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i n t r ac t ab l e ques t i on s o f m ode rn sc i ence : w ha t i slearning?

Selected re ferences1 B l iss , T. V . P , and Co l l ingr id ge, G. L. (1993 ) N a t u r e 3 6 1 ,3 1 - 3 92 Benzer, S . (196 7) P r o c . N a t l A c a d . S c i . U S A 5 8 , 1 1 1 2 - 1 1 1 93 Tu l l y , T . (1991) i n N e u r o b i o l o g y o f L e a r n i n g , E m o t i o n a n dA f f e c t (Madden, J . , ed . ) , pp . 29-66 , Raven P ress4 S i lva, A . J ., S tevens, C. F., To ne ga wa , S. and W an g, Y . (1992 )Sc ie n ce 2 5 7 , 2 0 1 - 2 0 65 S i lva, A . J . , Pay lor , R. , Wehner, J . M. and Tonegawa, S.( 1 9 9 2 ) Sc ie n ce 2 5 7 , 2 0 6 - 2 1 16 Gran t, S. G. N. e t a l . ( 1 9 9 2 ) Sc ie n ce 2 5 8 , 1 9 0 3 - 1 9 1 07 Capecch i , M. (1989 ) N a t u r e 2 4 4 , 1 2 8 8 - 1 2 9 28 B rad ley , A . (1993) R e c e n t P ro g . H o r m . R e s . 4 8 , 2 3 7 - 2 5 19 Ma l inow , R ., S chu lman, H . and Ts ien , R . W. (1989) Sc ie n ce2 4 5 , 8 6 2 - 8 6 610 Ma lenk a, P ,. C. e t a l . ( 1 9 8 9 ) N a t u r e 3 4 0 , 5 5 4 - 5 5 711 O 'D ell , T. J., Kan del, E. P,. an d Gran t, S. G. N. (19 91 ) N a t u r e3 5 3 , 5 5 8 - 5 6 O12 Greengard, P . , Val tor ta, F. , Czernick, A . J . and Benfenat i , F.( 1 9 9 3 ) Sc ie n ce 2 5 9 , 7 8 0 - 7 8 51 3 Z h o n g , Y . a n d W u , C - F . ( 1 9 9 1 ) Sc ie n ce 2 5 1 , 1 9 8 - 2 0 11 4 C o n s t a n t i n e - P a t o n , M . ( 1 9 9 0 ) C o l d S p r i n g H a r b o rS y m p o s i u m 5 0 , 4 3 1 ~ , 4 315 Orban, P . C . , Chub, D . and Mar th , J . D . (1992) Pro c . N a t lA c a d . S c i . U S A 8 9 , 6 8 6 1 - 6 8 6 516 Gr i f fi ths , L. C. , Versel is , L. M , , D anh o, W. and Green span,R. J. (1993) N e u r o n 1 0 , 5 0 1 - 5 0 917 P et ti t, D . L . and M al ino w, R . (1992) So c . N e u ro sc i . A b s t r . 3 2 318 S ommer , B . e t a l . ( 1 9 9 0 ) Sc ie n ce 2 4 9 , 1 5 8 0 - 1 5 8 419 S taub li , U ., Th ibau l t , O . , D iLorenzo, M. a nd Lynch, G . (1989)

Behavio ra l NeuroscL 1 0 3 , 5 4 - 6 020 Davis , S ., Butcher, S . P . an d Mo rr is , R. G. M . (1 99 2)J. Neurosc i . 1 2 , 2 1 - 3 421 B ohme, G . A . , B on, C . , S tu t zmann, J -M. , DoNe, A . andB lanchard , J -C . (199 1) Eur . J . Pharmaco l . 1 9 9 , 3 7 9 - 3 8 122 O'D el l , T. J. , Hawkins, R. D. , Kand el , E . R. and A ranc io, O.(1991 ) Pro c . N a t l Aca d . 5 c i . U SA 8 8 , 1 1 2 8 5 - 1 1 2 8 923 S chuman, E . M. and Mad ison, D . V . (1991) Sc ie n ce 2 5 4 ,1 5 0 3 - 1 5 0 624 Haley, J . E . , W i lcox, G . L. and C hap ma n, P. F. (19 92) N e u r o n8 , 2 1 1 - 2 1 6

