The Brain and Behavior

1 The Brain and Behavior Eric R. Kandel THE LAST FRONTIER OF THE biological sciences—th eir ultimate challen ge—is to understand the bio lo gical basis of conscio usness and the mental processes by wh ich we perceive, act , learn , and remember.In the last two decades a remarkable unity has emerged wit hin bio lo gy. The abilit y to sequence genes and infer the amin o acid sequences for the protein s they enco de has revealed unant icipat ed similarit ies between protein s in the nervous system and those encountered elsewhere in the bo dy. As a resu lt , it h as beco me possible to est ablish a gen eral plan for the funct io n of cells, a plan that provides a commo n concept ual framework for all o f cell biology, inclu din g cellu lar neu robio lo gy. The next an d even more challen gin g step in this un ifyin g process with in bio lo gy, which we outlin e in this boo k, will be the un ification of the study of beh avio r—th e scien ce of the min d—an d neural scien ce, the science o f the brain. This last step will allo w us to achieve a unified scient ific approach to the study o f behavior. Such a comprehensive approach depends on the view th at all behavior is the result of brain function . What we commonly call the mind is a set of operation s carried out by the brain . The actions of the brain underlie not only relatively simple motor behaviors such as walking or eating, but all the co mplex co gnitive action s that we believe are qu in tessentially human, such as thinkin g, speaking, and creating wo rks of art . As a co rollary, all the behavioral disorders th at characterize psychiat ric illness—disorders o f affect (feeling) and cogn it io n (thou ght)— are disturbances o f brain funct io n. The task o f neural science is to explain beh avio r in terms o f the act ivit ies of the brain . How does the brain marshal its millio ns o f in dividu al nerve cells to produce behavior, an d how are these cells in fluenced by the environ ment, which in cludes the action s of other people? The progress of neural scien ce in explain ing human behavior is a major theme of th is book. L ike all science, neu ral scien ce must cont in ually con front certain fun dament al questions. Are part icular ment al processes localized to specific regio ns o f the brain, or does the min d represent a collect ive an d emergent pro pert y o f the wh ole brain ? If specific ment al processes can be localized to discrete brain region s, what is the relation sh ip bet ween the an atomy an d ph ysio lo gy of one region and its specific funct io n in perception , thou ght, o r movement ? Are such relat io nships mo re likely to be revealed by examining the regio n as a who le or by studying its in dividu al nerve cells? In th is chapt er we consider to wh at degree mental function s are located in specific regions of the brain and to what degree such lo cal ment al processes can be understood in terms of the propert ies of specific nerve cells an d their int erco nnect io ns. To answer these question s, we loo k at how modern neu ral scien ce appro ach es one of the most elaborate cogn it ive behaviors—language. In do in g so we necessarily (P.6) focus on the cerebral co rtex, the part of the brain concerned with the most evolved human behaviors. Here we see how the brain is organized int o regio ns o r brain compart ments, each made up of large groups of neurons, and how h ighly co mplex behaviors can be traced to specific regio ns o f the brain and underst ood in terms o f the funct io ning of grou ps of neu ron s. In the next ch apter we consider how these neural circuits funct io n at the cellu lar level, usin g a simple reflex beh avio r to examine the way senso ry sign als are transformed int o motor acts. Two Opposing Views Have Been Advanced on the Relationship Between Brain and Behavior

11 Ou r cu rrent views about nerve cells, the brain , and behavior h ave emerged over the last centu ry from a convergence of five experiment al tradition s: an atomy, embryology, ph ysio lo gy, ph armacology, an d psycho lo gy. Befo re the invent io n of the compoun d microsco pe in the eighteent h century, nervous t issue was thought to fu nction like a gland—an idea that goes back to the Greek ph ysician Galen , who proposed that nerves con vey flu id secreted by the brain an d spinal cord to the body's periph ery. The microscope revealed the t rue structure of the cells o f nervous tissue. Even so, nervous t issue did not become the subject of a special scien ce until the late 1800s, when the first detailed descript io ns of nerve cells were un dert aken by Camillo Golgi and Sant iago Ramón y Caj al. Go lgi developed a way of stain in g neurons wit h silver salts that revealed their ent ire st ructu re under the micro scope. He could see clearly th at neurons had cell bodies and two major types of project io ns or processes: bran ching dendrites at one end and a lo ng cable- like axon at the other. Usin g Go lgi's techniqu e, Ramón y Cajal was able to stain individu al cells, thus sho wing th at nervou s tissue is not one continu ous web but a network of discrete cells. In the cou rse o f this work, Ramón y Cajal develo ped some of the key concepts and much of the early evidence for the neuron doctrine —the principle that individual neu rons are the element ary signalin g elements o f the nervou s syst em. Addition al experimen tal suppo rt fo r the neu ron doct rine was pro vided in the 1920s by the American embryo lo gist Ross Harriso n, wh o demon strated that the two majo r projection s of the nerve cell—t he dendrites and the axo n—gro w out fro m the cell body and that they do so even in tissue cu lt ure in wh ich each neu ron is isolat ed fro m other neuro ns. Harriso n also con firmed Ramón y Cajal's suggest io n that the t ip of the axon gives rise to an expansio n called the growth cone , which leads the developin g axon to it s target (wheth er to other nerve cells o r to mu scles). Physiological in vest igat io n of the nervous system began in the lat e 1700s when the Italian ph ysician an d physicist Lu igi Galvan i disco vered th at livin g excitable muscle an d nerve cells produ ce elect ricity. Modern electroph ysio lo gy grew out o f wo rk in the n in eteenth cent ury by th ree German ph ysio lo gist s—Emil DuBo is-Reymo nd, Joh annes Mü ller, and Herman n von Helmh oltz—who were able to show that the electrical activit y of one nerve cell affects the activit y of an adj acent cell in predict able ways. Pharmaco lo gy made its first impact on ou r underst andin g of the nervous system and behavior at the end of the n in eteenth century, wh en Claude Bernard in France, Pau l Ehrlich in German y, and John L angley in Englan d demonst rated th at dru gs do not interact with cells arbit rarily, but rather bin d to specific recepto rs typically lo cated in the membrane on the cell su rface. This disco very became the basis o f the all- import ant stu dy of the chemical basis o f commun icat io n bet ween nerve cells. The psycho lo gical in vest igat io n o f behavior dates back to the begin nings o f Western science, to classical Greek philosoph y. Man y issues cent ral to the modern invest igat io n of beh avio r, part icularly in the area of perception , were subsequent ly reformulat ed in the seventeent h centu ry first by René Descartes and then by John Locke, of whom we sh all learn more lat er. In the midnineteenth century Ch arles Darwin set the st age fo r the study o f an imals as models o f human act io ns an d behavior by publish in g h is observation s on the continu it y of species in evolut io n. Th is new approach gave rise to etho lo gy, the stu dy of an imal beh avio r in the n atural environ ment , an d later to experimen tal psycho lo gy, the study of human an d an imal beh avio r under cont ro lled condition s. In fact , by as early as the end o f the eighteenth centu ry the first attempt s had been made to bring together bio lo gical an d psycho lo gical concepts in the study of behavior. Fran z Jo seph Gall, a German physician and neu ro anatomist , proposed three radical new ideas. First, he advocat ed that all behavior eman ated fro m the brain. Seco nd, he argued that part icular

