neurotransmitters: Brain chemicals that carry information from the axon of a sending neuron to the dendrites of a receiving neuron.

axons: Fibers that extend from neurons and transmit electrochemical impulses from that neuron to the dendrites of other neurons.

synapses: The intersection between the axon of one neuron and the dendrites of other neurons.

head-sparing: A biological mechanism that protects the brain when malnutrition disrupts body growth. The brain is the last part of the body to be damaged by malnutrition.

co-sleeping: A custom in which parents and their children (usually infants) sleep together in the same room.

transient exuberance: The great but temporary increase in the number of dendrites that develop in an infant’s brain during the first two years of life.

norm: An average, or standard, calculated from many individuals within a specific group or population.

neurons: Nerve cells in the central nervous system, especially in the brain.

cortex: The outer layers of the brain in humans and other mammals. Most thinking, feeling, and sensing involve the cortex.

pruning: When applied to brain development, the process by which unused connections in the brain atrophy and die.

prefrontal cortex: The area of the cortex at the very front of the brain that specializes in anticipation, planning, and impulse control.

REM (rapid eye movement) sleep: A stage of sleep characterized by flickering eyes behind closed lids. REM indicates dreaming.

dendrites: Fibers that extend from neurons and receive electrochemical impulses transmitted from other neurons via their axons.

percentile: A point on a ranking scale of 0 to 100. The 50th percentile is the midpoint; half the people in the population being studied rank higher and half rank lower.

bed-sharing: When two or more people sleep in the same bed.

binocular vision: The ability to focus both eyes in a coordinated manner in order to see one image. Depth perception requires it.

stunting: The failure of children to grow to a normal height for their age due to severe and chronic malnutrition.

protein-calorie malnutrition: A condition in which a person does not consume sufficient food. This can result in illness, severe weight loss, and even death.

sensation: The response of a sensory system (eyes, ears, skin, tongue, nose) when it detects a stimulus.

perception: The mental processing of sensory information when the brain interprets a sensation.

sudden infant death (SIDS): An infant’s unexpected, sudden death; when a seemingly healthy baby, usually between 2 and 6 months old, stops breathing and dies while asleep.

experience-dependent: Brain functions that depend on particular, variable experiences and therefore may or may not develop in a particular person.

shaken baby syndrome: A life-threatening injury that occurs when an infant is forcefully shaken back and forth, a motion that ruptures blood vessels in the brain and breaks neural connections.

gross motor skills: Physical abilities involving large body movements, such as walking and jumping. (The word gross here means “big.”)

immunization: A process that stimulates the body’s immune system to defend against attack by a particular contagious disease. Immunization may be accomplished either naturally (by having the disease) or through vaccination (often by having an injection). (Also called vaccination.)

motor skills: The learned abilities to move some part of the body, in actions ranging from a large leap to a flicker of the eyelid. (The word motor here refers to movement of muscles.)

fine motor skills: Physical abilities involving small body movements, especially of the hands and fingers, such as drawing and picking up a coin. (The word fine here means “small.”)

experience-expectant: Brain functions that require certain basic common experiences (which an infant can be expected to have) in order to develop normally.

marasmus: A disease of severe protein-calorie malnutrition during early infancy, in which growth stops, body tissues waste away, and the infant eventually dies.

wasting: The tendency for children to be severely underweight for their age as a result of malnutrition.

child-directed speech: The high-pitched, simplified, and repetitive way adults speak to infants. (Also called baby talk or motherese.)

mean length of utterance (MLU): The average number of meaningful sound combination in a typical sentence (called utterance, because children may not use conventional words). MLU is often used to indicate a child’s language development.

grammar: All the methods—word order, verb forms, and so on—that languages use to communicate meaning, apart from the words themselves.

deferred imitation: A sequence in which an infant first perceives something done by someone else and then performs the same action hours or even days later.

little scientist: The stage-five toddler (age 12 to 18 months) who experiments without imagining the consequences, using trial and error in active and creative exploration.

holophrase: A single word that is used to express a complete, meaningful thought.

sensorimotor intelligence: Piaget’s term for the way infants think—by using their senses and motor skills—during the first period of cognitive development.

kwashiorkor: A disease of chronic malnutrition, in which a protein-calorie deficiency makes a child more vulnerable to other diseases, such as measles, diarrhea, and influenza.

naming explosion: A sudden increase in an infant’s vocabulary, especially in the number of nouns, that begins at about 18 months of age.

object permanence: The realization that objects (including people) still exist even if they can no longer be seen, touched, or heard.

babbling: The extended repetition of certain syllables, such as ba-ba-ba, when babies are between 6 and 9 months old.

information-processing theory: A perspective that compares human thinking processes, to computer analysis of data, including sensory input, connections, stored memories, and output.

language acquisition device (LAD): Chomsky’s term for a hypothesized mental structure that enables humans to learn language, including the basic aspects of grammar, vocabulary, and intonation.