25 Chapman, P. F. , A tk ins, C. M. , A l len, M. T. , Haley, J . E . andSteinmetz, J . E . (199 2) N e u r o R e p o r t 3 , 5 6 7 - 5 7 026 Dudek , S . M. and B e ar , M. F . (1992) Pro c . N a t l Aca d . Sc i .U S A 8 9 , 4 3 6 3 - 4 3 6 727 Mulkey , P , . M. and Malenka, P , . C . (1992) N e u r o n 9 ,9 6 7 - 9 7 528 O t to , T . , E ichenbaum, H . , Wiene r , S . I . and Wib le , G . W.( 1 9 9 1 ) H i p p o c a m p u s 1 , 1 8 1 - 1 9 229 Buzsaki , G. , Horv ath, Z. , Ur ioste, R. , He tke, J . and W ise, K .( 1 9 9 2 ) Sc ie n ce 2 5 6 , 1 0 2 5 - 1 0 2 730 Morr is , R. G. M. , G arrud, P . , Rawl ins, J . N. P . and O 'Kee fe, J.( 1 9 8 2 ) N a t u r e 2 9 7 , 6 8 1 - 6 8 331 Suther land, R. J . , Kolb, B . and Whishaw, I . Q. (1982)Neurosc i . Le t t . 3 1 , 2 7 1 - 2 7 632 Tu l l y , T . and Go ld , D . J . N e u ro g e n e t i cs ( in press)33 Bar ley, C. H. and Kande l , E . R. ( 1 9 9 3 ) A n n u . R e v . P h y s io l . 55,3 9 7 - 4 2 634 Gr ov e l L . M. a nd Teyle r, T . J . (1990 ) N a t u r e 3 4 7 , 4 7 7 - 4 7 935 Zalutsky, R. A . an d Nicol l , R. A . (199 0) Sc ie n ce 2 4 8 ,1619- -162436 Jaf fe, D. and Johnston, D. L. (1990) J . N e u ro p h ys io l . 6 4 ,9 4 8 - 9 6 037 Larson, J . , Wong, D . and Lynch, G . (1986) Bra in R e s . 3 6 8 ,3 4 7 - 3 5 0

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R e cen t bra i n- i m agi ng an d neurophys io l og ica l da t aindicate tha t at tent ion i s nei ther a properly o f a s inglebra i n area , nor o f t he en ti re bra i n . W h i l e a t ten t i ona lef fects seem me diate d by a relative am pl i f icat ion of bloodf low an d electr ical act iv i ty in the cort ical areas processingt he a t t ended comp ut a t i on , t he de t a il s o f how t h i s i s donet hrough e nha nce me nt o f a t t ended or suppress ion o funa t t ended i t ems , o r bo th , appear t o depen d on t he t askan d bra i n-area s t ud i ed . T he or ig i ns o f t hese ampl if wati onef fects are to be fo un d in special ized cort ical areas of thef ron t a l an d par i e t a l l obes t ha t have been descr ibed as t hean t er i or and pos t er i or a t t en t i on sys t ems . T hese resu l t srepresent substant ial progress in the ef for t to determinehow brain act iv ity i s regu lated through at tent ion. W hi lema ny ph i losophica l an d prac t i ca l i s sues remai n i ndeve lop i ng an und ers t an d i ng o f a t t en t i ona l r egu l a ti on ,the new tools avai lable s ho uld provide the basis fo rprogress .T h e s t u d y o f s e l ec t i v e a t t e n t io n h a s b e e n a n i m p o r t a n ta r e a o f r e s e a r c h s i n ce t h e i n c ep t io n o f p s y c h o l o g y i nthe l a t e 1800s . Ho we ve r , i t ha s r ema ined con t ro -v e r s ia l w h e t h e r t h e r e a r e a n y s e p a r a t e b r a i n m e c h a n -i s m s t h a t s u b s e r v e a t t e n ti o n . A t t e n t io n d o e s n o t g iv er ise to a unique qual i ta t ive exper ience l ike vision ortouch , no r does i t au toma t i ca l ly p rodu ce m oto rre sponses . Whi l e we appea r t o be ab l e t o se l ec tsen sor y s timuli , i n fo rm a t ion i n m em ory o r moto rre spo nses , t h i s m igh t no t ind i ca t e a sepa ra t e a t t en t i onsys t em, s i nce a l l b ra in sys t ems p l ay a ro l e i nse lec t ion.

