Crow - Is schizophrenia the price that Homo sapiens pays for language.pdf

8/14/2019 Crow - Is schizophrenia the price that Homo sapiens pays for language.pdf

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E L S E V I E R Schizophrenia Research 28 (1997) 127-141

SCHIZOPHRENIRESE RCH

Is schizophrenia the price that om o sapiens pays for languageimothy J . C r o w

Prince of Wales Centre University Department of Psychiatry Warneford Hospital Oxford OX3 7J X UKReceived 7 March 1997; accepted 17 July 1997

AbstractThe dichotomy between schizophrenia and manic-depressive il lness is, as E. Kraepelin suspected, flawed; nounequivocal separation can be achieved. There are no categories o f psychosis, but only continua of variation. However,the definition of nuclear symptoms by K. Schneider reveals the fundamental characteristics of the core syndrome--itis independent of the environment and constant in incidence across populations that have been separated for thousandsof years. The associated genetic variation must be as old as Homo sapiens and represent a component of diversitythat crosses the population as a whole. The fecundity disadvantage that accompanies the syndrome requires a balancein a substantial and universal advantage; this advantage, it is proposed, is the speciation characteristic of language;language and psychosis have a common evolutionary origin. Language, it is suggested, originated in a critical changeon the sex chromosomes (the 'speciation event'--the genetic change that defined the species) occurring in East Africabetween 100 and 250 thousand years ago that allowed the two hemispheres to develop with a degree of independence.

Language can be understood as bi-hemispheric with one component function--a linear output sequence--confinedto the dominant hemisphere--and a second--parallel distributed sampling occurring mainly in the non-dominanthemisphere. This mechanism provides an account of the generativity of language. The significance of nuclear symptomsis that these reflect a breakdown of bi-hemispheric coordination of language, perhaps specifically of the process of'indexicalisation' (the distinction between ' I' and 'you') of self- versus other-generated references. Nuclear symptomscan be described as 'language at the end of its tether'; the phenomena and population characteristics of the nuclearsyndrome of schizophrenia thus yield clues to the origin of the species. 1997 Elsevier Science B.V.Keywords: Nuclear; Schizophrenia; Speciation; Language; Dominance

1 Is schizophrenia an entity?There are serious doubts about the reality of

'sch izop hren ia' as a discrete categor y (Crow, 1986,1995d; Boyle, 1990). The origin of the concept liessquarely in the distinction that E. Kraepelin (1919)drew between dementia praecox and manic-depres-sive insanity. On the one hand, Kraepelin argued,

* Tel: +44 (1865) 226474; Fax +44 (1865) 244990.0920-9964/97/ 17.00 1997 Elsevier Science B.V. All rights reserved.PII S0920-9964(97)00110-2

there are diseases in which mood change (depres-sion or elation) is prominent, and psychotic symp-toms (delusions and hallucinations) can be seen ascongruent with and perhaps secondary to themood change. These illnesses Kraepelin groupedtogether under the heading of manic-depressiveinsanity, from which a complete recovery canusually be expected. On the other hand, there aredisease states in which the psychotic phenomenacannot be understood in this way. He groupedthese illnesses, in which the outcome was not as


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128 T.J. Crow / Schizop hrenia Research 28 19972 127- 141

g o o d , t o g e t h e r a s d e m e n t i a p r a e c o x a n d p a r a p h r e -n ia. Un t i l r ecen t ly , the s epara t ion o f dem en t iap raeco x and m an ic -depres s ive insan i ty ( the s ch izo -phren ic and a f f ec tive g roup s o f psychoses ) hasgone l a rge ly uncha l lenged . Indeed , i t can be s a idto unde rp in m os t , i f no t a ll , m o der n psych ia t r i cc lass i fi ca tions , and exer t s a p ro fou nd in f luence onae t io log ica l th ink ing . Sch izophren ia , a s dem en t iap raecox fo l lowing E . B leu le r (1950) has com e tobe cal led, and af fect ive (manic-depress ive) psycho-ses are general ly regarded as dis t inct ent i t ies withseparab le pa t t e rns o f sym ptom s , t r ea tm en ts andou tcom es . By im pl ica t ion , they have s epara teaet io logies .

H o w e v e r , K r a e p e l i n ( 1 9 2 0 ) h i m s e l f d e v e l o p e ddoub ts . T hus , he wro te o fthe difficulties which prevent us from distinguishing reliablybetween manic-depress ive insani ty and dement ia praecox. Noexper ienced psychia t r i s t w il l deny there i s an a larmingly largenumb er o f cases in which i t seems impossible in spi te of themost careful observat ion to make a f i rm diagnosis . .. . i t i sbecom ing increas ingly c lear that we canno t dis t inguish sa t i s fac-tor i ly between these two i l lnesses and this br ings home thesuspic ion that our form ulat ion o f the problem m ay be incorrect .

I t i s unden iab le tha t the re a re p ro found d i f f e r -ences between dif ferent forms of psychot ic i l lness .T here is a r e la t ionsh ip be tween fo rm of i llnes s andou tco m e - -p syc ho t ic i l lnesses w i th a f f ec t ive f ea -tu res genera l ly have a be t t e r ou tcom e than thosethat lack such features , and th is general isat ion isKraepe l in s l egacy . How ever , i t i s f a r f rom es tab -l i shed tha t any ca tegor ica l d i s t inc t ion can bedrawn . Schizo-a ffect ive il lnesses (K asa nin , 1933)a re com m o n and , in the absence o f p r inc ip les bywhich they can be subc las s i f i ed , they underm inet h e K r a e p e l i n i a n d i c h o t o m y .

2 A continuum of psychotic illnessT h at s ch izophren ia i s an e lus ive en t i ty i s dem on-s t r a ted by a l ack o f ag reem en t as to how i t shou ld

be def ined. Endicot t e t a l . (1982) appl ied dif ferentsets of d iagnos t ic cr i ter ia to a ser ies of 46 pat ientsadm i t t ed to the Psych ia t r i c Ins t i tu te in New Yorkwho m et a t l eas t one o f these de f in i tions . By themost l iberal cr i ter ia , 44 pat ients suffered f romsch izophren ia , bu t by the m os t r es t r i c t ive , the rewere only s ix . Such f indings engender scept ic ism

regard ing the ex i s tence o f any such d i s ease en t i ty .However , c lose r s c ru t iny r evea l s tha t the d i f f e r -ences in th is s tud y betwe en the sets of cr i ter ia areto a l a rge ex ten t accoun ted fo r by the ex ten t towhich d if ferent sets a l locate pat te rns of i llness tothe ca tegor ies o f s ch izo -a f fec t ive and a f f ec t ivepsychos i s . T he m ore l ibe ra l c r i t e r i a a l loca te m orepa t ien t s to the ca tegory o f s ch izophren ia andfewer to these diagnoses ; the s t r ic ter cr i ter ia a l lo-ca te the cases exc luded f rom a d iagnos i s o f s ch izo -phren ia to the ca tegor ies o f s ch izo -a f fec t ive o reven a f f ec tive psychos i s . T he ca tegory boundar iesa re a rb i t r a ry . T he f ind ings f i t m ore r ead i ly w i ththe no t ion tha t the re ex i s t s a con t inuum (Crow,1990c, 1994b, 1995b) that s t re tches f rom moreunder s tandab le m an ic -depres s ive psychoses a t

one end to the l e s s under s tandab le s ch izophren icpsychoses a t the o the r .W h a t d o e s t h e e x i s t e n c e o f s u c h a c o n t i n u u m

im ply? W hi le a ca tegor ica l concep t i s com pat ib lewi th an exogenous ( env i ronm enta l ) causa t ion , acon t inuum sugges t s r a the r tha t the d i so rder r ep re -sents a component that is in tr ins ic to the indivi-dua l , i .e . an ex t r em e o f va r ia t ion in the norm a lpopu la t ion . Here , i t w i l l be a rgued tha t the sym p-tom s co ns t i tu te a c lue to the gene t ic va r ia t ion tha tep i tom ises the popu la t ion o f Homo sapiens in thesense tha t the va r ia t ion was genera ted in thet r ans i t ion f rom a p recur so r hom in id , and tha t there levan t d im ens ion r e la tes d i r ec t ly to the func t iontha t charac te r i s es Homo sapiens as a species, thatis language.

