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

“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 for pain “empathy” (in the anterior cingulate).

MOTOR NEURON A neuron that carries information from the central nervous system to a muscle. Also loosely used to include motor-command neurons, which program a sequence of muscle contractions for actions.

MU WAVES Some specific brain waves that are affected in autism. Mu waves may or may not be an index of mirror-neuron function, but they get suppressed both during action performance and action observation, suggesting a close link with the mirror-neuron system.

NATURAL SELECTION Sexual reproduction results in shuffling genes into novel combinations. Nonlethal mutations arise spontaneously. Those mutations or gene combinations that make some species better adapted to their current environment are the ones that survive more often because the parents survive and reproduce more often. The term is used in opposition to creationism (which holds that all species were created at once) and in contrast to artificial selection by humans to improve livestock and plants. Natural selection is not synonymous with evolution; it is a mechanism that drives evolutionary change.

NEURON Nerve cell. It is specialized for the reception and transmission of information, and is characterized by long fibrous projections called axons and shorter, branchlike projections called dendrites.