Joyce Blue Kettle shows her kindness to Anna by: com limenting Anna's magical powers, sen ing her family to help Anna when Emery dies. ACT.$8É 31 GO ON TO THE NEXT PAGE. Passage11 the same shapc, and gain near-aduli ability to focus at varying distances, SOCIAL SCIENCE: Thls passaga Is adapied from Morton Hunt's The sfory of Psychology (Q1993 by Morton Hunt). In tha passaga, the term maturatlon refers to the process ol growth and devalopmani, and the term percaptual abilify refers to the capaclty to recognlzo somelhlng ihrough tha s£nsas (slghl. smell, touch, etc.). Exactly how maturation and experience interact in S5 the brain tissues lo produce such developmenial changes is becoming clear from neuroscience research. Microscopic examination of the brains of infants shows that as Ihe brain triples in size during the first two years of life, a profusion of dendrites (branches) grow from 60 its neurons and make contact with one another. Much maturation research is concerned with phys- ical skills and physical allributes, and adds little to our knowledge of the growth of the mind. But research on the development of perceptual abilities begins to pro- 5 vide solid factual answers 10 the ancient central ques- tion of psychology: How much is due to nature and how much to nurture (or, in developmental terms. to maiura- tion and to learning)? By ihe time a human is twelve. the brain has an estitnated hundred Irillion synapses (connections between nerve cells). Those conncctions are ihe wiring plan ihat establishes the brain's capabilities. Some of 65 the synaptic connections are made automatically by chemical guidance, but others are made by the stimulus of experience during the period of rapid dendrite growth. Lacking such stimulus. the dendrites wither away without forming the needed synapses. Mice 70 reared in the dark develop fewer dendritic spines and synaptic connections in the Visual coricx than mice reared in ihe light, and even when exposed io light never attain normal vision. The work has been focused on early infancy. when 10 perceptual abilities evolve rapidly; its aim is to dis- cover when each new ability first appears, the assump- tion being that at its first appearance. the new abilily arises not from learning but from maiuration of ihe optic nervous structures and especially of that pari of 15 the brain corlex whLrc visual signals are received and interpreted. Why should nature have done that? Why should 75 pcrceptual developmLni bL possiblL only ac a critical pcriod and not later? It docs not evolutionary sensc for the organism to bc pcrmancnily impaircd in scnsory performance just becausc it fails to have the propcr cxpericnces at speci fic times in lis dcvelopmcni. eo Bul somc brain rcscarchcrs say that there is an offset- ting advantage: Ihc essential exp¢ricnccs orc almost always available oi thc right time,'and they fine-tune thc brain structurc so as lo provid¢ far morc spccifiL pcrccptual powers than could rcsult from genetic con- 85 Irol of synapse formation. Much has been learned by simply waiching infants. Whal. exactly. do very young infants SLC? Sincc wc cannot ask them what they scc, how can wc 20 find oui? In 1961, Ihc psychologist Roberi Fantz dcvised an ingenious mcthod of doing so. Hc dcsigncd u stand in which. on ihe botiom Icvel, tho baby lies on her back, looking up. A few feet above is o display arca where 25 the cxperirncnter puts two large cards, each conlaining a design a whitc circle, o ycllow circlc. a bull's-eyc, a simple skcich of a face. Thc rcsearchcr. pcering down through L liny pcepholc. can walch the movement of thc baby's cyes and timc how long Ihcy arc dir¢ctcd at 20 one or Ihc other of each pair of paiterns. Fantz found Ihal al two monihs babies looked iwice as long at a bull's-eyc as at a circle of solid color, and twice as long at a skeich of a face as at a bull's-eye. Eyidcnily, eycn a two-month-old can,distinguish major differences and 35 direct her gaze toward what she finds more interestin8. Using this technique, developmental psychologists have leartted a great deal about what infants see and when they begin to sce it. In the first weck infant5 dis- tinguish light and dark patterns; during the first month 40 they begin lo track slowly moving ob'ects; by the second month they begin to have dept perception, coordinate th¢ mov¢ment of the eyes, and differ¢ntiatc among hues and levels of brightness. by three months they can glance from one object to another. and can dis- 45 tinguish among fatnily member5; by four months they focus at varying distances, make increasingly fine dis- tinctions, and begin to recognize the meaning of what they see (they look longer at a normal sketch of a face than ai one in which the features have been scrambled); 50 and from four io seven mon(hs they achieve sler¢opsis, recognize that a shape held at different angles is still With that, the vague old tcrms nalure and nurturc take on turies o recise new meaning. Now, aft¢r so many cen- speculation, we catch thc first glimpsc of how mind is constructed out of mattcr by expcricllcc.