I t was the pa r t i cu la r con t r ib u t ion o f K .Schne ider (1957) to de f ine a s e t o f sym ptom s( those desc r ibed as f i r s t r ank o r nuc lea r ) tha tiden t i f i e s the m os t charac te r i s t i c co re o f the syn-d rom e (T ab le 1 ) . Nuc lea r sym ptom s ( fo r exam pleg e d a n k e n l a u t w e r d e n , t h o u g h t i n s e r t i o n a n drem ova l ) a r e no tab le by the i r incom prehens ib i l i ty .T hey r ep resen t som e so r t o f lo ss o f the bou nda rybe tween the s e l f and the ou t s ide wor ld o r , m orespecif ical ly , o ther persons . Crucial ly , they cons t i -tu te a pa tho log y o f the r e la t ionsh ip be tweenthough t and l anguage . W hi le they f a i l to iden t i fya ca tegory d i s t inc t f rom psycho t ic i l lnes ses tha tl ack such f ea tu res , these sy m ptom s def ine a th resh -o ld tha t t e l ls us the m ean ing o f the popu la t iond is t r ibu t ion o f psychos is .


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T.J. Crow/Schizophrenia Research 28 1997) 127-141Table 1Some nuclear symptoms of schizophrenia

129

Symptom Description Putative non-domin ant hemisphere origin

Experience thatfeelings, actions are externally controlledH e a r i n g

1. one s thoughts spoken aloud2. voices discussing one in the third person3. a commentary on one s actions

Experience that thoughts are1. removed from one s head2. inserted into one s head3. broadcast to others

Delusions of controlThought echoThird-person hallucinationsRunning commentaryThought: withdrawal

insertionbroadcast

FrontalFronto-parieto-temporal

Parieto-temporal

3 T h e W or l d He a l t h Or gan i s a t i on T e n C ou n t r yS t u d y o f I n c i d en c e

A s c o m p r e h e n s i v e a n a n s w e r t o th e q u e s t i o n o ft h e i n c i d e n c e o f n u c l e a r s c h i z o p h r e n i a a s w e h a v ec o m e s f r o m t h e W H O T e n C o u n t r y S t u d y o fI n c i d e n c e ( J a b l e n s k y e t a l. , 1 9 9 2 ) . I n e a c h o f t e nc e n t r e s d i s t r i b u t e d a c r o s s p o p u l a t i o n s a s d i f f e r e n ta s t h o s e o f J a p a n , I n d i a, N o r t h e r n E u r o p e a n dH a w a i i , t h e s e a u t h o r s d e f i n e d a c a t c h m e n t a r e aa n d i d e n t i fi e d e a c h o f t h o s e f a c i li t ie s t o w h i c hi n d i v id u a l s e x p e r i e n ci n g p s y c h o t i c s y m p t o m s f o rt h e f i r s t t i m e m i g h t p r e s e n t . W i t h s t a n d a r d i s e di n t e rv i e w i n g p r o c e d u r e s , t h e y w e r e a b le t o d e m o n -s t r a t e g o o d r e l i a b i l i t i e s b e tw e e n c e n t r e s i n e l i c i t i n gs y m p t o m s a n d r e a c h i n g a d i a g n o s i s ( s e e F i g . 1 ) .

A a r husruralChancl igarhurban

D u b l i nH o n o l u l uM os c owN a ga s a k i

1 4

EEEb

Not t inghamI ; o ; o ;o s ; oC a s e s

l Restrict ive defini t ion: [ ] Broaa de f in i t ion : C l in ica lC a t e g o S + d i a g n o s is o r C a t e g o S , P , 0WHO t99

Fig. 1. Incidence of schizophren ia in seven centres according tobroad and restrictive definitions in the WHO Ten CountryStudy (Jablensky et al., 1992).

T h e m a i n f i n d i n g c o n s t r a i n s o u r c o n c e p t s o fo r i g in s . W h e r e a s w i t h a b r o a d d e f i n i t i o n ( t h a ti n c l u d e d d i a g n o s e s a l l o c a t e d b y t h e h o s p i t a l c l i n i -c i a n s a s w e ll as t h o s e o f t h e r e s e a r c h e r s a d o p t i n gl ib e r a l c r i t e r i a ) , t h e r e w e r e s i g n i f i c a n t d i f f e r e n c e sb e t w e e n c e n t r e s i n i n c i d e n c e , w h e n t h e c r i t e r i aw e r e m o r e n a r r o w l y d e f i n e d , sp e c i fi c a ll y b y t h ep r e s e n c e o f t h e n u c l e a r o r f i rs t r a n k f e a t u r e s , t h ed i f f e r e n c e s b e t w e e n c e n t r e s b e c a m e l e s s a n d , i nt h is c o m p a r i s o n , w e r e n o t s i g n if ic a n t. T h e m e a n i n gb e c o m e s c l e a r e r w h e n o n e c o n s i d e r s p s y c h o s i s a sa n e n t i t y t h a t c a n b e d e f i n e d b y d i f f e r i n g ( b r o a d e ro r n a r r o w e r ) d i a g n o s t i c th r e s h o l d s . I f t h e r e w e r er e a l d i f f e r e n c e s i n i n c i d e n c e b e t w e e n p o p u l a t i o n s ,a s t h e c r i t e r i a w e r e d r a w n m o r e r e s t r i c t i v e l y , o n ew o u l d e x p e c t t h e d i f fe r e n c e s i n i n c id e n c e t o b e c o m eg r e a t e r . H o w e v e r , t h i s i s n o t w h a t i s s e e n . I n f a c tt h e y b e c o m e l e s s ( a n d t h e v a r i a n c e i s r e d u c e d ) .T h i s f i n d i n g i s c o n s i s t e n t o n l y w i t h a s e c o n d i n t e r -p r e t a t i o n - - - t h a t i n c i d e n c e i s c o n s t a n t a c r o s s p o p -u l a t i o n s , a n d t h a t t h e d i f f e re n c e s w i t h b r o a dd i a g n o s t i c c r i t e r i a a r is e f r o m d i f f e re n c e s i n t h el e v e l s a t w h i c h t h e t h r e s h o l d i s d r a w n ( F i g . 2 ) .

S c h i z o p h r e n i a , i t s e e m s , i s c o n s t a n t a c r o s s p o p -u l a t i o n s t h a t d i f f e r w i d e l y i n g e o g r a p h i c , c l i m a t i c ,i n d u s t r i a l a n d s o c i a l e n v i r o n m e n t , a n d t h e u t i l i tyo f f ir s t r a n k s y m p t o m s h a s b e e n t o d e m o n s t r a t et h i s f a c t. T h e s e s y m p t o m s d e f i n e a l ev e l o f s e v e r i t yo r n o n - u n d e r s t a n d i b i l i t y a t w h i c h i t i s h i g h l y l ik e l yt h a t a n i n d i v i d u a l w h o e x p e r i e n c e s t h e m f o r t h ef i r s t t i m e w i l l p r e s e n t t o a p s y c h i a t r i c o r r e l a t e ds e r v i c e , a n d t h u s w i l l b e e n u m e r a t e d a s i n t h eW H O s t u d y .


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130 T.J. Crow / Schi zophrenia Research 28 1997) 127-141Re a l va r i a t i o n s i n i n c i d e n cei i i i i iB C

P o p u l a t i o n

Co n s t a n t i n c i d e n ce V a r i a t i o n si n t h re sh o l d o f dete t ion

A B CP o p u l a t i o n

F i g 2 T w o p o s s i b le in t e r p r e t a t i o n s o f th e f in d i n g s o f t h e W H OT e n C o u n t r y s t u d y

T h e c o n c l u s i o n i s c h a l l e n g i n g . I f s c h i z o p h r e n i ai s i n d e p e n d e n t o f t h e e n v i r o n m e n t , i t d i f f e r s f r o mc o m m o n p h y s ic a l d is e as e s s u c h a s c o r o n a r y a r t e r yd i s eas e , d i abe t es and a r t h r i t i s . Thes e r ep res en t ani n t e r a c t i o n b e t w e e n g e n e t i c a n d e n v i r o n m e n t a la e t i o l o g i c a l f a c t o r s i n a w a y t h a t s c h i z o p h r e n i adoes no t . S ch i zo ph ren i a , i t s eem s , is a cha ra c t e r i s t i co f h u m a n p o p u l a t i o n s . I t i s a d i s ea s e ( p e r h a p s t h ed i s e a s e ) o f h u m a n i t y .