12 region s of the cerebral co rtex contro lled specific fu nction s. Gall asserted that the cerebral co rtex did not act as a single organ but was divided in to at least 35 o rgans (others were added later), each co rrespon ding to a specific ment al facu lt y. E ven the most abst ract of human behaviors, such as generosit y, secretiveness, and religio sit y were assigned their spot in the brain. Th ird, Gall proposed that the cent er for each ment al funct io n grew with use, much as a mu scle bulks up wit h exercise. As each center (P.7) grew, it pu rportedly caused the o verlying skull to bu lge, creat in g a pattern of bu mps and ridges on the sku ll th at indicat ed wh ich brain regions were most developed ( Figu re 1-1 ). Rather than loo king wit hin the brain , Gall sou ght to establish an an atomical basis for describing ch aracter traits by co rrelat in g the personalit y of in dividuals wit h the bumps on their sku lls. His psycho lo gy, based on the distribut io n o f bumps on the outside of the head, became kn own as ph reno logy. In the lat e 1820s Gall's ideas were subjected to experimen tal an alysis by the French ph ysio lo gist Pierre Flou rens. By systemat ically removin g Gall's funct io nal centers from the brain s o f experiment al animals, Flou rens attempt ed to isolat e the cont ribu tion s of each “cerebral o rgan” to beh avio r. From these experimen ts he conclu ded that specific brain region s were not respo nsible for specific behaviors, but th at all brain regio ns, especially the cerebral hemisph eres of the fo rebrain, part icipat ed in every mental operat io n. An y part of the cerebral hemisph ere, he proposed, was able to perfo rm all the funct io ns of the hemisphere. Inju ry to a specific area of the cerebral hemisphere wou ld therefore affect all higher function s equ ally. In 1823 Flo urens wro te: “All perception s, all vo lit io ns occupy the same seat in these cerebral) organs; the faculty of perceiving, o f conceivin g, of willing merely const it utes therefore a facu lt y which is essentially one.” The rapid accept ance o f this belief (later called the aggregate-field view of the brain ) was based only part ly on Flo urens's experiment al work. It also represented a cultu ral react io n again st the reduct io nist view that the human min d has a biological basis, the not io n that there was no soul, that all ment al processes cou ld be reduced to action s with in differen t regio ns in the brain! The aggregate-field view was first serio usly challen ged in the mid-n in eteenth century by the British neu rologist J. Hugh lin gs Jackson . In h is stu dies of focal epilepsy, a disease ch aracterized by con vu lsio ns that begin in a particu lar part of the body, Jackson showed that different moto r an d senso ry funct io ns can be traced to different part s of the cerebral cort ex. These stu dies were lat er refined by the German neurologist Karl Wernicke, the English ph ysio lo gist Ch arles Sherrington , and Ramón y Caj al in to a view o f brain funct io n called cellu lar connect ionism . According to th is view, in dividual neu rons are the sign aling un it s of the brain ; they are generally arran ged in funct io nal groups and conn ect to one another in a precise fash io n. Wernicke's wo rk in part icular showed th at different behaviors are pro duced by different brain regions in terconnect ed by specific neu ral path ways. The differences between the aggregate-field theory and cellu lar- connect io nism can best be illustrat ed by an analysis of how the brain pro duces lan gu age. Befo re we consider the relevan t clin ical and an atomical studies concerned with the localizat io n of lan gu age, let us briefly loo k at the overall structu re o f the brain. (Th e an atomical organ izat io n of the nervo us system is described in detail in Ch apter 17 .)

13 Figure 1-1 According to the nineteenth- century doctrine of phrenology, complex traits such as combativeness, spirituality, hope, and conscientiousness are controlled by specific areas in the brain, which ex pand as the traits develop. This enlargement of local areas o f the brain was thought to produce characterist ic bumps an d ridges on the o verlying skull, from wh ich an in dividual's ch aracter cou ld be determin ed. This map, taken fro m a drawin g of the early 1800s, purport s to sho w 35 in tellectu al and emot io nal facu lt ies in distinct areas o f the skull and the cerebral co rtex underneath.

The Brain Has Distinct Functional Regions

The cent ral nervo us system is a bilateral an d essent ially symmet rical structu re wit h seven main parts: the spinal co rd, medu lla oblo ngata, pons, cerebellum, midbrain, dienceph alon , and the cerebral hemispheres ( Bo x 1-1 and Figures 1-2A ,1-2B and 1-3 ). Radio graphic imagin g techn iques have made it po ssible to visu alize these structu res in livin g subjects. Th rough a variety o f experimen tal methods, such images o f the brain can be made while subj ects are engaged in specific t asks, which then can be related to the activit ies of discret e regio ns o f the brain . As a result, Gall's original idea th at differen t regio ns are (P.8 P.9 ) specialized fo r differen t funct io ns is now accepted as one of the co rnerstones o f mo dern brain science. Box 1-1 The Central Nervous System

The cent ral nervo us system has seven main parts ( Figure 1-2A ).

 The spinal cord, the most cau dal part of the cent ral nervous system, receives and processes senso ry in fo rmat io n from the skin , jo in ts, and muscles of the limbs and trunk and contro ls movement of the limbs and the trunk. It is su bdivided in to cervical, tho racic, lu mbar, and sacral regio ns. The spinal co rd cont in ues rostrally as the brain

14 stem, which consists of the medulla, pons, and midbrain (see belo w). The brain stem receives sensory in fo rmat io n from the skin and muscles of the head and pro vides the moto r cont rol for the mu scles of the head. It also con veys information from the spin al co rd to the brain and fro m the brain to the spinal co rd, and regu lates levels of arousal and awareness, thro ugh the reticu lar fo rmation . The brain stem cont ains several co llection s of cell bo dies, the cran ial nerve nuclei. Some of these nuclei receive information fro m the skin an d muscles of the head; others cont rol motor output to muscles of the face, neck, and eyes. Still others are specialized fo r information fro m the special senses: hearing, balan ce, an d taste.

