The Structure of the Human Brain

Precise studies of the size and shape of the brain have yielded fresh insightsinto neural development, differences between the sexes and human evolution

John S. Allen, Joel Bruss and Hanna Damasio

I f you lived in the 19th century, yourentire character—attributes such as

ambition, tenderness, wit and va lor -might have been judged by the size andshape of your skull, this practice, calledphrenology, was developed by FranzJoseph Gall and Johann Spurzheim itiVienna during the early 1800s. Adher-ents claimed different mental "faculties"were localized to different parts of thebrain, and these regions would be big-ger if you possessed the traits in abun-dance. Phrenologists also believed thebrain determined the shape of the skull,so they reasoned an external examina-tion of the cranium would detect re-gional brain development. This led tothe popular (arid not inaccvirate) charac-terization of phrenology as the "sci-ence" of bumps on the head.

We are right to be skeptical of theseearly explorations of brain size and itsfunctional correlates. However, therewas a nugget of truth in the phreno-logical view of world: Brain structure isa fundamental aspect of neurosciencebecause brain functions take place inspecific combinations of brain regions.

/D/IIJ S. Alleii is a biolo<i!cal anthropologist wbo re-ceived his Ph.D. from the ibiii'ersity of Cntifoniin,Berkeley in J 989. He is n rcsctjrch scientist mid ad-junct nssocinte profi'ssor in tlic Dcpurtnwnt ofNeurology, Uiiivcrsitif of loivn Colicgc of Medi-cine, joe! Br»ss is (7 refienrch assistiuit in the Hu-man Nenroanakmn/ ami Ni'iioriinaging Lahornto-ry at the Uiiivcrsiti/ of Iowa College of Medicine.Hanna Damasio received her M.D. from the Uni-versity of Lisbon School of Medicine. She is fheUniversity of him Foniidntion Distinguished Pro-fessor of Neurology, and diraior of the HumanNcnroanatomii and Naiorimagiug Ijiboratonf.Address for Allen: Department of Neiirologi/. 2RCP, University of km\i Hospitals and Clinics.200 Hawkins Drive, ioim City, JA 52242. Inter-net: john-s~alkn@uiowa.edn

In complex ariimals, the size and shapeof the brain reflect a host of evolution-ary, developmental, genetic, pathologi-cal and functional processes that inter-act to produce an individual organism.

Because many factors influence neur-al structures, the study of brain volume,or volumetrics, has the potential to of-fer insights from many perspectives. Inan evolutionary context, studies ofbrain volume across species can linkanatomical, behavioral and ecologicaldata. Species that have unpredictablylarge or small brains are useful forstudying the forces of evolution that in-fluence brain size. For example,Katharine Milton at the University ofCalifornia, Berkeley has suggested thatfruit-eating primates have a higherbrain-to-body mass ratio than leaf-eat-ing primates because locating widelydispersed, seasonally available fruitmakes greater cognitive demands thanfinding more convenient foods, such asleaves. Volumetrics can also illuminatedevelopmental patterns within andacross species, which in turn suggesthow evolution might be constrained byimplicit rules of neurological growth.The study of neurological diseases alsodepends on a systematic analysis ofbrain size and shape. For instance,some children with autism have atypi-cally large brains, and Alzheimer's dis-ease causes progressive brain atrophy.In both cases, the pathological process-es that underlie these conditions mani-fest as changes in brain volume. So vol-umetric studies are both a means tounderstanding brain function and anend in themselves.

Tools of the TradeNeuroanatomy has undergone a re\'olu-tion in the past 30 years. The leap be-

came possible with the introduction ofnew imaging technologies such as x-raycomputed tomography (CT, also calledCAT scanning), magnetic resonanceimaging (MRl) and positron emissiontomography (PET). With these tot)ls, sci-entists can view the structure and activi-ty of the living human brain in unprece-dented detail. For the structural andvolumetric study of the brain, CT andMRI have been of critical importance.

Computed tomography is the oldertechnology. It uses the variable absorp-tioti of X rays by different brain compo-nents to visualize structures inside theskulls of living subjects. A single CTimage is the product of thousands ofindividual measurements, which aremade as the x-ray source swivels in afull circle around the head.