4 H o w o l d i s t h e s c h i z o p h r e n i a m u t a t i o n ?I f t h e v a r i a t i o n u n d e r l y i n g t h e s e p s y c h o l o g i c a l

p h e n o m e n a i s g e n e t i c , o n e c a n a s k ' h o w o l d i s t h es c h i z o p h r e n i a m u t a t i o n ? ' E i t h e r t h e m u t a t i o n ( i . e .g e n e t ic v a r i a t i o n ) o r t h e m e c h a n i s m t h a t g i ve s ri set o i t m u s t h a v e p r e c e d e d t h e s e p a r a t i o n o f th ep o p u l a t i o n s i n w h i c h i t is n o w p r e s e n t. G i v e n t h a tt h e J a p a n e s e , In d i a n a n d N o r t h E u r o p e a n p o p u l a -t io n s h a v e b e e n s e p a r a t e d f o r t h o u s a n d s ( p r o b a b l ya t l eas t 10 000 ) yea r s , t he ' m u t a t i o n ' i s c l ea r l y o l d .W h e n o n e c o n s i d e r s i n a d d i t i o n t h a t s c h i z o p h r e n i awi t h e s s en t i a l l y t he s am e fea t u res i s p res en t i n t heA u s t r a l i a n a b o r i g i n a l p o p u l a t i o n t h a t s e p a r a t e df r o m t h e re s t o f m o d e r n Homo s ap iens a t l eas t50 000 yea r s ago (Mowry e t a l . , 1994 ) , i t i s appar -e n t t h a t t h e v a r i a t i o n m u s t i n d e e d b e a n c i e n t - - i nf a c t , i t m u s t h a v e p r e c e d e d o r b e e n c o i n c i d e n t w i t ht h e o r i gi n o f m o d e r n Homo sapiens a n e v e n t d a t e d[ o n th e b a s is o f m i t o c h o n d r i a l D N A e v id e n c e(S t on ek i n g e t a l ., 1992 )] t o b e t wee n 137 000 and

2 5 0 00 0 y e a r s a g o t h a t o c c u r r e d s o m e w h e r e i n E a s tAf r i ca (S t r i nge r an d M cK i e , 1996 ) ( see F i g . 3 ) .

T h e g e n e t i c c h a n g e t h a t p r e c e d e d t h e d i a s p o r ao f m o d e r n Homo s ap iens c a n b e r e g a r d e d a s t h e' s p e c i a t i o n e v e n t ' , t h e e v e n t t h a t e n a b l e d t h i ss p e c i e s t o e x p a n d i n p o p u l a t i o n s i z e t o o c c u p y ar a n g e o f e c o l o g ic a l n ic h e s in a w a y t h a t n o p r e v i o u sp r i m a t e s p e c i e s h a d d o n e , t o s h a p e t h e e n v i r o n -m e n t t o i t s o w n a d v a n t a g e a n d t o t h r e a t e n t h es u rv i va l o f m a n y o t h e r o r g a n is m s . I t m u s t p r e s u m -ab l y r e l a t e t o t he s pec i es cha ra c t e r i s t i c o f l anguag e .

A s e c o n d q u e s t i o n c a n b e a s k e d : w h o ( a p a r tf rom s u f fe re r s t hem s e l ves ) ca r r i e s t he s ch i zo -p h r e n i a p r e d i s p o s i t i o n ? T h i s q u e s t i o n i s r e l e v a n tt o g e n e t i c l i n k a g e s t u d i e s ( t h a t a t t e m p t t o l o c a t ea gene o r genes ) bu t has a l s o been as ked i n ae u g e n ic c o n t e x t b y t h o s e w h o h a v e t h o u g h t t h a t i tm i g h t b e p o s s i b l e t o e l i m i n a t e t h i s p r e d i s p o s i t i o nf r o m t h e p o p u l a t i o n . A s t h e f o r e r u n n e r t o t h eN a z i p o l ic y o f g e n o c id e d u r i n g t h e S e c o n d W o r l dW a r , t h is v ie w h a s a n u n f o r t u n a t e h i s t o r ic a l p r e ce -d e n t ( M e y e r - L i n d e n b e r g , 1 9 9 1 ) .

T h e W H O s t u d y c a s t s l i g h t o n t h i s i s s u e . I tc a n n o t b e t h a t t h e r e is a f r a c t i o n o f th e p o p u l a t i o nt h a t c a r r i e s a g e n e t h a t i s a b s e n t f r o m t h e r e m a i n -d e r b e c a u s e i f su c h a f r a c t i o n e x i s te d , t h e r e i s n or e a s o n w h y i t s h o u l d r e m a i n c o n s t a n t i n p o p u l a -t i o n s t h a t h a v e b e e n s e p a r a t e f o r t e n s o f t h o u s a n d so f y e a rs . V a r i a t i o n b e t w e e n p o p u l a t i o n s , e i t h e r a sa r e s u l t o f d i f f e ren t i a l s e l ec ti on o r gene t i c d r i f t ,w o u l d b e e x p e c t e d . O n e m u s t c o n c l u d e t h a t t h ev a r i a t i o n o f w h i c h p r e d i s p o s i t i o n t o s c h i z o p h r e n i af o r m s a p a r t i s n o t c o n f i n e d t o a s u b f r a c t i o n - - i tm u s t c r o s s t h e p o p u l a t i o n a s a w h o l e .

A l o n g s i d e p o p u l a t i o n d i s t r i b u t i o n a s a p e c u l i a ra n d u n e x p l a i n e d c h a r a c t e r i s t i c i s a g e o f o n s e t .O n s e t s o f p s y c h o s is o c c u r ( a m e a n 2 - 3 y e a r s e a r li e ri n m a le s t h a n f e m a le s ) th r o u g h o u t t h e r e p r o d u c t iv ephas e o f l if e (P en ros e , 1991 ). A s t h i s is a l s o theh e a l t h i e s t p h a s e , i n w h i c h t h e e x p e c t a t i o n o f p h y s i-ca l d i s eas e i s l owes t , t hes e f ac t s aga i n d raw a t t en -t i o n t o t h e s i n g u l a r i ty o f p s y c h o si s . G i v e n t h a t t h ei ll n es s is a s s o c i a t e d w i t h a p r o c r e a t i v e d i s a d v a n t a g e[ s u f fe re r s a re l e s s l i ke l y t han t he popu l a t i on i ngenera l t o have ch i l d ren (MacS or l ey , 1964 ; Voge l ,1979 ) pe rha ps by a f ac t o r o f 50 ] t h i s f ac t po i n t sa f i n g e r a t t h e c e n t r a l p a r a d o x - - i f t h e d i s e a s e i s


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132 T..L Crow / Schizophrenia Research 28 1997) 1 2~ 1 4Il a n g u a g e a p p a r e n t l y h a s e v o l v e d b y a p r o c e ss o fi n c r e a s i n g h e m i s p h e r i c s p e c i a l i sa t i o n , c e r e b r a l c o n -t r o l ( ' d o m i n a n c e ' ) f o r l a n g u a g e b e i n g l o c a l i s e d i no n e ( m o s t o f t e n t h e l e f t ) h e m i s p h e r e . S u c h s p e c i a l -i s a t i o n i s a c c o m p a n i e d b y a p r e f e r e n c e t o u s e t h er i g h t h a n d f o r t a s k s r e q u i r i n g f in e m o t o r s k i ll .T h i s c h a n g e m a y h a v e b e e n p r e s a g e d i n H o m oe r e c t u s o r e v e n i n H o m o h a b i l i s ( S t e e l e , 1 9 9 7 ) , b u td i r e c ti o n a l a s y m m e t r y f o r m o t o r s k il l c e r t ai n l y isg r e a t e r i n m a n t h a n o t h e r p r i m a t e s . I t m a y b e as p e c i e s c h a r a c t e r i s t i c , b u t o n e f o r w h i c h v a r i a t i o nw i t h i n t h e p o p u l a t i o n i s m a i n t a i n e d ( M c M a n u s ,1 99 1; P e re l l e a n d E h r m a n , 1 9 9 4 ): s o m e w h e r eb e t w e e n 6 a n d 1 2 ( d e p e n d i n g u p o n t h e c r i t e r i o n )o f a ll p o p u l a t i o n s t h a t h a v e b e e n s t u d i e d h a s ap r e f e r e n c e f o r t h e u s e o f t h e l e f t h a n d .