 The medulla oblongata , wh ich lies direct ly abo ve the spin al cord, in cludes several centers respo nsible fo r vit al aut onomic fun ctions, such as digestion , breat hing, and the cont rol of heart rate.

 The pons, which lies above the medulla, conveys in fo rmat io n about movement from the cerebral hemisph ere to the cerebellu m.

 The cerebellum lies beh in d the pons an d is connect ed to the brain stem by several major fiber tract s called pedun cles . The cerebellum modulat es the force and range of mo vement an d is involved in the learn in g of moto r skills.

Figure 1-2A The cent ral nervous system can be divided in to seven main part s.

 The midbrain , which lies rostral to the po ns, cont ro ls man y sen sory and moto r function s, in cludin g eye movement and the coo rdination of visual and audito ry reflexes.

 The diencephalon lies rostral to the midbrain an d contain s t wo structures. One, the th alamus , processes most of the information reaching the cerebral cortex fro m the rest of the cent ral nervous system. The other, the h ypoth alamu s, regu lates autonomic, endocrin e, an d visceral fun ction .

15  The cerebral hemispheres consist of a heavily wrin kled outer layer—th e cerebral co rtex —and th ree deep-lying struct ures: the basal ganglia, the hippocampus , an d the amygdalo id nuclei . The basal gan glia part icipat e in regulat in g motor performance; the h ippocampus is involved wit h aspect s of memory sto rage; and the amygdaloid nuclei coo rdin ate the autono mic and endocrine responses of emot io nal stat es. The cerebral co rtex is divided in to fou r lobes: frontal, pariet al, temporal, an d occipital ( Figu re 1- 2B ).

The brain is also commonly divided in to three bro ader regio ns: the hindbrain (th e medu lla, pon s, and cerebellu m), midbrain , an d fo rebrain (dien cephalo n an d cerebral hemispheres). The h in dbrain (excludin g the cerebellum) an d midbrain comprise the brain stem.

Figure 1-2B Th e fou r lobes of the cerebral co rtex.

Figure 1-3 The main divisions are clearly visible when the brain is cut down the midline between the two hemispheres. A. Th is schemat ic drawing sho ws the po sition of major structu res of the brain in relation to external landmarks. Students of brain anato my quickly learn to dist in gu ish the major in ternal

16 lan dmarks, such as the co rpus callosum, a large bun dle of nerve fibers that connects the left and right hemisph eres. B. Th e majo r brain division s drawn in A are also evident here in a magnet ic resonance image o f a livin g human brain .

On e reason th is conclusio n elu ded in vestigato rs for so many years lies in anot her o rganization al prin ciple of the nervous system known as parallel dist ributed processing . As we sh all see below, many sensory, moto r, and cogn it ive funct io ns are served by mo re than one neu ral pathway. When one funct io nal region or pathway is damaged, others may be able to compensate partially fo r the lo ss, thereby obscu ring the beh avio ral evidence fo r localizat io n. Nevertheless, the neu ral path ways fo r certain h igher funct io ns h ave been precisely mapped in the brain . Cognitive Functions Are Localized Within the Cerebral Cortex

The brain operat io ns respon sible fo r ou r cognitive abilit ies occur primarily in the cerebral co rtex —the furro wed gray matt er co vering the cerebral hemispheres. In each of the brain 's t wo hemispheres the overlyin g co rtex is divided in to fou r an atomically distinct lo bes: front al, parietal, tempo ral, and occipit al (see Figure 1-2B ), o riginally n amed for the sku ll bon es that encase them. These lobes h ave specialized fu nction s. The front al lobe is largely concern ed wit h plan ning futu re action and wit h the cont ro l of mo vement; the pariet al lobe wit h somat ic sensat io n, wit h forming a bo dy image, and wit h relat in g one's bo dy image wit h extraperso nal space; the occipit al lobe wit h vision ; the tempo ral lobe with hearin g; an d throu gh its deep st ructu res—the h ippocampus and the amygdalo id nuclei—wit h aspects of learn in g, memo ry, and emot io n. Each lobe has several ch aracterist ic deep in fo ldings (a favo red evo lu tion ary st rategy for packing in more cells in a limited space). The crests of these convolut io ns are called gyri, wh ile the int erven in g groo ves are called su lci o r fissures. The mo re promin ent gyri an d sulci are qu it e similar in everyone and have specific names. For example, the cent ral su lcus separat es the precentral gyrus , which is concerned with motor funct io n, from the post central gyrus , which is concerned with senso ry funct io n ( Figure 1-4A ). The organ izat io n of the cerebral cort ex is characterized by two impo rt ant featu res. First, each hemisph ere is concerned primarily wit h sensory an d motor pro cesses on the cont ralat eral (oppo site) side of the bo dy. Thus senso ry information th at arrives at the spin al cord from the left side of the bo dy—fro m the left han d, say—crosses over to the righ t side of the nervous system (eit her with in the spinal co rd or in the brain stem) on it s way to the cerebral cortex. Similarly, the motor areas in the righ t hemisphere exert control o ver the movements o f the left half (P.10) of the bo dy. Secon d, althou gh the hemisph eres are similar in appearance, they are not completely symmet rical in struct ure nor equ ivalent in function . To illu st rate the role of the cerebral cortex in cognition , we will t race the development o f our underst an ding of the neu ral basis of language, using it as an example of ho w we have progressed in localizin g mental funct io ns in the brain. The neural basis of language is discussed mo re fu lly in Ch apter 59 . Much o f wh at we know about the lo calizat ion of language comes fro m studies of aphasia, a lan gu age diso rder fo und most often in patien ts who have su ffered a st roke (th e occlusio n or ruptu re of a bloo d vessel supplyin g blo od to a po rtion o f the cerebral hemisphere). Many of the important disco veries in the study of aph asia occu rred in rapid successio n durin g the last h alf of the n in eteenth cent ury. Taken togeth er, these advances fo rm one o f the most exciting ch apters in the stu dy of human beh avio r, because they offered the first in sigh t into the bio lo gical basis of a complex ment al funct io n.