Unlike CT, MRI does not use x rays,relying instead on powerful magnets tomomentarily align the nuclei of hydro-gen atoms in body tissues, most ofwhich are within water molecules.Wlien the tnagnet is turned off, the in-finitesimal spinning {or resonating) nu-clei fall back to a normal state, releasingenergy in the form of radio waves. Tliefrec|uency of these waves provides ameasure of local hydrogen concentra-tion, which varies according to tissuetype, such as bone or fat. This producesa very fine-grained map—often as goodas a postmortem analysis. The tech-tiique clearly distinguishes gray matter(mostly neuronal cell bodies), whitematter (mostly nerve fibers insulated byfatty myelin, plus supporting cells) andcerebrospinal tluid or CSF (the liquidthat fills the spaces within and aroundthe brain). In addition, individual MRscans can be stacked to form a virtualthree-dimensional model, then reslicedalong any plane or angle.

246 American Scientist, Volume 92

Figure 1. Computed lomography (CT) and magnetic resonance imaging (MRI) allow unprecedented access to the living human brain, as seen inthis colored, three-dimensional scan, which combines images from both techniques. The neuroanatomical study of the size and shape of thebrain—long relegated to the autopsy tabie—has been tremendously invigorated by these advances.

Draw the LineThe process of dividing the brain intodifferent regions is known us piircclln-lion, and there are many ways to do itdepending on the goals of the investi-gators and the methods available. MRIparcellation uses visible anatomicallandmarks, such as the sulci (folds)and gyri (bulges) on the surface of thebrain to create "regions of interest" orROIs. They can include broad strvictur-<il divisions—for example, the tempo-ral, parietal and occipital lobes—aswell as smaller structures such as thehippocampus or corpus callosum. Thelocations of specific brain activities,when they are known, can also guideanatomical parcellation.

A three-dimensional MR scan ismade from a series of separate, con-tiguous images. A typical high-resolu-tion analysis might have a slice thick-ness of 1.5 millimeters, meaning that anaverage brain would be compiled frommore than 100 sections. Specialized im-age-processing software can then "ex-tract" the brain from the skull and visu-alize it as a solid object. It can be slicedin any plane, rotated or resized tomatch a standard model. At this point,ROIs can be defined by marking theboundary limits of the structure on thesurface of the brain. These marks arethen transferred to "coronal" slices(parallel to the plane of a person's face)to define the region on each image. The

ROI volume (area multiplied by slicethickness) from each section is summedto give an overall value. The studiesmentioned in this article, like others inthe field, were done through a labori-ous process of manually tracing ROIsonto each image. Several methods arecurrently being developed to automatethis painstaking process, but to datenone exists that can match the precisionof hand tracing with expert knowledgeof anatomy.

One of the most useful aspects of anMR scan for imaging neural structuresis that it sharply defines gray matter,white matter and cerebrospinal fluid.Many research groups are studying therelative gray:white composition of var-

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Figure 2. Although computed tomography (CT or CAT, left) scans were revolutionary when in-troduced in Hie 1970s, magnetic resonance imaging (MR], riglit) provides a much more detailedview of the brain and its substructures. This technique clearly differentiates gray matter, whitematter and cerebrospinal fluid (which appears black) with high resolution. The CT and M Rlscans are from different subjects. (Photographs courtesy of the authors.)

iotis structures, aided by automatedmethods (vvhicli do work well for thispurpose) for segmenting; MRIs ititothese categories.

Genes and BrainsCjenetic processes underlie the develop-ment and evolution of the brain, andseveral research teams are studying thegenetics of human brain volume andstritcture. One strategy is to use MI J tolook at the brain volutnes of identicalanci fraternal twins. Tlie studies iiuiicatethat human cranial capacity is a stronglyinherited trait, and most ot the variationin total or hemispheric volutne can beexplained by genetic factors. In one re-port, by William Baare and his col-leagues at the University Medical Centerof Utrecht in the Netherlands, genes ac-counted for the large majority t)f brain\-olume differences: 90 percent for thebrain as a whole, S2 percent for graymatter and 88 percent for white-matter.