T h e r e h a s b e e n e v i d e n c e o f m o r e g e n e r a l m e c h a n -i sms re l a t ed t o v i sua l se l ec t i on . One way to se l ec ti n fo rma t ion i s t o o r i en t t o it . Ey e m ove me nt s t ofove a t e a v i sua l s timulus i s a c lea r ca se , s i nce wi thou tfovea t i on t he re i s l i t tl e ab i li t y t o exam ine de t a i l s o f t hev i s u a l s c e n e . T h e r e a r e a l s o b r a i n m e c h a n i s m s f o rv i sua l o r i en t i ng t ha t do no t invo lve any ove r t chang esin head o r eye . In t he 1970s , i t was found tha t c e ll s inthe pa r i e t a l l obe i nc rea sed t he i r f i ri ng r a t e i n r e spon seto s t imula t i on o f t he i r r ecep t ive f i e ld when monkeysa t t e n d e d t o p e r i p h e r a l s t i m u l i e v e n w h e n n o e y em o v e m e n t s w e r e a l lo w e d ~ . I t w a s a l s o s h o w n t h a thumans cou ld cove r t l y sh i f t a t t en t i on t o pe r iphe ra ls t i m u l i a n d w h e n t h e y d i d s o t h e y r e s p o n d e d m o r erap id ly , a t l ower t h re sho ld and wi th enhanced e l ec -t r i c a l a c t i v i t y t o s t imul i a t t he a t t ended l oca t i on .Les ion s of the p ar ie ta l lobe speci fical ly dam age d thiscove r t o r i en t i ng ab i l i t y on t he s i de o f space oppos i t ethe l e s ion . These f i nd ings suppor t ed t he i dea t ha tpor t i ons o f t he pa r i e t a l l obe were i nvo lved i n cove r tor ient ing to visual s t imul i2.S ince 1987 , wh en w e l a s t r ev i ew ed th i s t op ic 2 , ith a s b e c o m e p o s s ib l e t o u s e n e u r o i m a g i n g m e t h o d s t oo b s e r v e t h e n e t w o r k s o f br a in a r e a s t h a t b e c o m eac t ive when peop le pe r fo rm c omp lex t a sks 3 '4 , Ce r t a inb r a i n a r e a s a p p e a r a c t i v e w h e n s u b j e c t s h a v e t oo r i en t , se l ec t o r t r ans fo rm in fo rma t ion i n way s t ha twou ld be sa id , by mo s t psycho log i ca l mode l s , t oinvo lve a t t en t i on . T he se b ra in a rea s app ea r t o f i t qu i t ewel l wi th the idea of se lec t ion and cont rol as prop er t ieso f a t t en t i on ou t l i ned i n ou r p rev ious a r t i c l e . In t h i s

M ich a d L Po sn e r s a tt he D e N o fPsychology,Un ive rs i ty o f Oregon ,Eugene , OR 9 7403 ,USA, and StanislasDehaene s a tthe Laboratoirede SciencesCo&nit ives andPsycholinguist ique,CNRS, EHESS ndINSERM, P aris,France.