17 The French neuro lo gist Pierre Pau l Broca was much influenced by Gall and by the idea that function s could be lo calized. But he exten ded Gall's th in king in an impo rt ant way. He argu ed th at phrenology, the attempt to localize the funct io ns of the mind, shou ld be based on examining damage to the brain produced by clin ical lesio ns rather th an by examin in g the distribut io n of bumps on the outside o f the head. Thus he wro te in 1861: “I had thought th at if there were ever a ph reno lo gical scien ce, it wou ld be the phrenology o f convolut io ns (in the co rtex), and not the phreno lo gy of bu mps (on the head).” Based on this insight Broca founded neuropsycho logy , a new scien ce of ment al processes th at he was to dist in gu ish fro m the ph reno lo gy of Gall. In 1861 Broca described a patien t named Lebo rgne, who cou ld underst and lan gu age but could not speak. The patien t had non e of the con vent io nal moto r deficits (o f the tongu e, mout h, o r vocal cords) that wou ld affect speech. In fact , he cou ld utter isolat ed words, wh istle, and sin g a melo dy wit hout difficu lt y. But he cou ld not speak grammatically or creat e complete sentences, nor cou ld he express ideas in writ in g. Postmo rtem examination of th is pat ient's brain sho wed a lesion in the post erio r regio n of the fro ntal lo be (now called Bro ca's area; Figure 1-4B ). Broca studied eight similar patien ts, all with lesion s in th is region , an d in each case fou nd that the lesion was located in the left cerebral hemisphere. This disco very led Broca to announ ce in 1864 one of the most famous prin ciples o f brain funct io n: “No us parlons avec l'hémisphère gauche!” (“We speak wit h the left hemisphere!”) Broca's wo rk st imulat ed a search for the co rtical sites of other specific behavioral funct io ns— a search soon rewarded. In 1870 Gustav Fritsch an d Eduard Hitzig galvan ized the scient ific commun it y by sho wing th at characteristic and discrete limb mo vements in dogs, such as extendin g a paw, can be pro duced by electrically stimu lating a localized region of the precent ral gyrus of the brain. Th ese discrete regions were invariably located in the cont ralateral moto r co rtex. Thus, the right h and, the one most humans use for writ ing and skilled movements, is cont ro lled by the left hemisphere, the same hemisph ere that cont rols speech. In most people, therefo re, the left hemisphere is regarded as dominant .

18 Figure 1-4 The major areas of the cerebral cortex are shown in this lateral view of the of the left hemisphere. A. Out line o f the left hemisphere. B. Areas involved in language. Wernicke's area processes the au dito ry in pu t for language and is important to the underst an ding of speech. It lies near the primary audit ory cortex an d the angu lar gyru s, which combin es audito ry input with information fro m other senses. Broca's area controls the produ ction o f in telligible speech. It lies near the regio n of the moto r area th at contro ls the mouth and tongue movement s that fo rm wo rds. Wern icke's area commun icates with Broca's area by a bidirect io nal pathway, part of wh ich is made up of the arcuate fasciculus . (Adapted fro m Gesch wind 1979 .)

The next step was taken in 1876 by Karl Wernicke. At age 26 Wernicke publish ed a no w classic paper, “The (P.11) Symptom-Co mplex of Aph asia: APsychological Study on an An atomical Basis.” In it he described another type of aph asia, one in vo lving a failure to comprehend language rather than to speak (a recept ive as opposed to an expressive malfu nct io n). Whereas Broca's patien ts could understand lan gu age but not speak, Wern icke's patient could speak but cou ld not underst and language. Mo reover, the locus of th is new type o f aph asia was different fro m that described by Broca: the crit ical co rtical lesion was located in the posterio r part of the temporal lo be where it jo in s the pariet al and occipit al lo bes (Figure 1-4B ). On the basis of th is disco very, and the work of Broca, Fritsch, and Hit zig, Wernicke formulat ed a theory of language that attempt ed to reco ncile and exten d the two theories of brain fu nction holding sway at that t ime. Phrenologists argued th at the cortex was a mosaic o f funct io nally specific areas, whereas the aggregate-field schoo l argued that mental

19 function s were dist ribu ted homo geneo usly throughout the cerebral co rtex. Wernicke propo sed th at only the most basic mental funct io ns, those concern ed wit h simple perceptu al and moto r activit ies, are lo calized to sin gle areas o f the cortex. More complex cognitive funct io ns, he argued, resu lt from int erconnect io ns bet ween several funct io nal sites. In placing the principle o f lo calized funct io n with in a connect io nist framework, Wern icke appreciat ed th at differen t components of a sin gle behavior are processed in differen t regio ns o f the brain. He was thus the first to advance the idea o f dist ributed processin g , now cent ral to our understanding of brain fu nction . Wern icke postu lated that lan gu age in vo lves separate motor and sensory programs, each go verned by separate cort ical region s. He pro posed that the motor pro gram, which govern s the mout h movements for speech , is located in Broca's area, suitably situat ed in front of the moto r area that cont rols the mouth, tongue, palate, an d vocal cords ( Figu re 1-4B ). And he assign ed the sensory program, which go verns word percept io n, to the temporal lo be area he discovered (now called Wernicke's area). Th is area is con ven iently su rrounded by the audito ry cort ex as well as by areas collect ively kno wn as associat ion co rtex , areas th at int egrate audit ory, visu al, and somat ic sensation int o complex percept io ns. Thus Wern icke fo rmulat ed the first coherent model fo r lan gu age organization that (wit h mo dificat io ns and elaboration s we shall soon learn abo ut) is still of some use today. Acco rdin g to th is model, the init ial steps in the processin g o f spoken or writ ten wo rds by the brain occu r in separate sensory areas o f the cortex specialized for audit ory or visu al information . This information is then con veyed to a co rtical association area specialized fo r both visu al and audit ory information , the an gular gyrus. Here, acco rdin g to Wern icke, spoken o r writ ten wo rds are transformed into a commo n neural representat io n shared by both speech and writ in g. From the angu lar gyrus this represent ation is con veyed to Wern icke's area, where it is recognized as language an d associated wit h mean in g. Witho ut that associat io n, the ability to comprehen d language is lo st. The common neu ral represent ation is then relayed fro m Wernicke's to Broca's area, where it is tran sfo rmed from a sensory (audit ory or visu al) represent ation in to a moto r represent ation that can potentially lead to spoken or writt en lan gu age. When the last-stage transformat io n from sensory to moto r represent ation cannot take place, the ability to express lan gu age (eith er as spo ken words o r in writing) is lo st. Based on th is premise, Wern icke co rrectly predicted the exist ence of a third type of aph asia, one th at resu lt s from discon nection . Here the receptive an d motor speech zones themselves are spared but the neu ron al fiber path ways that connect them are dest ro yed. This con ductio n aphasia, as it is no w called, is characterized by an incorrect use of words ( paraphasia). Pat ients wit h con duct io n aph asia un derstan d wo rds that they hear an d read and have no moto r difficult ies wh en they speak. Yet they cannot speak coherent ly; they omit parts of words or substitu te in co rrect sounds. Painfu lly aware of their own errors, they are un able to put them righ t. Inspired in part by Wernicke, a new scho ol of cortical lo calizat ion arose in Germany at the beginn in g o f the twentieth century led by the an atomist Ko rbin ian Brodmann . This sch ool sou ght to dist in gu ish different fun ction al areas o f the cortex based on variat io ns in the st ructu re of cells and in the charact eristic arrangement of these cells in to layers. Usin g th is cytoarch itecton ic metho d, Brodmann dist in gu ished 52 an atomically an d fun ction ally dist in ct areas in the hu man cerebral co rtex ( Figu re 1-5 ). Thus, by the begin ning of the t went ieth centu ry there was compelling biological evidence for many discrete areas in the cortex, some wit h specialized ro les in beh avio r. Yet du ring the first half o f this centu ry the aggregate-field view of the brain , not cellu lar connection ism, cont in ued to do minate experimental th in king an d clin ical practice. This su rprising state of affairs owed much to the argu ments o f several promin ent neural scientist s, among them the British neu rologist Henry Head, the German neuro psycho lo gist Ku rt Go ldstein , the Russian