However, two major neuroanatomi-cal features appear to be free of stronggetietic control. In the same paper,Baare stated that the lateral ventricles—CSF-filled cavities itiside the brain—were only mildly influenced by heredi-ty. A separate study by Alycia Bartleyand her colleagues at the National In-stitute of Mental Health explained howpatterns of sulci and gyri were moresimilar in monozygotic (identical)twins than in dizygotic (fraternal)twins. Interestirigly, siblitigs from bothgroups were still very different fromeach other, especially in the smaller sul-ci. Thus, while overall volumes of ma-jor brain sectors are under strotig ge-netic control, smaller regions may be

more responsive to environmental in-fluence. These insights into the relativecontributions of genes and environ-ments to this phenotype are useful itiframing another area of volumetrics re-search—the evolution of the modernhuman brain.

Lobe Row over Low BrowsScientists have debated for decades thehypothesis that frontal lobe expansionaccelerated during hominid evolution.When we compare our own high fore-heads to the low brows of our closestliving kin (the chimpanzee) and extinctcousins (the Neandertals), the ideaseems obvious. In terms of brain func-tions in which parts of the frontal lobeplay a critical role, language, predictionand judgment represent importantcognitive differences between us atidother animals. So the idea that thefrontal lobe expanded disproportion-ately during hominid evolution makesintuitive sense.

The equation of a big frtintal lobewith intelligence is also embedded inthe popular imagination. The 1955 sci-ence-fiction movie Tliis Island Earth fea-tured three intelligent species: humans,Metahnian aliens {similar to humansbut more advanced, with unnervinglylarge foreheads) and the menacing buthighly advanced Zagons. The mutantalien brains of the Zagt^ns had appar-ently become so large that they literallyburst through their foreheads. The im-plicit notion in this hierarchy is thatbrain size is linked with mental acuity.Mtire specifically, the increasing size ofthe foreheads (especially in the human-like Metalunans) highlights a belief thatcognitive ability is tied to the frontal re-gions. But is this assumption true?

Several recent studies have turnedthe tools of neuroimaging to the issueof relative frontal lobe expansion dur-ing hominid evolution. Our colleagueKaterina Semendeferi, now at the Uni-versity of California, San Diego, usedMRI to compare the proportional sizeof the frontal lobe in people and otherprimates. She found that the frontalcortex (gray matter) and the entirefrontal lobe (including gray and whitematter) had very similar relative pro-portions in humans, orangutans, goril-las and chimpanzees. In these fourspecies the frontal lobe as a whole com-prised between 33 and 36 percent ofthe total volume of the cerebrum, andthe frontal cortex made up 36 to 39 per-cent of the cerebral gray matter. Al-though the human brai)i is approxi-mately three times larger than thebrains of the great apes, regressionanalyses of the data indicated that theproportion of the frontal lobe is notgreater than expected for an ape withour size brain. By contrast, our brainproportions aredifferent than those of a

Figure 3. A three-dimensional MR image (left) renders the living br,iin at least as accurately asthe view during an actual surgery (ri^tit). Major sulci (folds) within the surgical window are in-dicated with yellow for the Sylvian fissure, green for the superior temporal sulcus, and blueand red to mark fwo parts of the precentral sulcus. (Photographs courtesy of the authors andMatthew Howard, University of Iowa Hospitals and Clinics.)

24H Americiin Scientist, Volume '•)2

lateral view

central sulcus

corona) view

cingulate frontal

mesial view

central sulcu$

parietal

parietal temporal

Figure 4. The process of dividing an MR scan into regions of interest is known as parcellation. It proceeds in two steps, which are shown in a.Analysts first identify suici and other landmarks on the outer and inner surfaces of the three-dimensional model of the whole brain. In the sec-ond step, they manually trace so-called "regions of interest" onto computer-generated, coronal slices. The heavy white line indicates the coro-na! plane. Part h shows a brain on which the major lobes and the cingulate gyms are color coded. Ten coronal sections (i? through;') are shownbelow, representing fewer than 10 percent of all the hand-traced slices. The frontal lobe is colored red; the temporal is blue, parietal is green, oc-cipital is yellow, and cinguiate is purple. (Photographs courtesy of the authors.)

"lesser ape" (the small-bodied gibbon)and fwo monkey species (rhesusmacaque and cebus monkey), whichhave significantly smaller fronfal lobes.