T r a n s m i s s i o n o f h a n d e d n e s s ( a n i n d e x o f a s y m -m e t r y i n t h e b r a i n ) w i t h i n f a m i l i e s c a n b ea c c o u n t e d f o r b y a s i n g l e a d d i t i v e g e n e ( t h e ' r i g h ts h i f t f a c t o r ' ) t h a t b i a s e s t h e l e f t h e m i s p h e r e a n dr i g h t h a n d t o w a r d s d o m i n a n c e o v e r t h e i r c o n t r a -l a t e ra l p a r t n e r s ( A n n e t t , 1 9 8 5 ) . A s a t h e o r y o f t h et r a n s m i s s i o n o f h a n d e d n e s s w i t h i n f a m i li e s ,A n n e t t ' s h a s p o i n t s i n c o m m o n ( e . g . t h e p o s t u l a t eo f a si n g le g e n e c o m b i n e d w i t h a r a n d o m i n f l u en c e )w i t h t h a t o f M c M a n u s ( 1 98 5 ). M o r e c o n t r o v e r -s i a l l y , A n n e t t h a s a r g u e d t h a t t h e d i f f e r e n t g e n o -t y p e s a t t h e p u t a t i v e ' r i g h t s h i f t ' l o c u s ( e s t i m a t e df r o m t h e i r p o s i t i o n o n a c o n t i n u u m o f re l a ti v eh a n d s k i l l) a r e a s s o c i a t e d w i t h d i f fe r e n t c o g n i t i v ea b i l i ti e s , sp e c i f i c a ll y t h a t h e t e r o z y g o te s ( _ +) f o r t h er i g h t s h i f t f a c t o r a r e a t a n a d v a n t a g e w i t h r e s p e c tt o h o m o z y g o t e s ( - / - a n d + / + ) . T h e g e ne ti cv a r i a t i o n a t t h i s lo c u s w o u l d t h u s b e a c a se o f a' b a l a n c e d p o l y m o r p h i s m ' , i n w h i c h h e t e r o z y g o t e sh a v e a s u r v i v a l a d v a n t a g e o v e r h o m o z y g o t e s , as i t u a ti o n t h a t c a n m a i n t a i n v a r i a t io n i n t h e p o p u l a -t i o n a g a i n s t s e l ec t iv e p r e s s u r e . E v i d e n c e t o s u p p o r tt h is v ie w co m e s f r o m a t h e U K N a t i o n a l C h i l dD e v e l o p m e n t c o h o r t . O n a t es t o f h a n d s ki ll a t th ea g e o f 1 1 y e a r s , t h o s e w h o w e r e s t r o n g l y l a t e r a l is e di n e i t h e r d i r e c t i o n w e r e a t a d i s a d v a n t a g e c o m -p a r e d t o t h o s e w h o w e r e l e s s s t r o n g l y l a t e r a l i s e d ,i n a g r e e m e n t w i t h A n n e t t ' s t h e o r y , b u t t h o s e w h ow e r e m o s t d i s a d v a n t a g e d ( o n v e r b a l ; a n d n o n -v e r b a l a b i l i t y a s w e l l a s m a t h e m a t i c s a n d r e a d i n gs k i l l s ) w e r e t h o s e w h o w e r e c l o s e s t t o t h e p o i n t o f

e q u a l h a n d s k il l, o r t h e p o i n t o f ' h e m i s p h e r i cin d e c i s io n ' ( C r o w e t a l . , 1 9 9 6 ) .

D o m i n a n c e f o r la n g u a g e a n d h a n d e d n e s s i sr e f l e c te d i n a n a t o m i c a l a s y m m e t r y i n t h e b r a i n . I nm o s t i n d i v i d u a l s , t h e w i d t h o f t h e b r a i n i s g r e a t e ro n t h e r i g h t i n t h e f r o n t a l r e g i o n a n d o n t h e l e f ti n t h e o c c i p i t o - p a r i e t a l r e g i o n , a n d t h e l a t e r a l( S y l v i a n ) f i s s u r e ( t h a t d i v i d e s t h e t e m p o r a l f r o mt h e p a r i e t a l l o b e s ) i s l o n g e r o n t h e l e f t ( W i t e l s o na n d K i g a r , 1 9 8 8 ) . T h e s e a s y m m e t r i e s a r e l e s s i nt h o s e w h o a r e l e f t - h a n d e d o r a m b i d e x t r o u s ( B e a re t a l ., 1 9 8 6; F o u n d a s e t a l ., 1 9 9 5 ) . T h e v a r i a t i o nc o u l d r e p r e s e n t m o d u l a t i o n o f t h e i n fl u e n ce o f as in g l e g r o w th f a c to r ( s e e F ig . 4 ) .

S e x d i f f e re n c e s i n a s y m m e t r y a r e p r e s e n t . M a l e sa r e m o r e l i k e l y t o b e l e f t - h a n d e d - - 1 2 v s . 1 0 .5( M c M a n u s , 1 9 9 1 ) - - a d i f f er e n ce t h a t w a s p r e s e n ti n t h e U K N a t i o n a l C h i ld D e v e l o p m e n t S a m p l e - -a n d h a v e a g r e a t e r m e a n a s y m m e t r y i n t h e b r a i nt h a n f e m a l e s ( B e a r e t a l ., 1 9 8 6 ). T h e s e d i f f e re n c e sp r e s u m a b l y a r e r e l a t e d ( s e e b e l o w ) t o t h e s m a l l ,b u t w e l l - e s ta b l i s h e d , m e a n d i f f e r e n c e i n t h e d i s t r i -b u t i o n o f i n t e l le c t u a l a b i l i t i e s - - w i t h s u b s t a n t i a lo v e r l a p b e t w e e n t h e s e x e s f e m a l e s h a v e a g r e a t e rv e r b a l f l u e n c y a n d m a l e s h a v e a g r e a t e r s p a t i a la b i li t y ( M a c c o b y a n d J a c k l in , 1 97 5; M c G l o n e ,1 9 8 0 ; H a lp e r n , 1 9 9 2 ) .

symm etry o f the human b ra innterior

Posterior

Frontal

Occipital

Fig. 4. W idth asymm etries in the h um an brain.


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T.J. Crow / Schi zophrenia Research 28 1997) 1 2~ 14 1 1337 . T h e h y p o t h e si s o f a n X Y h o m o l o g o u s g e n e

T h e o r i g i n o f s u c h s e x d if f e r e n c e s i s o f c o n s i d e r -a b l e i n t e r e s t . I h a v e s u g g e s t e d ( C r o w , 1 9 9 3 , 1 9 9 4 a ,1 9 9 5 c ) t h a t t h e y r e f le c t t h e o p e r a t i o n o f a n X a n dY - l i n k e d g e n e , t h a t i s a g e n e t h a t i s r e p r e s e n t e d i nh o m o l o g o u s f o r m o n b o t h X a n d Y c h r o m o s o m e s .T h e s i m p l e s t h y p o t h e s i s i s t h a t t h i s g e n e i t s e l f ist h e a s y m m e t r y d e t e r m i n a n t , i .e . A n n e t t s r i g h t s h if tf a c t o r . T h e e v i d e n c e i s( 1 ) t h a t i n d i v id u a l s w i th s e x c h r o m o s o m e a n e u -

p l o i d i e s h a v e r e l a t i v e h e m i s p h e r i c i m p a i r m e n t so r d e l a y s . I n d i v i d u a l s w h o l a c k a n X ( X O ,T u r n e r s s y n d r o m e ) h a v e r i g h t -h e m i s p h e r e( n o n - v e r b a l ) d e f ic i ts , w h e r e a s i n d i v i d u a l s w i t ha n e x tr a X ( X X Y = K l i n e f e l te r s , a n d X X Xs y n d r o m e s ) h a v e l e f t - h e m i s p h e r e ( v e r b a l )d e fi c it s . T h i s s u g g e s ts t h a t t h e r e i s a n a s y m m e -t r y d e te r m i n a n t o n t h e X c h r o m o s o m e , b u tt h e f a c t t h a t m a l e s ( X Y ) d o n o t h a v e d e f i c i t sc o m p a r a b l e t o t h o s e p r e s e n t i n T u r n e r s s y n -d r o m e e v e n t h o u g h t h e y h a v e o n l y o n e Xi n d i ca t e s t h a t t h e r e m u s t b e a g e n e o f c o m p a r a -b l e e f f ec t o n t h e Y c h r o m o s o m e ( C r o w , 1 9 8 9,1 9 9 3 , 1 9 9 4 a , 1 9 9 5 e ) ( s e e F ig . 5 ) . I t s h o u ld b en o t e d t h a t G e s c h w i n d a n d G a l a b u r d a sh y p o t h e s i s ( G e s c h w i n d a n d G a l a b u r d a , 1 9 8 5)t h a t s e x d i f f e re n c e s i n c e r e b r a l a s y m m e t r y a r es e c o n d a r y t o a h o r m o n a l i n fl u e nc e is r u le d o u t

1 0 0

8 06 0

IQ4O

2 0

0

p : < 0 0 1 < 0 0 0 2 < 0 0 0 2

Tum er s Kl inefe lter sX O X X Y X X X

n = 3 5 4 3I I Verbal IQ ~ Performance Q

N e t l e y R o v e t , 1 9 8 2 ; 1 9 8 7Fig . 5 . Neuro psycho log ica l impa i rm ents a ssoc ia ted wi th sexchrom osom e aneup lo id ies [ f rom Crow 1993)] .