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Fig. 1. Visua l search fo r a ver t ica l rec tang le am ong a la rge nu mb er o f d is t rac tors (ho r izonta l andver tica l e ll ipses and rec tang les) . The ta rge t is de f ined by a con junction o f fo rm a nd or ien ta tion . Inmo st cases, se lec tive a t ten t ion is used to sequent ia l ly search the d isp lay un t i l the ta rg e t is foun d 5 .a r t ic l e , we u se t he neuro im ag ing da t a t o ou t li ne wh a ti s k n o w n a b o u t n e t w o r k s o f b r a in a r e a s i n v o lv e d ins e l e c t i o n d u r i n g v i s u a l s e a r c h a n d t h e n e x t e n d t h ed i s c u s s i o n t o m e c h a n i s m s o f s e l e c t i o n a n d c o n t r o lm o r e g e n e r a ll y .V i s u a l s e a r c h

T h r o u g h o u t t h i s a rt ic l e, w e u s e t h e e x a m p l e o f as imple v i sua l - sea rch t a sk i n o rde r t o i l lu s t r a t e t he ro l eo f a t t en t i on i n f ind ing ob j ec t s 5 . Da ta i nd i ca te t h a t t het ime fo r a sub j ec t t o l oca t e a t a rg e t ( fo r example , t heve r t i c a l r ec t ang l e i n F ig . 1 ) i nc rea se s a lmos t l i nea r lyw i t h t h e n u m b e r o f d is t r a c to r s , a s t h o u g h t h e s u b j e c ta t t e n d e d t o e a c h i t e m i n tu r n . T h e r e a c t io n t i m e w h e nt h e r e i s n o t a r g e t h a s t w i c e t h e s l o p e f o u n d fo r ' y e s 'r e s p o n s e s , i n d ic a ti n g t h a t o n e s t o p s a s s o o n a s t h et a rge t i s found5.A s impli fi ed v i ew o f t he p rob l em faced by t he ~ra inin t h i s t a sk i s p re se n t ed i n F ig . 2 . T he v i sua l a r r ay i sp r o c e s s e d t h r o u g h a d i v e r g i n g t r e e o f c o rt ic a l a r e a s 6 .

Cor t i ca l c e l l s e a r ly i n t he s t r eam have na r rowr e c e p t i v e f i e ld s a n d r e s p o n d m e c h a n i ca l ly w h e n e v e rt h e i r p r e f e r r e d s t im u l i a r e p r e s e n t , w h e t h e r t h e yc o r r e s p o n d t o a t a r g e t o r n o t . A s o n e m o v e s u p i n t h eh i e ra rchy , howeve r , r ecep t ive f i e lds widen and t hece l l s become more sens i t i ve t o a t t en t i ona l and i n t en -t i onal in f l uences , and rea c t l e ss e f fec t i ve ly t o t hepass ive p re sen t a t i on o f a s t imulus .Th e s imple i ll u s t r a ti on i n F ig . 2 ma kes i t c lea r t ha tse l ec t i ve a t t en t i on se rves a t l e a s t two d i s t i nc t i vef u n c ti o n s . F i r s t, t h e b r a i n m u s t e n h a n c e t h e p r o c e s s -ing o f t he se l ec t ed s t imulus r e l a t i ve t o o the r s t imul ip re sen t , o the r wi s e a ll s t imul i wou ld be p roces sed t o as imi la r deg ree (F ig . 2 , t op ) . Second , p roces s ing o f t hes e l e c t e d s t i m u l u s m u s t b e a c t iv e l y o r ie n t e d o r g u i d e dtowards t he co r t i c a l a rea s appropr i a t e fo r a g ivent a sk . T he d ive rg ing t r ee o f co rt i c a l a rea s , e ach o fwhich spec i a l i z e s fo r a d i s t i nc t a spec t o f t he s timuli ,m u s t b e s e l e c t iv e l y 'p r u n e d ' t o f o c u s o n ly o n t h o s e