Semencieferi suggests fhe evolutionof a proportionally larger frontal lobehappened after the human and greatape lineage splif off from fhe other an-thropoid primates (20 to 25 millionyears ago), but before the divergence

of hominids during fhe late Miocene (5fo 10 million years ago). Therefore,frontal lobe expansion is not a recentdevelopment in humans. She offersseveral hypotheses about the evolu-tionary origins of brain enlargementand cognitive change In the hominidline. These traits may have arisen fromcortical reorganization within smallsubsectors of the lobe, enriched con-

necfivify between selected regions, re-gional changes in cytoarchitecture orsome combination of these features.The evidence from comparative anafo-my supports all three possibilities.

Lobal FormingOur most recent work on proportion-ate volume also relates to the debateover frontal lobe expansion. We found

u' v\' w. a m or icti 11 SC it'n t ist. t 2004 Mav-June 249

Figure 5. Automated segmenfntion of MR images is a valuable tool for determining the volumeof different types of tissue in the brain. An original MRI is shown in IT, followed by computer-generated images of the cerebrospinal fluid (h), white matter (c) and gray matter (i1). (Pho-tographs courtesy of the authors.)

that variation in total braiti size ismuch greater than variation in the pro-portions of the major lobes. In otherwords, people vary rnvrv in brain sizethan in how the major regions of thebrain are apportioned. This is sfriking-ly evident when we compare men andwomen. Although men have largerbrains, the proportions of the major

lobes are similar. In both sexes, thefrontal lobe comprises about 38 percentof the hetnisphere (ratiging from 36 fo43 percent), the temporal lobe 22 per-cent (ratiging from 19 to 24 percent),the parietal lobe 25 percent (rangingfrom 21 fo 28 percent), and the occipitallobe 9 percent (ranging from 7 to 12percent). (Note that these values differ

Th.'Fv,T,.trC.

I igure 6. Along with a cast of humans (left), the classic science-fiction movie Tills Isliiud Earth(1955) features representatives of fwo advanced alien species—a toweringly browed Metalu-nan (center) and a menacing Zagon (ri^ht). The film embodies the popular notion that bigbrains, particularly a big frontal lobe, convey intelligence.

slightly from those of Semendeferi be-cause of a parceilation scheme in thisstudy that includes more of the whitematter core,)

Comparing frontal- and pariefal-lobe volumes has added another twistto the story, As we expected, peoplewith large fronfal lobes also havelarge parietal lobes, since they bothreflecf large overall brain size. How-ever, after controlling for overall di-mensions, we found that there was ahighly significant, negative correla-tion between fronfal and parietal lobevolutiie: People with larger fronfallobes had smaller parietal lobes andvice versa. We concluded that this in-verse relation probably reflects genet-ic rather than environmental factors,because the boundary between theselobes, the central sulcus, appears ear-ly in the developing brain, and itscourse and position are strongly in-fluenced by itiheritance.

The negative correlation indicatesthat frontal lobe expansion duringhimiinici evolution likely would havecome at the cost of a smaller parietallobe. And the contraction of the pari-etal lobe makes little sense from a cog-nitive standpoint. After all, associationcortices in the parietal lobe serve manyimportant language functit)ns, and tooluse, a hallmark of hominid cognitiveevolution, depends on the connectionsbetween parietal and frontal lobes.Thus if is possible that there couldhave been selection against relativefrontal lobe expansion if if compro-mised the functiotis of the parietal lobe.In light of this evidence, the frontallobe probably grew at the same time asother major regions of the cerebrumduring the past 2 million years.

A third perspective on fronfal lobeevolution comes from a CT study offhe skulls of several hominid fossilsfrom the past half-million years. FredBookstein at the University of Michi-gan and his colleagues compared theskulls of our exfincf hominid cousinswith those of modern human beings.Archaic mernbers of genus Homo arecharacferized by cranial capacifies thatequal or exceed those of modern Homosapiens sapiens. However, the bones ofthe cranium and face are very thickand strong, and most specimens havelarge brow ridges and some degree ofmid-facial prognafhisni (protrudingnose), which together give the impres-sion of a low, sloping forehead. But de-spite these external differences, Book-

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stein etal. showed that the inside of thecranial vault was identical by using astatistical method known as Procrustesanalysis. Tbis strategy uses a series offloating intervals between fixedanatomical landmarks to standardizethe measurement of size, position, ori-entation and, ultimately, shape. (Pro-crustes was the highwayman of Greekmythology who forced each victim tofit the same terrible bed—stretching oraxing the unfortunates as necessary.)The authors determined that the interi-or shape of the frontal bone (and pre-sumably the shape of the frontal lobeitself) had tiot changed over the past500,000 years—despite substantialchanges in the external morphology ofthe face.