20 beh avio ral ph ysio lo gist Ivan Pavlov, and the American psychologist Karl Lash ley, all advocat es of the aggregate-field view. The most influent ial of th is grou p was Lashley, who was deeply skeptical of the cytoarch it ecton ic approach to funct io nal delineat io n of the cort ex. “The ‘ideal’ architectonic map is nearly wo rth less,” Lashley wro te. (P.12 ) “The area subdivisio ns are in large part anato mically mean in gless, and misleading as to the presumpt ive fu nction al division s of the co rtex.” Lash ley's skepticism was rein fo rced by his att empts, in the t radition of Flo urens's work, to find a specific seat of learning by studying the effect s of vario us brain lesion s on the ability of rats to learn to run a maze. But Lashley found that the severit y of the learn in g defect seemed to depend on the size of the lesion s, not on their precise site. Disillu sio ned, Lashley—and, after h im, many other psychologists —conclu ded that learn in g and other mental function s have no special locus in the brain an d consequ ently cannot be pin ned do wn to specific co llect io ns o f neurons. On the basis of h is observat io ns, Lash ley reformulat ed the aggregate-field view int o a theory o f brain funct io n called mass actio n, wh ich fu rther belit tled the importance of individual neu rons, specific neu ron al conn ection s, and brain regions dedicated to particu lar tasks. Acco rdin g to th is view, it was brain mass, not its neu ron al component s, th at was crucial to its function . Applyin g th is logic to aphasia, Head and Goldst ein asserted th at language disorders cou ld result fro m in ju ry to almost any cort ical area. Co rtical damage, regardless of site, caused patien ts to regress from a rich, abstract lan gu age to the impoverished utteran ces of aphasia. Lashley's experiments with rats, and Head's observation s on human pat ients, have gradu ally been rein terpreted. A variet y of stu dies h ave demon strated that the maze-learn in g task used by Lashley is unsuited to the stu dy of lo cal co rtical funct io n because the task in vo lves so many motor and sen sory capabilit ies. Deprived o f one senso ry capabilit y (such as visio n), a rat can still learn to run a maze usin g another (by following tact ile or olfacto ry cues). Besides, as we shall see, man y ment al funct io ns are h andled by mo re than one regio n or neu ron al pathway, an d a sin gle lesio n may not eliminate them all. In addit io n, the eviden ce fo r the localizat io n of fun ction soon became overwh elming. Beginn in g in the late 1930s, Edgar Adrian in England an d Wade Marsh all and Philip Bard in the Un it ed Stat es discovered that applying a tactile st imulus to different parts of a cat's body elicit s elect rical activit y in dist in ctly different subregions of the cort ex, allo win g fo r the est ablish ment of a precise map o f the body su rface in specific areas of the cerebral cortex described by Brodmann. These studies established th at cyto architectonic areas o f cortex can be defined un ambiguo usly according to several indepen dent criteria, such as cell t ype an d cell layering, connect io ns, an d—mo st impo rt ant—physiological fun ction . As we shall see in lat er chapters, local funct io nal specializat ion has emerged as a key prin ciple of cort ical o rganization , exten ding even to in dividual co lu mns o f cells with in a fun ction al area. Indeed, the brain is divided in to many more funct io nal region s than even Brodman n envisaged!

21 Figure 1-5 In the early part of the twentieth century Korbinian Brodmann divided the human cerebral cortex into 52 discrete areas on the basis of distinctive nerve cell structures and characteristic arrangements of cell layers . Brodmann's scheme of the co rtex is st ill widely used today an d is cont in ually updated. In th is drawin g each area is represented by it s o wn symbo l and is assigned a un ique number. Several areas defin ed by Brodmann have been found to cont ro l specific brain funct io ns. For in st ance, area 4, the moto r co rtex, is responsible for volun tary movement. Areas 1, 2, and 3 comprise the primary somatosensory co rtex, wh ich receives in fo rmat io n on bodily sensation . Area 17 is the primary visual cortex, wh ich receives sign als from the eyes an d relays them to other areas fo r fu rther deciphering. Areas 41 and 42 comprise the primary au dito ry cort ex. Areas not visible from the outer su rface o f the cortex are not sho wn in this drawing.