s t imulus cha rac t e r i s t i c s t ha t a rere l evan t t o t he t a sk . Th i s ' p run ing 'i s dep i c t ed on t he bo t to m of F ig . 2in a s t a t i c manne r , bu t i n complext a sks such a s se r i a l sea rch , t hea p p r o p r i a t e p r o c e s s i n g s t r e a mmigh t change a s a func t i on o f t ime( fo r example , t he sub j ec t m igh ta t t e n d t o e a c h s t i m u l u s i n t u r n o rto a l l ve r t i c a l i t ems f i r s t and t h en t oa l l hor izonta l i tems) .Recen t advances i n func t i ona lb ra in image ry , such a s pos i t rone m i s s i o n t o m o g r a p h y ( P E T ) s c a n ,h a v e p r o v i d e d n e w e v i d e n c e c o n -ce rn ing t h e co r t ic a l and subcor t i c aln e t w o r k s r e s p o n s i b l e f o r t h e s etw o fun ct ions of se lec t ive a t tent ion.As i t t u rns ou t , t he two func t ionss e e m t o b e c a r r i e d o u t b y t w od i s t i nc t sys t emsT: t he pos t e r io ra t t e n t i o n s y s t e m ( s u p e r io r p a r i e ta lco r t ex , pu lv ina r and supe r io r co l -l iculus) i s la rgely respo nsible forse l ec t i ng one s t imulus l oca t i ona m o n g m a n y a n d f o r s h i ft in g f ro mo n e s t i m u l u s t o t h e n e x t , w h e r e a st h e a n t e r i o r a t t e n t i o n s y s t e m (an-t e r i o r c ingu la t e and basa l gang l i a ) se rves a moreexecu t ive func t i on ( a t t en t i on fo r ac t i on ) and i s i n -vo lved i n t he a t t en t i ona l r ec ru i tmen t and con t ro l o fb ra in a rea s t o pe r fo rm complex cogn i t i ve t a sks . Th i sne twork i s a l so i nvo lved i n se l ec t i ng v i sua l ob j ec t sw h e n t h e i n s t r u c t i o n s e m p h a s i z e p r o p e r t i e s o f t h eob jec t . Ul t ima te ly , howe ve r , t he e f fec t o f bo th

systems is re la t ive ampl i f ica t ion of ac t ivi ty wi thin thecor t i c a l a rea s r e l evan t t o a g iven t a sk .A t t e n t i o n a l a m p l i f i c a t i o nIn t he v i sua l d i sp l ay o f F ig . 1 , i f you a re a t t end ing t othe c o r rec t l oca t i on i t i s e a sy t o se e t h e t a rge t .O t h e r w i s e , t h e t a r g e t i s n o t s e e n . H o w d o e s t h e b r a ini m p l e m e n t s u c h t a r g e t d e t e c ti o n ? I t a p p e a r s t h a t b r ai nac t i v i t y , i n many co r t i c a l a rea s , c an be se l ec t i ve lyampl i fi ed o r su ppre ssed a s a func t i on o f a t t en t i ona ls e t . W h e n s u b j e c t s a r e i n s t r u c t e d t o p a y a t t e n t i o n toone pa r t i cu l a r d imens ion o f the s t imul i , t he b ra in a rea stha t spec i a l i z e i n t he p rocess ing o f t h i s s t imulusd i m e n s i o n ar e s e l e c t iv e l y e n h a n c e d . T h e s e f in d in g sa re s t r i k ing in t he p re s t f i a t e a rea s o f t he v i sua ls y s t e m . I n o n e e x p e r i m e n t , s u b j e c ts v i e w e d p a s s i v e lye i t h e r s t a t io n a r y o r m o v i n g o b j e c ts , p r e s e n t e d e v e r ysecon d fo r a minu te s . W hen b lood f l ow dur ing t hes t a t io n a r y c o n t r o l c o n d it io n w a s s u b t r a c t e d f r o m t h a tfound in t he m oving cond it i on , a p re s t r i a t e a rea o f themid t empora l l obe was found to have a s i gn i f i can tinc rea se i n b lood f l ow. Th i s migh t be t he humane q u i v a le n t o f a r e a V 5 o r M T w h i ch , i n th e m o n k e y ,con ta ins ce l l s wi th a s t rong se l ec t i v i t y fo r mov ings timuli . In a d i f f e ren t expe r im en t 9 , sub j ec t s a lwaysv i e w e d m o v i n g o b je c t s o f v a r y i n g co l o r a n d f o rm , b u ti n o n e s e t o f tr ia ls t h e y w e r e i n s t r u c t e d t o d e t e c td i f f e rences i n ve loc i t y , whe reas i n a con t ro l se t t heyh a d t o d e t e c t c h a n g e s i n o t h e r s t i m u l u s p a r a m e t e r s .E v e n t h o u g h t h e p h y s i c al s ti m u l i w e r e t h e s a m e in th etwo cond i t i ons , t he i ns t ruc t i on t o a t t end t o ve loc i t yac t i va t ed a b ra in a rea t ha t was s imi la r t o t h a t found in