Sex in the BrainPostmortem and MRI studies showfbat on average, men's brains are largerfhan women's brains, even after cor-recting for body size. This dimorphismis unlikely to be a recently evolvedtrait, as other primafes bave similarpatterns. But size is not the only differ-ence. It turns out that women tend tohave a higher proportion of gray mat-ter than men.

We recently published a pair of pa-pers that examined differences in brainstructures of men and women. On av-erage, male brains (mean 1,241 cubiccentimeters) were about 12 percentlarger than female brains (mean 1,100cubic centimeters), although there wassignificant overLip between the twogroups. Tbis dissimilarity did notseem to involve sex-specific differ-ences in hemispheric volume, as themajority of men and women had larg-er right hemispheres. In general, sexdifferences for each of the major lobesof the brain reflected those of the brainas a whole. However, tbe occipitallobe, which processes visual informa-tion, was less sexually dimorphic thanother regions.

Our segmentation of the brain intogray and white matter revealed thatwomen have a mean gray:white ratioof 1.35 compared with 1.26 for men.This higher ratio in women appears tobe caused by less white matter ratherthan more gray matter. Men had, onaverage, 9.3 percent more gray matterthan women, but the increase in whitematter volume was almost twice asbig—17.4 percent. When we analyzedthe covariance in this data set, the ratiodifference disappeared with white-

cfiimpanzee

human

gibbon

Figure 7. Human brains are substantially larger Ihan those of chimpanzees, but the majorsectors of the brains occupy similar proportions—despite differences in the way those struc-tures are used. However, the relative proportions of human and chimp brains are differentthan those of a "lesser ape," the gihbon. The precentral sulcus is marked in yellow, the cen-tral sulcus in red and the Sylvian fissure in blue. The brains are presented at approximatelythe same scale. (Photographs courtesy of the authors and Katerina Semendeferi, Universityof California, San Diego.)

Figure 8. Despite the differences between fossil skulls from early hominids, the inner curve ofthe front of the cranium is nearly identical, suggesting the shape of the frontal lobe has notchanged in recent hominid evolution. The oldest, a. is the so-called Bodo skull, an example ofHomo hciiielbergensis from about 600,000 years ago. The Kabwe <h) and Petralona (c) specimensare H. heiiielbergensis skulls from more than 200,000 years ago. The Atapuerca skull (d) is a300,000-year-old "proto-Neanderfal" and the Guattari fossil (c) a "classic" example of Homo ne-andertalensis at 50,000 years old. Panel / shows a modem human (Homo sapiens sapiens).(Reprinted from Bookstein et al 1999, by permission of Wiley-Liss, Inc.)

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Figure 9. Investigators can manipulate MR images to visualize "hidden" structures on the cortical surface of the brain. In a lateral view (left),Heschl's gyrus (red) is obscured, and the planum temporale (blue) is barely visible along the lower edge of the Sylvian fissure. Removing thefrontal and parietal lohes (center) exposes these areas on the upper surface of each temporal lobe (right). (Photographs courtesy of the authors.)

matter volume normalized. This analy-sis indicated that the variability inwhite-matter volume had the most in-fluence on sex differences.

Of all brain structures, the corpuscallosum has probably drawn the mostattention over the years for putativedifferences between the sexes. Thislarge band of white matter connectsthe right and left hemispheres, and ear-ly research suggested that it might belarger in women than men. However,the current generation of studies hasfound tbe opposite to be tn.ie—it is ac-tually larger in men, reflecting thegreater overall size of male brains. Inour ongoing studies, we observe thatthe corpus callosum is about 10 percentlarger in men; however, it constitutes asignificantly greater percetitage of thetotal white matter in women (2.4 per-cent versus 2.2 percent).