More refined metho ds have made it possible to learn even more about the funct io n of different brain region s in vo lved in language. In the late 1950s Wilder Penfield, and mo re recent ly George Ojeman n used small elect rodes to stimu late the co rtex of awake patient s du ring brain surgery for epilepsy (carried out under lo cal anesthesia), in search of areas that produ ce language. Patient s were asked to name objects o r use lan gu age in other ways wh ile different areas of the co rtex were stimu lated. If the area of the co rtex was crit ical for lan gu age, applicat io n o f the elect rical stimu lu s blo cked the patient 's abilit y to name objects. In th is way Pen field and Ojemann were able to con firm—in the living conscio us brain —t he lan gu age areas of the cort ex described by Bro ca and Wern icke. In addit io n, Ojemann discovered other sites essential for language, in dicating (P.13 ) th at the neural networks for lan gu age are larger th an those delineat ed by Broca and Wernicke. Ou r underst an ding of the neu ral basis of language has also advan ced th rough brain localizat io n studies that combine lingu istic an d cognitive psycho lo gical appro aches. From these studies we have learned that a brain area dedicated to even a specific component of lan gu age, such as Wern icke's area for language comprehensio n, is fu rther subdivided function ally. These modu lar subdivision s o f wh at had previo usly appeared to be fairly element ary operation s were first discovered in the mid 1970s by Alfonso Caramazza and Edgar Zurif. They fo und that different lesion s with in Wernicke's area give rise to different failures to compreh end. Lesions of the front al-temporal regio n of Wernicke's area result in failures in lexical processing , an inabilit y to un derstan d the meaning of words. By cont rast,

22 lesio ns in the pariet al- tempo ral region of Wern icke's area resu lt in failures in syntactical processing , the abilit y to un derstan d the relation sh ip bet ween the wo rds of a sentence. (Thu s syntactical kno wledge allows one to appreciat e that the sentence “Jim is in love wit h Harriet” h as a different meaning from “Harriet is in lo ve with Jim.”) Unt il recently, almo st everyth in g we knew about the an atomical organ izat io n of lan gu age came from studies of patients who had su ffered brain lesion s. Posit ron emissio n tomo graph y (PE T) and fu nction al magn etic reson ance imaging (MRI) have exten ded th is appro ach to no rmal peo ple (Ch apter 20 ). PE T is a noninvasive imaging techniqu e fo r visualizin g the local ch anges in cerebral bloo d flow and met abo lism that accompan y ment al activit ies, such as reading, speakin g, and th in king. In 1988, using th is new imagin g fo rm, Michael Posner, Marcus Raich le, an d their colleagues made an in teresting discovery. Th ey found that the incoming senso ry in fo rmat io n that leads to language product io n an d underst andin g is processed in mo re th an one pat hway. Recall that Wern icke believed that both writt en and spo ken wo rds are tran sfo rmed into a represent ation of lan gu age by both audit ory and visual inputs. Th is information , he thought , is then conveyed to Wernicke's area, where it beco mes associat ed wit h mean in g befo re bein g transfo rmed in Broca's area int o output as spoken language. Posner and h is colleagues asked: Must the neu ral code for a wo rd that is read be translat ed in to an audito ry represent ation befo re it can be asso ciat ed wit h a mean in g? Or can visual information be sent direct ly to Broca's area with no in vo lvement of the audit ory system? Using PET, they determin ed how individual words are coded in the brain of no rmal subj ects when the words are read on a screen o r heard th rough earphon es. Thus, when words are heard Wernicke's area becomes active, but wh en words are seen but not heard or spoken Wernicke's area is not activated. The visual information from the occipit al co rtex appears to be con veyed directly to Broca's area with out first bein g transformed in to an audito ry represent ation in the posterior tempo ral co rtex. Posner an d his co lleagu es concluded th at the brain path ways and sensory codes used to see words are different from those used to hear words. They propo sed, therefo re, that these path ways have independent access to h igher-order regio ns o f the cortex concern ed wit h the mean in g of wo rds and wit h the ability to express lan gu age ( Figure 1-6 ). Not on ly are reading an d listening processed separately, but the act o f thinkin g abo ut a word's meaning (in the absence o f senso ry in put s) activates a st ill different area in the left front al cortex. Thu s language processin g is parallel as well as serial; as we shall learn in Ch apter 59 , it is considerably mo re complex th an in it ially en visaged by Wernicke. Indeed, similar conclu sion s have been reached from studies of behavior other than language. These studies demonst rat e that information processing requires many in dividu al co rtical areas th at are appropriat ely int erconnected—each of them respon ding to, an d therefo re codin g for, on ly some aspects of specific sensory st imuli or moto r mo vement, and not for others. Studies o f aph asia afford unusu al insight in to ho w the brain is organ ized fo r lan gu age. One of the most impressive in sigh ts comes from a study of deaf people wh o lo st their abilit y to speak American Sign Lan guage after suffering cerebral damage. Un like spo ken language, American sign in g is accomplish ed with han d gestu res rather than by sound an d is perceived by visual rather th an audit ory path ways. Nonetheless, sign in g, which has the same structural complexities ch aracterist ic o f spoken lan gu ages, is also lo calized to the left hemisphere. Thus, deaf peo ple can beco me aphasic fo r sign language as a result of lesion s in the left hemisph ere. Lesio ns in the right hemisphere do not produce these defects. Mo reo ver, damage to the left hemisphere can have qu it e specific consequen ces, affect in g eit her sign comprehensio n (fo llo win g damage in Wern icke's area) or grammar (fo llo wing damage in Broca's area) o r sign in g fluency. These observation s illu st rate th ree po in ts. First, the cognitive processin g for lan gu age occurs in the left hemisphere and is in dependent of pathways that process the sensory o r motor

23 mo dalit ies used in language. Second, speech and hearin g are not necessary con dition s fo r the emergence o f language capabilit ies in the left hemisphere. Th ird, spoken lan gu age represent s on ly one of a family of cognitive skills mediated by the left hemisphere.

