with apes, especially on the left. In humans the IPL split into two entirely new structures: the supramarginal gyrus (on top), which is involved in skilled actions such as tool use; and the angular gyrus, involved in arithmetic, reading, naming, writing, and possibly also in metaphorical thinking.

INHIBITION In reference to neurons, a synaptic message that prevents the recipient cell from firing.

INSULA An island of cortex buried in the folds on the side of the brain, divided into anterior, middle, and posterior sections, each of which has many subdivisions. The insula receives sensory input from the viscera (internal organs) as well as taste, smell, and pain inputs. It also gets inputs from the somatosensory cortex (touch, muscle and joint, and position sense) and the vestibular system (organs of balance in the ear). Through these interactions, the insula helps construct a person’s “gut level,” but not fully articulated, sense of a rudimentary “body image.” In addition, the insula has mirror neurons that both detect disgusting facial expressions and express disgust toward unpleasant food and smells. The insula is connected via the parabrachial nucleus to the amygdala and the anterior cingulate.

KORO A disorder that purportedly afflicts young Asian men who develop the delusion that their penises are shrinking and may eventually drop off. The converse of this syndrome—aging Caucasian men who develop the delusion that their penises are expanding—is much more common (as noted by our colleague Stuart Anstis). But it has not been officially given a name.

LIMBIC SYSTEM A group of brain structures—including the amygdala, anterior cingulate, fornix, hypothalamus, hippocampus, and septum—that work to help regulate emotion.

MIRROR NEURONS Neurons that were originally identified in the frontal lobes of monkeys (in a region homologous to the Broca’s language area in humans). The neurons fire when the monkey reaches for an object or merely watches another monkey start to do the same thing, thereby simulating the other monkey’s intentions, or reading its mind. Mirror neurons have also been found for touch; that is, sensory touch mirror neurons fire in a person when she is touched and also when she watches another person being stroked. Mirror neurons also exist for making and recognizing facial expressions (in the insula) and