2 )b y t h e s i m i l a r it y o f t h e h e m i s p h e r i c d e f i c it s i nX X Y ( m a l e ) a n d X X X ( f e m a l e ) i n d i v i d u a l s .t h a t w i t h i n f a m i l i e s t h e r e i s a n a s s o c i a t i o nb e t w e e n s e x a n d h a n d e d n e s s , s i b li n g s o f t h es a m e s e x b e i n g m o r e l i k e ly t h a n i n d i v i d u a l s o fo p p o s i t e s e x t o b e o f t h e s a m e h a n d e d n e s s( C o r b a l l i s e t a l . , 1 9 9 6 ) . T h e e f f e c t i s s ma l l( b e in g s i g n i f i c a n t a t t h e 0 . 0 2 l e v e l i n a s a m p leo f 1 5 0 0 0 s i b s h i p s ) , b u t i ts m a g n i t u d e i s a se x p e ct e d f r o m t h e c o m b i n a t i o n o f r a n d o m a n dr i g h t s h i f t e f fe c t s i n A n n e t t s t h e o r y .

X - Y h o m o l o g o u s g e n e s c o n s t i t u t e a r e c e n t l yr e c o g n i s e d c l a ss ( L a m b s o n e t al ., 1 9 9 2 ) . It i n c lu d e sg e n e s w i t h i n t h e p s e u d o - a u t o s o m a l ( e x c h a n g e )r e g io n s a t t h e t e l o m e r e s o f t h e s h o r t a n d l o n g a r m so f t h e X a n d Y c h r o m o s o m e s , a s w e l l a s a sm a l lb u t i n c r ea s i n g n u m b e r o f g e n e s o u t s id e t h i s r e g i o n .T h e g e n e c o p i e s o n t h e X c h r o m o s o m e a r e n o ts u b j e c t t o i n a c t i v a t i o n , a p r o t e c t i o n t h a t e n s u r e sg e n e d o s a g e e q u i v a l e n c e b e t w e e n t h e s e x es .W h e r e a s f o r g e n e s w i t h i n t h e p s e u d o - a u t o s o m a lr e g i o n ( w h e r e t h e r e i s r e c o m b i n a t i o n b e t w e e n Xa n d Y c h r o m o s o m e s i n m a l e m e i o s i s ) , s t r i c ts e q u e n c e h o m o l o g y b e t w e e n t h e c o p y o n t h e Xa n d t h a t o n t h e Y w i l l b e e x p e c t e d , o u t s i d e t h i sr e g i o n , r e c o m b i n a t i o n d o e s n o t t a k e p l a c e , a n dd i v e r g e n c e b e t w e e n X a n d Y c o p i e s w i ll o c c u r .S u c h d i v e r g e n c e , w h e t h e r i n a p r o t e i n - c o d i n g o rc o n t r o l s e q u e n c e , c o u l d a c c o u n t f o r a s e x u a ld i m o r p h i s m . I n e v o l u t i o n a r y te r m s , t h e s e ge n e sw i l l a l s o b e s u b j e c t t o t h e f o r c e o f s e x u a l s e l e c t io n( C r o w , 1 9 9 6 a ) .

8 . T h e o r i g i n s o f g e n e r a t iv i t yW h a t i s m o s t c h a r a c t e r i s t i c a b o u t l a n g u a g e i s

i t s g e n e r a t i v i t y - - t h e c a p a c i t y f o r r e c o m b i n a t i o nt h a t d i s t i n g u i s h e s t h e c o m m u n i c a t i v e a b i l i t y o fh u m a n s f r o m t h a t o f o t h e r o rg a n i sm s ( C h o m s k y ,1 9 65 ; B i c k e r to n , 1 9 9 0 ; C o r b a l l i s , 1 9 9 1; M a y n a r d -S m i t h a n d S z a t h m a r y , 1 9 9 5 ) a n d t h a t m a d eD a r w i n s t h es i s ( D a r w i n , 1 8 7 1) o f c o n t i n u i t y i nT h e D e s c e n t o f M a n s o c o n t r o v e r si a l . H o w c o u l da n a p p a r e n t l y g r a d u a l p r o c e s s a c c o u n t f o r w h a ts e e m s t o b e a n e n t i r e l y n o v e l c a p a b i l i t y ?

T h e a n s w e r m u s t l ie in t h e a l l o c a t i o n o f f u n c t i o n


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134 T.J. Crow / Schizophrenia Resea rch 28 1997) 127-141to one or other of the two hemispheres. It is thispotential that appears either to be restricted to, orat least to have greatly developed in, the hominidprogression. Whereas previously, the two hemi-spheres were closely matched in their functionsand these were related in a parallel and topographi-cal manner to their respective motor and sensoryfields, in Homo sapiens there has been a radicaldepar ture [see Annett (1985); McManus (1991);Corballis (1991) and Steele (1997) for discussionsof this discontinuity].

Why was the potential for a degree of functionalindependence of the hemispheres so important?Following Dax (1865) and Broca (1861), it is oftenstated that language is localised in the left ordominant hemisphere with the implication thatsomething else is localised in the non-dominanthemisphere, but this clearly cannot be the case.What other function could this be? Cook (1986)asks:What is the right hemisphere doing while the left hemisphereis busy with the chores of linguistic communicati on...seriousconsideration of the nature of hemispheric interactions via theforebrain commissures demand that we ask what one hemi-sphere is doing simultaneously with the dom ina nt functionsof the o ther hemisphere. If the cerebral hemispheres are indeedas yoked to one another as the massive commissural connec-tions suggest, then the activity on one side must somehowresult in complementary activity (in an unknown, but physio-logically precise way) on the other side.

If language is the facul ty by which Homo s ap ienshas speciated and hemispheric specialisation is theprocess by which this has taken place, it followsthat what brain modifications there have been inHomo s ap iens are attributable to this fact. Theincreased interconnectivity of diverse cortical areasrelates to this core function; specific componentsof language must be located in each hemisphere.Language, therefore, is a whole brain function; itmust be bi-hemispheric. The key to the in teractionbetween the hemispheres rests in the mysteriousprocess of the establishment of dominance . Whatcomponent is present in the dominant hemisphere,and why does it have to be so segregated? Whatcomplementary component is present in the non-dominant hemisphere, and how is it tapped fromthe other side?Answers to these questions, it is suggested, mustlie in the relation between temporal and spatial

aspects of language, for example in the spatialisa-tion of form hypothesis (Lakoff, 1987; Deane,1993), the concept that language is, in part, spatialas well as temporal. Sign language provides cluesto the neural basis; in particular, its transmissionthrough visual and motor modalities underminesany claim for phonological and acoustic primacy,and therefore for any overwhelming significancefor Wernicke s area in speech perception.Armstrong et al. (1995) suggest that this fact shiftsthe focus from the fine temporal organisation ofthe acoustic modality to the spatial and action-orientated nature of language. Sentence structure,they suggest, can be unders tood as gestures relatingto the body and to external space. Other authors(e.g. Jackendorff , 1996; Bierwisch, 1996; Johnson-Laird, 1996) have considered how some semanticand morphologic relations can be understood interms of spatial constructs. The question ariseswhether spatial organisation is in some sense fun-damental to syntax. This has at times been sug-ges ted -se e, for example, Anderson (1971), Lyons(1977, pp. 718-724), Lyons (1995, Ch. 10),Jackendorff and Landau (1992) and Deane(1993).The specific hypothesis being developed here isthat there are both temporal and spatial aspectsto language, that the two are segregated (in thetwo hemispheres), and that the interaction betweenthem is central to the mode of operation of thehuman brain.Why the output sequence is restricted to onehemisphere can be explained by the physiology ofcallosal transmission, specifically from limits tothe ways in which the two hemispheres caninteract. The time delay of approximately 25 msfor transmission through the corpus callosum pre-cludes multiple inter-hemispheric passes in thecourse of single actions (Ringo et al., 1994):these temporal limits will be avoided if the neural apparatusnecessary to perform each h igh-resolution, time critical t ask isgathered in one hemisphere. If the, presumably overlap-ping, neural assemblies needed to handle overlapping tasksare clustered together, this would lead to hemisphericspecialization.