Figure 1-6 Specific regions of the cortex involved in the recognition of a spoken or written word can be identified with PET scanning. Each of the fou r images of the hu man brain sho wn here (from the left side of the cortex) actu ally represents the averaged brain activit y of several no rmal subj ects. (In these PE T images wh it e represents the areas o f h ighest activit y, red and yellow qu it e high activit y, an d blue and gray the areas of minimal activit y.) The “in put ” compon ent of lan gu age (reading or hearin g a wo rd) activates the region s of the brain sho wn in A an d B. The moto r “out put” component o f language (speech or thou ght) activates the region s shown in C and D. (Cou rtesy o f Cath y Price.) A. Th e readin g of a single word pro duces a response bot h in the primary visu al cort ex and in the visual association cort ex (see Figu re 1-5 ). B. Hearin g a wo rd activates an entirely different set of areas in the temporal cortex and at the junct io n of the temporalparietal cort ex. (To contro l fo r irrelevan t differen ces, the same list of words was used in bot h the reading an d listening tests.) A and B sho w that the brain uses several discrete pat hways for processing language an d does not transfo rm visual sign als for processing in the au dito ry path way. C. Subject s were asked to repeat a wo rd presented eit her throu gh earpho nes or on a screen. Speakin g a word act ivat es the supplement ary motor area of the medial front al cortex. Bro ca's area is act ivat ed wh ether the wo rd is presented orally o r visu ally. Th us both visual and audito ry pat hways con verge on Broca's area, the co mmon sit e for the motor art iculat ion of speech. D. Su bjects were asked to respo nd to the wo rd “brain” with an appropriat e verb (fo r example, “to th in k”). This t ype o f thinkin g act ivat es the frontal cortex as well as Broca's and Wern icke's areas. These areas play a ro le in all cogn it io n and abstract representat io n.

(P.14)

24 Affective Traits and Aspects of Personality Are Also Anatomically Localized

Despite the persu asive evidence fo r lo calized languagerelat ed funct io ns in the co rtex, the idea nevertheless persisted th at affective (emot io nal) funct io ns are not localized. Emotion , it was believed, must be an expression of wh ole- brain activit y. On ly recent ly has th is view been mo dified. Alt hough the emotion al aspects of beh avio r have not been as precisely mapped as sensory, motor, an d cogn it ive function s, distinct emotion s can be elicited by st imulat in g specific parts of the brain in hu mans or experimental an imals. Th e lo calizat ion of affect has been dramatically demon strated in pat ients wit h certain lan gu age diso rders and those wit h a part icular type of epilepsy. Aph asia patien ts not only manifest cogn it ive defect s in language, but also have t rouble with the affect ive aspects of lan gu age, such as inton ation (o r prosody). Th ese affective aspects are represent ed in the right (P.15) hemisphere and, rat her strikin gly, the neu ral organ izat io n o f the affect ive elements o f language mirro rs the organ izat io n o f the lo gical content o f lan gu age in the left hemisphere. Damage to the right tempo ral area correspo nding to Wern icke's area in the left temporal regio n leads to dist urbances in comprehending the emot io nal qu ality of lan gu age, fo r example, appreciat in g from a person's tone of vo ice whether he is describing a sad o r happy event. In cont rast, damage to the right front al area co rrespon ding to Broca's area leads to difficu lty in expressin g emot io nal aspects of language. Thus some lin gu istic fu nction s also exist in the right hemisphere. In deed, there is no w considerable evidence that an in tact righ t hemisphere may be necessary to an appreciation o f subt leties of lan gu age, such as iro ny, metaphor, an d wit, as well as the emot io nal content o f speech. Cert ain diso rders o f affective lan gu age that are localized to the righ t hemisphere, called apro sodias , are classified as sensory, moto r, or condu ction aprosodias, fo llo win g the classification used fo r aphasias. This pattern of localizat ion appears to be inborn, but it is by no means completely determin ed unt il the age of about seven o r eight . Yo ung ch ildren in whom the left cerebral hemisph ere is severely damaged early in life can still develop an essentially no rmal grasp of language. Fu rther clu es to the lo calizat ion o f affect come from patien ts with ch ronic tempo ral lo be epilepsy. These patien ts manifest ch aracterist ic emot io nal ch anges, some of wh ich occur on ly fleet in gly durin g the seizure itself and are called ict al phenomena (Lat in ict us, a blow o r a st rike). Common ictal pheno mena in clude feelings o f unreality and déjàvu (the sensation of h avin g been in a place before or of h avin g had a particu lar experien ce before); tran sient visual or au dito ry hallucination s; feelin gs o f deperson alizat io n, fear, or an ger; delu sion s; sexu al feelings; and paranoia. More endu ring emotion al chan ges, however, are evident wh en patients are not havin g seizures. Th ese int erictal pheno men a are interest in g because they represent a tru e psych iatric syndrome. A detailed study of such pat ients indicat es they lose all in terest in sex, and the declin e in sexual in terest is o ften paralleled by a rise in social aggressiveness. Most exhibit one o r more dist in ctive perso nality traits: They can be in tensely emotion al, ardently religiou s, extremely moralist ic, an d totally lacking in humor. In st rikin g cont rast , patients wit h epilept ic foci outside the tempo ral lo be show no abno rmal emot io n and beh avio r. On e impo rt ant structure fo r the expression and perception of emot io n is the amygdala, wh ich lies deep with in the cerebral hemispheres. The role o f this st ructu re in emot io n was discovered th rough studies of the effect s of the irritative lesion s of epilepsy wit hin the temporal lo be. The consequ ences o f such irritative lesion s are exactly the opposit e of those o f destructive lesio ns resu lt in g from a stroke or in jury. Whereas dest ruct ive lesion s bring about loss of fu nction , oft en throu gh the disconnect io n o f specialized areas, the electrical

25 sto rm of epilepsy can in crease act ivity in the region s affect ed, leadin g to excessive expressio n of emotion o r over-elabo ration of ideas. We consider the neu robio lo gy of emot io n in Part VIII of this boo k. Mental Processes Are Represented in the Brain by Their Elementary Processing Operations