This detail suggests an explanationfor why men have a greater proportionof white matter. In MR images, mostwhite matter includes myelinated axotifibers, glial cells and blood vessels. Bycontrast, the white matter of the cor-pus callosum is mostly just fiber tracts.Therefore, if the callosum is an indexof the axonal fraction of white matter,then men may have more non-axonalcomponents (giia, blood vessels) in theoverall makeup of their white matter.In other words, the "excess" whitetnatter in men (underlying the lowergray:white ratio) probably doesn't rep-resent a big step up in the connectivityof male brains.

Dispelling an Old ClicheWhat do these differences in brain vol-ume tell us about the way that maleand female brains actually work?When the sexually dimorphic corpuscallosum was first suggested in the ear-ly 1980s, many scientists speculated

that the "larger" band in womenmeant they had a greater degree ofcommunication between the two hemi-spheres. This idea seemed to supportthe cliche that in women, the "emo-tional" right side and the "atialyfical"left side were more "in touch" witheach other. Of course, we now knowthat women do not have larger corpuscallosa than men. This fact doesn't pre-clude greater functional connectivitybetween the hemispheres (as thestereotype would have it), buf there isno anatomical evidence for the claim.

On average, the brains of men andwometi differ by more than 100 cubiccentimeters, or about two and a halfgolf balls. Should we expect this differ-ence fo bave direct cognitive effects?Not necessarily, for several importantreasons. First, although the sex differ-ence in brain volume is present aftercorrection for body size, sorne of thevariation can be attributed fo a person'sphysical dimensions. In a careful MRIstudy (in which equal attention waspaid to both brain and body size para-meters), Michael Peters of the Univer-sity of Guelph and his colleaguesfound that the difference in brain vol-ume between the sexes dropped bytwo-thirds after height was included asa covariate.

Next, volume differences betweenthe sexes are distributed fairly evenlythroughout the major lobes of thebrain; there is no "sex-specific" regionthat accounts for an utidue share of thedifference in total brain volume. Thisdiffuse pattern indicates that it will bedifficult to find a functional sex differ-ence that correlates with differences intotal brain volume. Furthermore, asimilar pattern of sexual dimorphismis seen in several other primatespecies: the buman sex difference itibrain volume evolved before the pro-

found changes in brain size and cogni-tion that occurred during hominidevolution.

Although we have argued agaitist astrong hmctional explatiafion for sexu-al ditnorphism in total brain volume—indeed, if may reflect primate aticestryrather than cognitive adaptations—wedo not suggest that there are no struc-tural-functional differences in brainanatomy between men and women.Rather, we would expect the changesto exist in more subtle ways—particu-lar regions or networks of tbe brainfhat are associated with specific behav-iors (for example, visual-spatial tasks)that exhibit sexual dimorphism.

The Mark of SilenceHeschl's gyrus (HG) is a small structureon the top of fhe tetnporal lobe, buriedwithin the Sylvian fissure. It is impor-tant because it marks the approximateposition oi the primary auditory cor-tex—the place in the braiti where soundis initially processed. But how wouldIIG develop in peciple who had neverheard sounds in their lives?

The examination of HG in deaf indi-viduals is related to a series of now-classic animal studies that proved therequirement for senst)ry informationduring critical periods of neural devel-optiienf. When the atiimal's sensory in-put was blocked (by covering one eye,for example), the braiti structures thatnormally received tbose projectionsfailed to develop. Obviously, such ex-periments cannot be conducted in peo-ple, so we have little direct informationon sensory deprivation and the devel-opment of the human brain. With thisin mind, we collaborated with KarenEmmorey at the Salk Institute to recordgray and white matter volumes of HGin hearing and congenitally deaf indi-viduals using high-resolution MRI.

252 American Scientist, Volume 92

Figure 10. Congenitally deaf people have less white matter than do controls in Heschl's gyrus, the primary region for processing sound. Heschl'sgyms is outlined in green. The original MRI has been segmented into separate gray-matter and white-matter images for comparison.

We measured the volume of HG andotber regions in tbe brains of 25 con-genitally deaf individuals and 25 age-and sex-matclied controls. One of theseareas, the planum temporale, bordersHG and is involved with secondaryprocessing of sound. Tbis structure isone of tbe most reliably asymmetricparts of tbe human brain, being largerin tbe left hemisphere tban the right. Infact, many scientists once thought thatthe asymmetry might have evolvedwith spoken language. However, asimilar pattern also exists in chim-panzees, so hemispheric languagekinctions must have developed withinthe context of preexisting laterali/ation{at least in this area).