7 6 TINS, VoL 17, No. 2, 1994


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t h e c o m p a r i s o n o f m o v i n g a n d s t a t i o n a r y t a r g e t s . I no the r words , a t t en t i on t o ve loc i t y ac t i va t e s a b ra ina rea t ha t i s s im i l a r i n l oca t i on t o wha t i s found whenmot ion i s phys i ca l ly added t o t h e s t imulus .T h e s a m e g e n e r a l c o r r e s p o n d e n c e a l s o o c c u r sb e t w e e n b r a in a r e a s a c t i v a te d b y p r e s e n t i n g c o l o r a n df o r m s t i m u l i a n d t h o s e a c t i v a t e d w h e n t h e p e r s o n i si ns t ruc t e d t o se l ec t i ve ly p rocess co lo r o r fo rm in fo r -ma t ion9 . Inc rea se s i n b lood f l ow have a l so beenr e p o r t e d i n m o t o r a r e a s 1 w h e n s u b j e c t s a r e i n-s t r u c t e d t o a t t e n d t o m o t o r a c ti o n s . I n g e n e ra l , t h e r eappea r s t o be a r e l a t i ve i nc rea se i n b lood f l ow inn e a r l y a n y a r e a o f t h e b r a i n w h e n s u b j e c t s a r e i n -s t r u c t e d t o a t t e n d t o a s e n s o r y o r m o t o r c o m p u t a t i o nthough t t o be r e l a t ed t o t ha t b ra in a rea . A poss ib l eexcep t ion i s t he p r imary v i sua l co r t ex , wh ich does no tseem to be a f fec t ed by a t t en t i ona l man ipu l a t i ons 1'9.S o f a r w e h a v e l o o k e d a t t h e e f f e c t s o f a t t e n t io nw h e n p e o p l e a r e i n s t r u c t e d t o a t t e n d t o s e n s o r yd imens ions such a s co lo r , fo rm o r mot ion . Bu t i n t hes e a r c h t a s k a b o v e , n e i t h e r t h e o r i e n t a t io n n o r t h efo rm of t he t a rge t i s su f f i c i en t t o f i nd i t . On ly a