Why has the evidence fo r localizat io n, which seems so obvio us and compelling in ret rospect, been reject ed so often in the past? Th e reasons are several. First , ph reno lo gist s int rodu ced the idea o f lo calization in an exaggerated fo rm and wit hout adequ ate evidence. Th ey imagin ed each regio n o f the cerebral cortex as an in depen dent ment al organ dedicat ed to a complete and distinct mental fu nction (mu ch as the pancreas and the liver are in dependent digestive organ s). Flo urens's reject io n of ph reno lo gy an d the ensu in g dialectic between proponents o f the aggregate-field view (again st localizat io n) and the cellu lar connect io nist s (for lo calization ) were responses to a theory that was simplist ic and overweening. The concept of localizat ion that u lt imat ely emerged—an d prevailed—is mo re subt le by far than an yth in g Gall (or even Wern icke) ever envisio ned. In the aftermath of Wernicke's disco very that there is a modu lar organ izat io n for language in the brain consisting of a complex of serial an d parallel processin g cent ers with more or less independent funct io ns, we no w appreciate th at all cogn it ive abilit ies resu lt from the int eraction of many simple processing mechanisms dist ribu ted in man y different regio ns of the brain . Specific brain regio ns are not concern ed wit h facu lties of the min d, but wit h element ary processing operat io ns. Percept io n, movement , lan gu age, thought, and memory are all made possible by the serial an d parallel interlinkin g of several brain regions, each wit h specific funct io ns. As a result, damage to a sin gle area need not resu lt in the lo ss o f an ent ire faculty as man y earlier neurologists predicted. E ven if a beh avio r in it ially disappears, it may partially retu rn as undamaged part s of the brain reo rgan ize their linkages. Thus, it is not useful to represent mental pro cesses as a series of lin ks in a ch ain, fo r in such an arrangement the ent ire process breaks do wn when a single link is disru pted. Th e better, mo re realist ic metapho r is to think of mental pro cesses as several railroad lin es that all feed (P.16) int o the same termin al. The malfu nction o f a sin gle link on one path way affects the information carried by that path way, but need not in terfere permanently wit h the system as a who le. The remain in g parts of the system can modify their performan ce to acco mmodate extra traffic after the breakdo wn of a line. Models of localized function were slo w to be accepted because it is enormously difficu lt to demonst rate wh ich components of a ment al operation are represented by a particu lar pathway o r brain regio n. Nor h as it been easy to analyze mental operat io ns and come up with testable components. On ly durin g the last decade, wit h the convergence o f modern cogn it ive psych ology and the brain scien ces, have we begun to appreciate that all ment al funct io ns are divisible int o subfunct io ns. One difficulty with breaking do wn mental processes in to an alyt ical catego ries or steps is that ou r cogn it ive experien ce consists of instant aneo us, smooth operations. Actu ally, these processes are composed of numerou s in dependent in fo rmat io n- processing components, and even the simplest task requ ires coordin at io n o f several distinct brain areas. To illu st rate this point , consider ho w we learn about , store, an d recall the knowledge that we h ave in our mind about objects, people, an d events in ou r wo rld. Our common sense tell us th at we store each piece of ou r kn owledge of the wo rld as a sin gle representat io n that can be recalled by memo ry-joggin g st imuli or even by the imagination alo ne. Everyth in g we know about ou r grandmother, for example, seems to be sto red in one complete representation o f “grandmother” that is equally accessible to us wheth er we see her in person , hear her vo ice,

26 o r simply think about her. Ou r experien ce, however, is not a faith fu l gu ide to the knowledge we have stored in memo ry. Kno wledge is not stored as co mplet e representat io ns but rath er is subdivided int o dist in ct cat egories and sto red separat ely. For example, the brain sto res separately in fo rmat io n about an imat e and in an imat e object s. Thus selected lesion s in the left temporal lo be's association areas can oblit erat e a pat ient 's kno wledge of livin g th in gs, especially people, while leavin g the pat ient's knowledge o f in an imat e objects quite in tact. Representat io nal cat ego ries such as “living people” can be subdivided even furth er. A small lesio n in the left temporal lo be can dest roy a patient 's abilit y to recogn ize people by n ame wit ho ut affecting the abilit y to recogn ize them by sight . The most astonish in g example of the modu lar natu re o f representat io nal ment al processes is the fin ding th at our very sense of ourselves as a self- conscio us coherent bein g—the su m of what we mean when we say “I”—is achieved throu gh the connect io n of in dependent circuits, each wit h its own sen se of awareness, th at carry out separate operation s in ou r two cerebral hemisph eres. The remarkable disco very that even con sciou sness is not a unitary process was made by Roger Sperry and Mich ael Gazzan iga in the cou rse of studyin g epileptic pat ients in whom the corpu s callosum—the majo r tract connect in g the two hemisph eres—was severed as a treat ment for epilepsy. Sperry an d Gazzan iga found that each hemisphere had a consciousness that was able to fun ction independent ly of the other. The right hemisphere, which cann ot speak, also can not understand lan gu age that is well- understoo d by the isolat ed left hemisph ere. As a resu lt , opposing commands can be issued by each hemisph ere—each hemisph ere has a mind o f it s o wn! Wh ile one patien t was ho ldin g a favorit e boo k in h is left h and, the righ t hemisphere, which contro ls the left han d but can not read, foun d that simply loo king at the book was borin g. The right hemisph ere commanded the left han d to put the boo k do wn ! Another pat ient wo uld put on his cloth es with the left hand, wh ile t akin g them off wit h the other. Th us in some commissurotomized pat ients the t wo hemispheres can even int erfere wit h each other's fun ction . In addition , the dominant hemisphere somet imes comments on the performan ce of the non domin ant hemisphere, frequent ly exh ibit in g a false sense of con fidence regarding pro blems in which it cannot kno w the so lu tion , sin ce the information was projected exclu sively to the nondomin an t hemisphere. Thus the main reason it has taken so lo ng to appreciate which ment al activit ies are localized wit hin which regio ns o f the brain is that we are dealing here wit h bio lo gy's deepest riddle: the neu ral represent ation of conscio usness and self- awaren ess. After all, to study the relat io nship between a mental pro cess an d specific brain regions, we must be able to ident ify the components of the ment al process that we are attempt in g to explain . Yet , of all beh avio rs, high er mental pro cesses are the most difficu lt to describe, to measure object ively, and to dissect in to their element ary co mponent s and operation s. In addit io n, the brain 's anato my is immensely co mplex, an d the struct ure and int erconnect io ns of it s man y parts are st ill not fully un derstood. To analyze ho w a specific mental act ivity is represented in the brain , we need not only to determin e wh ich aspect s of the act ivity are represented in which region s of the brain , but also how they are represented and ho w such represent ation s int eract. On ly in the last decade h as that become possible. By co mbining the conceptu al too ls of cognitive psychology with new physiological techn iques and brain imaging met hods, we are beginn in g to visualize the regio ns of the brain involved in particu lar beh avio rs. And we are (P.17) just beginn in g to discern ho w these behaviors can be broken do wn int o simpler ment al operations and mapped to specific intercon nected modules of the brain . Indeed, the excitement evident in neu ral science today is based on the con vict io n th at at last we have in h and the proper too ls to explo re the ext rao rdin ary o rgan of the min d, so that we can eventu ally fat hom the bio lo gical principles that un derlie human cognition .

27 Selected Readings

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