The planum temporale proved to bethe same in deaf and hearing subjects,indicating that the stmcture of tbis re-gion is not critically influenced by sen-sory input. However, HG did chatige:The gray:white ratio was significantlyhigher in deaf subjects compared tohearing controls. This increase wascaused by a reduction in white mattervolume, as the amount of gray matter(after normalization) varied little be-tween deaf and hearing subjects. Wespeculated that the auditory depriva-tion from birth might have led to acombination of less myelination, fewerconnections with the auditory cortexand the gradual decay of unused ax-onal fibers. This part of the brain is notdead—it responds to nonauditorystimuli, according to functional imag-ing studies. But our results do indicatethat exposure to sound may influencethe anatomical development of thisprimary sensory region.

Mind the GapGiven the complexity of the subjectmatter and the number of issues thatneed to be addressed, the volumetricstudy of the human brain is still in its in-fancy We have not yet ascertained thefull scope of human-brain variability,and more normative research is neces-sary. And despite the fact that MKI hasbeen used in hundreds of studies ofschizophrenia, Alzheimer's disease andautism, quantitative volumetric data isnot yet a standard component of clinicaldiagnoses. We anticipate the next gener-ation of higher-resolution MRI studieswill add even more analytical power tofurther elucidate the links betweenbrain stmcture and function.

BibliographyAllon, J. S., H. Damasio and T. j . Crabowski.

2002. Normal neiirotinatomical varititiun inthe human brain: An MRI-volumetric study.American joiinial of Physical Anthivpolo^^y118:341-358.

Allen, J. S., H. Damasio, T. j . Crabciwski, J.Bruss and W. Zhang. 2003. Sexual dimor-phism and asymmetries in the gray-whitecomposition of the human cerebrum. Nenro-

18:88O-S94.Baare, W. F. C, H. E. Hulshoff Pol, D. I. Booms-

ma, D. Posthuma, E. J. C. de Geus, H. C.Sciinack, N. E. M. van I laren, C. |. van Oeland R. S. Kahn. 2001. Quantitative geneticmodeling ot variation in luiman brain miir-piiology. Cerebral Cortex 11:816-824.

Bartley, A. J., D. W. Jones and D. R. Weinberger.1997. Genetic variability of human brainsize and cortical gyral patterns. Brain120:257-269.

Bookstein, F., K. Schafer, H. Prossinger, H.Seid-ler, M. Eieder, C. Stringer, G. W. We-ber, J.-L. Arsuaga, D. E. Slice, E J- Rohlf, W.Recheis, A. ). Mariam and L. F. Marcus.1999. Comparing frontal cranial profiles inarchaic ancl modern Homo by morphomet-

ric analysis. Anatomical Record (NewAimtomist) 257:217-224.

Emniorey, K., J. S. Allen, |. Bruss, N. Schenkerand H. Damasio. 20Q3. A morphometricanalysis of auditory brain regions in con-j^c'mtally deaf adults. Proceeding's af the Na-tional Academy of Sciences of the U.S.A.100:10049-10054.

Grabowski, T. j . , R. J. Erank, N. R. Szumski, C.K. Brown and H. Damasio. 2IKX). Validationof partial tissue segmentation of single-channel magnetic resonance images of thebrain. Nnirolmage 12:64l.)-636.

Holloway, R. L. 1980. Witbin-species brain-body weight variability: A reexamination ofthe Danish data and other primate species.American journal of PInisical Anthropalw^^y53:109-121.

Milhm, K. 19S1. Distribution patterns ot' tropi-cal plant foods as an evolutionary stimulusto primate mental development. AmericanAnthropologist S3:534-54H.

Peters, M., 1.. Jancke, J. E Staiger, G. Schlaug, Y.Huang and H. Steinmetz. 1998. Unsolvedproblem.^ in comparing brain sizes in Homosapiens. Brain and Cognition 37:254-285.

Semendeferi, K., and H. Damasio. 2000. Thebrain and its main anatomical subdivisionsin living hominoids using magnetic reso-nance imaging, jonrna! of Hnman Evolution38:317-332.

Semendeferi, K., A. l,u, N. Schenker and H.Damasio. 2002. Humans and great apesshare a large frontal cortex. Nature Neuro-science 5:272-276.

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