c o m b i n a ti o n o f th e t w o d i m e n s i o n s a t t h e s a m eloca ti on wi ll wo rk s Un de r t he s e cond i t ions , s ub j ec t so f t en sea rch se r i a l ly by sh i f t i ng t he i r a t t en t i o n f roml o ca t io n t o l o ca t io n . I f t h e r e a r e n o e y e m o v e m e n t sa ll o w e d, t h e s e s h i f ts m u s t b e m e d i a t e d c o v e r t ly , a n dthey shou ld p roduce a seq uenc e o f ampl if ic a ti on a teach success ive l oca t i on . Indeed , e l ec t r i c a l r eco rd -i n g s o f t h e s c a l p h a v e b e e n u s e d t o m e a s u r e s u c hlocatio n-sp ecific amplification 11,~2. Fir st , the t im e forsub j ec t s t o f i nd a t a rge t con junc t ion was found . A t t h i st i m e , a w h i t e s q u a r e w a s p r e s e n t e d a s a p r o b es t imulus , e i t he r a t t h i s a t t ended t a rge t l oca t i on o r a tano the r un re l a t ed l oca t i on t ha t d id no t con t a in at a rge t , and t he b ra in even t - r e l a t ed po t en t i a l gene r -a t e d b y th i s p r o b e w a s r e c o r d e d . T h e r e s u l t s s h o w e dtha t by 80 ms a f t e r t he p robe , t he p o t en t i a l e l i c it edo v e r t h e p o s t e r i o r r e g i o n s o f th e s c al p w a s g r e a t e rw h e n t h e p r o b e o c c u r r e d a t t h e t a r g e t l o c a t i o n . T h ecove r t sh i f t o f a t t en t i on t o t he t a rge t l oca t i on hadampl i f i ed t he sca lp e l ec t r i c a l a c t i v i t y fo r p robespresented a t th is loca t ion. The sca lp ampl i f ica t ion wass t r o n g e s t o v e r p o s t e r i o r s i t e s c o n t r a l a t e r a l t o t h eh e m i fi e ld w h e r e t h e p r o b e a p p e a r e d . I n o t h e r s t u d i e s ,whe re a t t en t i on was de l i be ra t e ly cued t o a l oca t i on , as imi la r ampli fi ca ti on o f t a rge t ER Ps a t t he cuedloca t i on was a l so found 13. In th es e cuein g stu dies ~3,c u r r e n t s o u r c e d e n s i t y a n a l y s i s h a s s h o w n t h a t t h eampl if ica tion or ig inates f rom p res t r ia te ar eas s imi lar inloca t i on t o t hose shown to be ac t i va t ed i n t he abovec i te d P E T s t u d i e s 9

C e l l u l a r r e c o r d i n g s i n a w a k e m o n k e y h a v e s h o w ntha t t he r e l a t i ve ampl i fi c a ti on o f t he a t t e nde d s t imulusmigh t ac tua l l y i nvo lve suppre ss ion o f ac ti v i t y evok edb y u n a t te n d e d e v e n t s . W h e n m o n k e y s w e r e r e q u ir e dto a t t en d t o a pa r t i cu l a r loca t i on i n o rd e r t o r e s pon d toa change in color a t tha t loca t ion, ce l l s in area V4s t o p p e d r e s p o n d i n g t o a s t i m u l u s a t a n u n a t t e n d e dloca t i on t ha t wou ld o the rwi se be op t ima l i n i n i t i a t i ngcell firing M . Similar s u p p r e s s i o n e f f e c t s h a v e b e e no b s e r v e d i n t h e i n f e r i o r t e m p o r a l c o r t e x d u r i n g avisual searc h tas k: ce l l s tha t in i tia lly fared st rong ly to ag i v e n v i s u a l o b j e c t s t o p p e d r e s p o n d i n g w h e n t h em o n k e y a t t e n d e d t o a d i f f e r e n t o b j e c t a t a n o t h e rloca t i on , even t hough the p re fe r red ob j ec t was s t i l lp re sen t i n t he r ecep t ive f i e ldz~.

E a r l y T i m e L a t eautom at ic autom at ic ity requires a t tent ionnar row recept ive f ie ld wide

Act iva t ion leve lA t t e n d e d

Passiveunat tended

F i g . 2 . Schemat ic d iagram o f the e f fec ts o f a t ten t ion on v isua l p rocess ing .Boxes dep ic t the d ive rg ing t ree o f v isua l cor t ica l a reas, wi th ac t iva t ion f low ingfrom le f t to r igh t. Shad ing is used to represent the s t rength o f ac t iva t ion a teach s tage. Pass ive presenta tion o f tw o v isua l ob jec ts , a rec tang le an d anellipse ( t o p ) , induces a h igh leve l o f respond ing in ear ly v isua l a reas, wi th aprogress ive decrease fu r ther down the process ing s t ream as recept ive f ie ldswiden an d the neurons becom e less s t imu lus-dr iven and m ore sens i tive toa t ten t iona l in fluences. When a t ten t ion is se lec tive ly o r ien ted tow ards thevert ical rectangle (m iddle ) , there is an early suppression of the irre levante l lipse . Th is is fo l low ed by an ampl i f ica t ion o f the ac t iv i ty evoked by the ta rge trec tang le in the cor t ica l pa thw ays re levant to the task ( fo r example , thoseextrac t ing i ts loca tion , bu t no t those ex t rac t ing i ts co lo r) . The bot tom diagramsumm ar izes the t ime course o f suppress ion and ampl i fica tion e f fec ts insuccessive areas as comp ared to th e passive situation.