Such constraints must apply to sentences. Onecan postulate t hat the determining focus (a tempo-rally-organised sequence) is localised in one, pre-sumably the dominant, hemisphere (and acts as a


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T.J. C row / Schi zophrenia Research 28 1997) 127-141 135frame) but tha t this sequence also has accessthrough commissural fibres to neural traces(contents), perhaps at multiple sites, in the otherhemisphere. Such access could provide the basisfor the recombinational generativity of the process.9. Working memory the minimalist program andthe Saussurean sign

Hemispheric specialisation and the phenomenaof dominance can be related to the concept ofworking memory. Three componen ts--t he phono-logical loop, within which acoustically coded infor-mation is retained for a period of seconds(corresponding to primary or short-termmemory), the visuo-spatial sketch pad, from whichspatial informat ion can be retrieved and the execu-tive , which determines the direction of thought oraction--are recognised (Gathercole andBaddeley, 1993).In terms of the above theory of hemisphericspecialisation, it seems clear that, for the reasonsoutlined by Ringo et al. (1994), phonological loopactivity would necessarily be confined to one hemi-sphere, presumably the dominant one. Conversely,the functions performed by the visuo-spatialsketchpad correspond quite closely to those of thenon-dominant hemisphere. According to the spat-ialisation of form hypothesis [see Lako ff (1987);Deane (1993); Armstrong et al. (1995)] sentencestructure requires a reference frame that isextended in at least two-dimensional space,within which:(1) components, such as subject and object, canbe linked;(2) constituency relationships are understood aspart-whole correspondences; and(3) syntax can be related to the bodily schema.The parallel-processing capacity of the non-

dominant hemisphere provides such a refer-ence frame.The executive (a suspiciously homunculoidelement) must clearly interact with both compo-nents; as a term, it appears to describe the processof ordering sequences, for example in the case oflanguage within and between sentences. If thethread of continuity is in the phonological loop in

the dominant hemisphere, it seems tha t the execu-tive , which is often considered to be a frontal lobefunction, must be located, along with the linearoutput sequence, primarily within the dominanthemisphere. As a functional entity, it has affinitieswith Gazzaniga s left brain interpreter(Gazzaniga, 1992).

In the minimalist program of universal gram-mar (Chomsky, 1995), a distinction is drawnbetween logical form (L F) and phonetic form(PF), the former representing the assembly of thelexical and syntactic components of the sentence,and the latter its phonetic expression with criticalfunctions occurring at the interface. Logical formprecedes, and interacts with, phonetic form, theultimate configuration of the latter being achievedat spell-out at which point the two componentsseparate, and achieves its final output structure.Phonetic form bears a relationship to the phono-logical loop component of short-term memory. Inthe current theory, it is located in the dominanthemisphere and is assumed to have a purelysequential (unitary and temporally organised)form. The key functional characteristic is notphonological or phonetic but its sequence, i.e. itslinear form. According to de Saussure (1916):a primary characteristic of the spoken sequence is itslinearity...In itself it is merely a line, a continuous ribbonof sound.Logical form, however, would be expected to bespatially distributed, to be located, at least in part,in the non-dominant hemisphere, and to interactwith the domina nt hemispheric sequence ( phoneticform ) through commissural connexions. Thenotion that logical form has a neural representa-tion that is in some way different from that ofphonetic form, that it is located principally in thenon-dominant hemisphere and has a distributionthat is partly spatial (allowing an element of paral-lel processing) has implications for the neural basisof these distinctive components of the languageprocess. Paivio s (1991) dual coding theory pos-tulated that cognitions exist in two interconnectedforms--verbal ( logogens ) and non-verbal ( ima-gens ). In terms of the current theory, these areconsidered as the dominant and non-dominanthemispheric representat ions, respectively. In


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136 T.J. C row / Schi zophrenia Research 28 1997) 127-14 1Table 2T h e m e c h a n i s m o f c e r eb r a l h e m i s p h e ri c d o m i n a n c e

D o m i n a n t N o n - d o m i n a n t

de Saussure (1916)Paiv io ( 1991 )Cho msk y s min imal i s t theory (Chom sky , 1995)Work ing memory (Ga therco le and Badde ley , 1993)

SignitierL o g o g e n sP h o n o l o g i c a l f o r m ( P F )Phon olog ica l loo p (and execu t ive )1D , l i n ea r ( t e m p o r a l )

SignifiedI m a g e n sL o g i c a l f o r m ( L F )Visuo-spa t ia l ske tchpad2-D ( spa t ia l )

S a u s s u r e s t e r m i n o l o g y , i t is t h e n o n - v e r b a l i m a g et ha t r ep res en t s t he en t i t y t ha t is s i gn i f i ed ( s eeTab l e 2 ) .Eac h o f thes e concep t s i s cons i s t en t w i t h t he v i ewt h a t t h e m e c h a n i s m o f la n g u a g e i n c l u d es t w o c o m -p o n e n t s , o n e o f w h i c h is m o r e t e m p o r a l a n d t h eo t h e r m o r e s p a t i a l i n c h a r a c t e r. W h a t is a d d e dhere i s t he hypo t hes i s t ha t t hes e com ponen t s a ret he com pl em en t a ry r ep res en t a t i ons o f t he l i ngu i s t i cs i gn i n t he t wo hem i s pheres , t he d i f fe rences a r i s i ngf rom t he i n t r in s i c and ge ne t i ca l ly de t e rm i ned ana -t o m i c a l d e v i a ti o n s in g r o w t h o f t h e t w o h e m i s p h e re si n m a n , a n d t h a t t h e c r i t i c a l f u n c t i o n a l c o n s t r a i n ta r i ses f rom t he phys i o l og i ca l neces s i ty fo r t he l i nea ro r o u t p u t s e q u e n ce t o b e c o n f i n e d to o n e h e m i -s phere . G ene ra t i v i t y i s a s econ dary cons eq uenceo f acces s t o t he s pa t i a l l y d i s t r i bu t ed i n fo rm at i one n c o d e d i n t h e n o n - d o m i n a n t h e m i s p h e r e .

C r u c i a l i n f o r m a t i o n o n t h e g e n e t i c m e c h a n i s mis p ro v i d e d b y t h e p h e n o m e n a o f th e s e x c h r o m o -s o m e a n e u p l o i d i e s ( s e e S e c t i o n 7 a b o v e ) . L a c k o fa n X c h r o m o s o m e ( a s i n T u r n e r s s y n d r o m e ) i sa s s oc i a t ed wi t h de f i c i t s i n s pa t i a l p roces s i ng , anda n e x t r a X ( a s i n X X Y a n d X X X s y n d r o m e s ) i sa s s o c i a t e d w i t h d e f i c it s o f t e m p o r a l s e q u e n c i n g( M o n e y , 1 9 9 3) . T h e s e s y n d r o m e s t h u s d e fi n e t h eb o u n d a r i e s o f l a n g u a g e f u n c t i o n a n d i d e n t i f y i tsc r i t i c a l f u n c t i o n a l c o m p o n e n t s - - a t e m p o r a ls e q u e n c e i n t h e d o m i n a n t h e m i s p h e r e i n t o w h i c hi s i n t e g r a t e d t h e s p a t i a l l y d i s t r i b u t e d i n f o r m a t i o nf r o m t h e n o n - d o m i n a n t h e m i s p h e r e .

I O N u c l e a r s y m p t o m s a s a n o m a l i e s o f h e m i s p h e r i cs p e c i a l i s a t i o n

T h e c o n c e p t o f p s y c h o s i s as a f a i l u r e o f h e m i -s pher i c d i f f e ren t i a t i on has p receden t s [ fo r a r ev i ewo f c o n c e p t s i n t h e 1 9 th c e n t u r y , s ee H a r r i n g t o n

( 1 9 87 ) ]; i n A N e w V i e w o f I n s a n i t y : T h e D u a l i t yo f M i n d , A . L . W i g a n ( 1 8 44 ) p u t f o r w a r d t h e v ie wt h a t a s e p a r a t e a n d d i s t in c t p ro c e s s o f t h i n k i n g . . .m a y b e c a r r i e d o u t i n e a c h c e r e b r u m s i m u l t a -n e o u s l y a n d t h a t e a c h c e re b r u m i s c a p a b l e o f ad i s t i n c t a n d s e p a r a t e v o l i t i o n , a n d t h a t t h e s e a r ev e r y o f t e n o p p o s i n g v o l i t io n s . H e c o n s i d e r e d t h a tt h e i n t e r a c t i o n o f t h e t w o f u n c t i o n a l l y s e p a r a t eh e m i s p h e re s w a s a t t h e r o o t o f t h e s y m p t o m s o fi n s a n i ty . C r i c h t o n - B r o w n e (1 9 0 7 ) a n d S o u t h a r d( 1 9 1 0 ) , e a c h i n f l u e n c e d b y e v o l u t i o n a r y c o n s i d e r a -t i o n s , e n t e r t a i n e d t h e c o n c e p t o f s e r io u s m e n t a li ll n e ss a s a d i s o r d e r o f t h e d o m i n a n t o r l e f t h e m i -s p h e re . F l o r - H e n r y ( 1 9 6 9 ) o n t h e b a s i s o f h iso b s e r v a t i o n s o n t h e p s y c h o s e s a s s o c i a t e d w i t h e p i-l e p s y a l s o s u p p o r t e d t h is v ie w . H o w e v e r , t h e f i n d -i n g s f r o m a n a t o m i c a l ( C r o w e t a l . , 1 9 8 9 ; C r o w ,1990a, 1993, 19 97; De Lis i e t a l . , 1997) and fu nc-t i o n a l s t u d ie s ( G u r , 1 9 7 7 ; G r e e n e t a l. , 1 9 8 9 ; C r o wet a l . , 1996 ) a re cons i s t en t w i t h t he hyp o t he s i s t ha ts c h i z o p h r e n i a i s n o t a d i s o r d e r o f o n e o r t h e o t h e rh e m i s p h e r e , b u t o f t h e i n t e r a c t i o n b e tw e e n t h e m ,an d s pec i f ica l l y t ha t t he re i s a f a i l u re t o e s t ab l i s hu n e q u i v o c a l d o m i n a n c e . J a y n e s ( 1 9 9 0 ) i n h i s b o o kT h e O r i g in s o f C o n s c i o u s n es s i n t he B r e a k d o w n

o f th e B i c a m e r a l M i n d r e la t e s, a l b e it w i t h in a ni m p l a u s i b l e e v o l u t i o n a r y t h e o r y , h o w s c hi z o -p h r e n i a m i g h t r e p r e s e n t a r e g r e s s i o n t o a n e a r l i e rs t a te o f c o n s c i o u s n e s s ( t h e b i c a m e r a l m i n d ) i nw h i c h t h e t w o h e m i s p h e r e s w e r e l es s d if f e r e n t i a te da n d t h e i n t e r a c t i o n b e t w e e n t h e m w a s e x p e r i e n c e das vo i ces .N a s r a l l a h ( 1 9 8 5 ) p r o p o s e d t h a t a n o r m a l c o m -p o n e n t o f i n t e r h e m i s p h e r i c i n t e g r a t i o n i s:inh ib i t ion o f any awareness by the ve rba l ly express ive hemi-spheric consciousness (usually th e lef t) that i t actual ly receivesand sends though ts , in ten t ions and fee lings f rom and to ano the r( the r ight) consciousness.


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T.J. Crow / Schizophrenia Research 28 1997) 127-141 137I n s c h i z o p h r e n i a , t h i s f u n c t i o n i s d i s t u r b e d w i t ht h e r e s u l t t h a t t h e l e f t h e m i s p h e r i c c o n s c i o u s n e s sb e c o m e s a w a r e o f a n i n f lu e n c e f r o m a n e x t e r n a lf o r c e , w h ic h , i n f a c t , i s t h e r i g h t h e misp h e r e ( s e eT a b l e 1 ). A c c o r d i n g t o N a s r a l l a h , S c h n e i d e r ia nd e l u s i o n s , s u c h a s t h o u g h t i n s e r t i o n a n d w i t h -d r a w a l , a n d d e l u s io n s o f c o n t r o l m i g h t a r is e int h is w a y , a l t h o u g h t h e c o n c e p t o f a l os s o f n o r m a li n h i b i ti o n p r o v i d e s n o m o r e s p ec if ic a n e x p l a n a t i o nt h a n t h a t t h e r e i s a f a i lu r e o f i n t e r - h e m i s p h e r i ci n t e g r a t i o n .

I n t h e c o n t e x t o f th e p r e s e n t h y p o t h e s i s , f i r s tr a n k s y m p t o m s t a k e o n a n e w s i g n i f i c a n c e . T h e ya r e c l u es t o t h e c e r e b r a l o r g a n i s a t i o n o f l a n g u a g e ,t h e p r i m a r y f u n c t i o n o f w h i c h i s t o c o m m u n i c a t ew i t h a n o t h e r p e r s o n . A s a r g u e d a b o v e , t hi s p ro c e s sr e q u i re s a c o m p l e m e n t a r i t y o f f u n c t i o n b e t w e e nt h e h e m i s p h e r e s , w i t h o n e c o m p o n e n t t h e p h o n o -l o g i c a l l o o p , r e p r e s e n t i n g a l i n e a r a n d u n i n -t e r r u p t e d s e q u e n c e , n e c e s s a r i l y l o c a l i s e d i n t h ed o m i n a n t h e m i s p h e r e . F r o m t h e f a c t t h a t t h ef l e x ib i l it y ( o r g e n e r a t i v i t y ) o f l a n g u a g e m u s t b ec o n t r i b u t e d f r o m t h e o t h e r h e m i s p h e r e , a n d t h a t ,i n s o m e s e n s e , t h i s c o n t r i b u t i o n e x i s t s i n a s p a t i a lo r d i s t r i b u t e d f o r m , i t f o l l o w s t h a t a b n o r m a l i t i e so f i n t e r - h e m i s p h e r i c c o n n e c t i v i t y w il l b e a s s o c i a t e dw i t h d e v i a t io n s i n s e n t en c e p r o d u c t i o n a n d t h et r a i n o f t h o u g h t , a l t h o u g h t h e f o r m o f t h es e a n o -m a l i e s c a n n o t b e p r e d i c t e d w i t h o u t a m o r e s p e c i f i ct h e o r y o f t h e n a t u r e o f t h e i n t e ra c t i o n . T h e t r u es i g n if i ca n c e o f t h e f i r st r a n k s y m p t o m s o f s c h iz o -p h r e n i a i s t o c h a r t o u t t h e b o u n d a r y c o n d i t i o n s o fl a n g u a g e , t o d e p i c t l a n g u a g e a t t h e e n d o f i tst e t h e r .

O f p o s s ib l e r el e v a n c e a r e th e p h e n o m e n a o fi n d e x ic a l it y ( L y o n s , 1 9 9 5) . A f e a t u re o f h u m a nl a n g u a g e i s t h a t i t i s a t w o - w a y s y s t e m - - s o u n d sa r e d e c o d e d a n d g e n e r a t e m e a n i n g , a n d m e a n i n g sa r e e n c o d e d i n t o s o u n d - - t h e s o - ca l le d b i -d i re c -t i o n a l i t y o f t h e S a u s s u r e a n s i g n ( H u r f o r d , 1 9 9 2 ) .T h e g e n e r a l p r in c i p le o f li n g u is t ic c o m m u n i c a t i o ni s t h a t s y m b o l s a r e h e l d i n c o m m o n b y s p e a k e r so f a g i v en l a n g u a g e, a n d b y m e a n s o f t h eb i - d i re c t i o n al m e c h a n i s m c a n b e u s e d a s e x c h a n g e -a b l e t o k en s . H o w e v e r , a s H u r f o r d p o i n t s o u t , t h e rei s a c l a s s o f w o r d s , t h e d e i c t i c ( o r i n d e x i c a l )p r o n o u n s T a n d y o u , f o r w h i c h t h is is n o t t r u e.

T h e r e f e r e n t i s n o t f i x e d , a n d i n t h e c o u r s e o f at w o - w a y c o n v e r s a t i o n , t h e m e a n i n g t o b e a t t a c h e dt o t h e se s y m b o l s m u s t b e s w i t c h e d b a c k a n d f o r t h ,a c c o r d i n g t o w h o e v e r i s t h e s p e a k e r . I t is a n a s p e c to f th i s p r o c e s s t h a t h a s b e c o m e d e v i a n t i n a s s o c i a-t i o n w it h t h e fi rs t r a n k s y m p t o m s - - m e a n i n g s a n di n t e n t i o n s t h a t a r e i n t e r n a l l y g e n e r a t e d a r e a t t r i b -u t e d t o a n o t h e r p e r s o n o r o u t s i d e a g e n c y ( s ee al s oL a k o f f , 1 9 9 6 ) . I t i s a l s o r e l e v a n t t h a t s o m e c h i l -d r e n , p e r h a p s t h o s e w h o a r e a t r is k o f s e m a n -t i c - p r a g m a t i c d i s o r d e r , i . e . w h o a r e w i t h i n t h es p e c t r u m o f a u t i s m / A s p e r g e r s s y n d r o m e , h a v ed i f fi c u l ty i n e s t a b l i s h i n g t h e u s e o f t h e s e p r o n o u n s .D y s f u n c t i o n c a n o c c u r e a r ly , b u t i t ca n a ls o o c c u rl a t e w h e n t h e l a n g u a g e m e c h a n i s m i s r e a c h i n g i t sp l a t e a u o f d e v e l o p m e n t , a n d w h e n i t d o e s s o , i tu n c o v e r s s o m e t h i n g a b o u t t h e r o l e o f th e t w oh e m i s p h e r e s i n t h e b i - d i r e c t i o n a l i t y o f t h eS a u s s u r e a n s i g n .

1 1 T h e n a t u r e o f t h e b r a in c h a n g e sI f p s y c h o t i c i l ln e s s is a p a r t o f t h e g e n e t i c

v a r i a t i o n t h a t i s c o m m o n t o H o m o s a p i e n s , t h i sf a c t h a s i m p l i c a t i o n s f o r u n d e r s t a n d i n g t h e n a t u r eo f th e b r a i n c h a n g e s. I t c a n n o t b e e x p e c t e d th a tt h e r e w i l l b e a p a th o lo g i c a l p r o c e s s t h a t i s sp e c i f i ct o t h is c o n d i t i o n b u t r a t h e r t h a t t h e b r a i n c h a n g e sw i ll r e p r e s e n t a n e x t r e m e o f v a r i a t i o n t h a t i sp r e s e n t i n t h e p o p u l a t i o n a s a w h o l e . T h r e e g r o s sm o r p h o l o g i c a l c h a n g e s i n s c h i z o p h r e n i a a r e n o wr e la t iv e l y w e ll e s t a b l i s h e d - - a d e g r e e o f v e n t r i c u l a re n l a r g e m e n t , a s m a l l r e d u c t i o n i n c o r t i c a l m a s s ,a n d l o s s o f t h e a s y m m e t r i e s t h a t a r e c h a r a c t e r i s t i co f t h e h u m a n b r a i n ( C r o w , 1 9 9 0 b , 1 9 9 7 ). T h e s ec h a n g e s r e p r e s e n t a s h i ft f r o m t h e m e a n o f t h eg e n e r a l p o p u l a t i o n , w h i c h , i n t h e c a se o f t h e w e ll -s t u d ie d v e n t r i c u l a r e n la r g e m e n t , i s k n o w n t o o c c u rw i t h o u t a n i n c r e a s e i n v a r i a n c e . T h e t h r e e c h a n g e sm u s t b e r e l a t e d . A s i m p l e i n t e r p r e t a t i o n i s t h a ts y m m e t r y o f d e v e l o p m e n t o f th e c e r e b r a l c o r t e xi m p l i e s a s m a l l e r , a n d p e r h a p s l e s s c o n v o l u t e dc o r t e x , a n d t h a t t h i s i n t u r n m e a n s t h a t t h e v e n t r i -c l es ( w h i c h d e c r e a s e i n s i ze a s t h e c o r t e x d e v e l o p s )wi l l be la rge r .

T o w h i c h a n a t o m i c a l p a t h w a y s d o t h e s e v a r i a-


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T.J . Crow / Schi zophrenia Research 28 1997) 1 27-141 139f o r c e o f s e x u a l s e l e c t i o n , i .e . d i f fe r e n c e s i n t h ec r i t e r i a f o r m a t e c h o i c e i n t h e t w o s e x e s . T h i sp r e d i c t i o n c a n b e t e s t e d in i n v e s t i g a t i o n s o fl i nk a g e f o r a s y m m e t r y a n d p s y c h o s i s o n t h eX c h r o m o s o m e .( 6 ) A s a r e s u l t o f t h i s g e n e t i c c h a n g e , s o m e n e u r a lp r o c e s s , o n w h i c h t h e e v o l u t i o n o f l a n g u a g ew a s d e p e n d e n t , b e c a m e c o n f in e d t o o n e h e m i -s p h e r e . T h i s c o m p o n e n t , i t i s s u g g e s t e d , is th el in e a r o u t p u t ( p h o n o l o g i c a l ) s e q ue n c e .B e c a u s e i t i s a t e m p o r a l s e q u e n c e , i t is o n e -d i m e n s i o n a l , b u t e ac h c o m p o n e n t h a s a s s o c i a -t i o n s ( t h r o u g h t h e c e r e b r a l c o m m i s s u r e s ) i nt h e n o n - d o m i n e n t h e m i s p h e r e t h a t a r e n o t s oc o n s t r a i n e d , b u t a r e tw o - d i m e n s i o n a l a n ds p a t i a l . T h i s b i - h e m i s p h e r i c t h e o r y o f l a n -g u a g e c a n a c c o u n t f o r t h e c o n t r a s t i n g s y n t a g -m a t i c a n d p a r a d i g m a t i c a s p e c ts o f l a n g u a g et o w h i c h d e S a u s s u r e d r e w a t t e n t i o n , i . e. toi t s g e n e r a t i v i t y .

( 7 ) A s e v i d e n c e d b y a n a t o m i c a l ( i. e . r a d i o l o g i c a la n d p o s t - m o r t e m ) a n d f u n c t i o n a l ( e. g. h a n d e d -n e s s ) s tu d i e s , s c h i z o p h r e n i c i l l n e s s e s a r e a s s o c i -a t e d w i t h a f a i l u r e o f la t e r a l i s a t i o n ; t he s ei l ln e s s e s , i t s e e m s , r e p r e s e n t o n e e x t r e m e o ft h e v a r i a t i o n b y w h i c h t h e c r i t i c a l c o m p o n e n t so f la n g u a g e a r e a l l o c a t e d t o t h e t w o h e m i -s p h er e s. N u c l e a r s y m p t o m s t el l u s s o m e t h i n ga b o u t t h e n a tu r e o f t h e l a n g u a g e m e c h a n i s m ,t h a t i n a d d i t i o n t o t h e r e s t r i c t i o n o f t h e l in e a ro u t p u t s e q u e nc e t o th e d o m i n a n t h e m i s p h e r e ,s o m e c o m p o n e n t o f t h e p ro c e s s o f h e m i s p h e r i cs p e c i a l i s a t i o n r e l a te s t o i n d e x i c a l i s a t i o n , t h ed i s t i n c t i o n b e t w e e n s e lf - v e r s u s o t h e r - g e n e r -a t e d r e f e r e n c es . T h u s , t h e c o n s e q u e n c e o f t h ev i ew t h a t p s y c h o s i s a n d l a n g u a g e h a v ec o m m o n e v o l u t i o n a r y o r i g i n s is t h a t i t i s o n l yt h r o u g h t h e p h e n o m e n a o f p s y c h o s is th a t i tw i ll be p o s s i b le t o u n d e r s t a n d t h e m e c h a n i s mo f l a n g u a g e .

cknowledgmentI t h a n k A n n a S a l t m a r s h f o r s u g g e s t i n g t h e

e s s e n c e o f t h e t it le , a n d t h r e e a n o n y m o u s r e f e re e sa n d t h e e d i t o r f o r h e l p f u l s u g g e s t i o n s i n a c h i e v i n gt h i s f o r m u l a t i o n .

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