Based on t he mode l i n F ig . 2 , we specu l a t e t ha ta t t en t i ona l e f f ec t s shou ld gene ra l l y appea r a s as u p p r e s s i o n o f u n a t t e n d e d i n f o rm a t i o n e a r l y o n i n t h ep r o c e s s i n g s t r e a m , a n d a s a n e n h a n c e m e n t o f r e l e v a n tin fo rma t ion l a t e r on . The r ea so n i s tha t neuro ns i nea r l i e r v isua l a rea s a re a l r eady ac t i va t ed t o a nea rop t ima l l eve l by v i sua l s t imul i even i n a pa ss ives i t ua t i on (o r i n t he anes the t i zed an ima l ) , and t ha tt h e r e f o r e t h e r e i s li tt le r o o m f o r fi ri n g e n h a n c e m e n t o ft h e t a r g e t , b u t m o r e r o o m f o r f ir in g s u p p r e s s i o n o f t h e

TINS, Vol. 17, No. 2, 1994 7 7


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non- t a rge t s . Neurons i n t he r e t i na , l a t e ra l gen i cu l a t eand a rea V1 f i r e s t rong ly t o t he i r p re fe r red s t imul iw h e t h e r t h e y a r e a t t e n d e d o r n o t . E x t r a - s t r i a t e a r e a ss u c h a s V 4 s e e m t o b e t h e f i rs t v is u a l a r e a s t o s h o wthe i n f luence o f a t t en t i on 14. Becau se t he neu rons i nea r ly v i sua l a rea s a l r eady re spo nd a t a h igh- f ir i ng r a t eeven i n a pa ss ive cond i t i on , a t t en t i on has t he e f fec t o fse l ec t i ve ly suppre ss ing some o r a l l o f t h i s au toma t i cac t i va t i on . La t e r on , a s t he ce l l r e sponses becomele ss au toma t i c , a t t en t i on can have t he e f fec t o fboos t i ng t he ac t i va ti on o f ce l ls cod ing fo r t he a t t end eds t imulus , a s we l l a s suppre ss ing t hose cod ing fo runa t t ended s t imul i . F ina l l y , even fu r the r downs t ream,in co r t ic a l a rea s t ha t a c t i va t e on ly dur ing spec i f ic t a sksand no t i n a pa ss ive s i t ua t i on , a t t en t i on appea r snecessa ry fo r any ac t i va t i on t o occur .I n d e e d t h i s p r e d i c t e d s e q u e n c e o f s u p p r e s s i o nf o ll o w e d b y e n h a n c e m e n t h a s b e e n o b s e r v e d r e c e n t l yin s t ud i e s o f even t - r e l a t ed po t en t i a l s du r ing cueda t t en t i on t a sk s 12. Th e t a sk i nvo lved t h ree cond i t ions .In the co r rec t l y cued cond i ti on , a t t en t i on was b rou gh tto t he co r rec t l oca t ion p r io r t o p re s en t a t i on o f t het a rge t . In t he neu t ra l cond i t i on , t he cue p rov ided noin fo rma t ion on t a rge t l oca t ion . In t he mi scu ed con-d i t i on , a t t en t i on was b rough t t o a l oca t i on o the r t hant h a t a t w h i c h t h e t a r g e t w a s p r e s e n t e d . T h e P 1componen t o f t he evoked po t en t i a l , an ea r ly pos i t i vee l ec t ri c a l even t wh ich appea r s abou t 80 ms fo l lowings t im u l u s p r e s e n ta t i o n , w a s t h e s a m e f o r c o r r e c t l yc u e d a n d n e u t r a l c o n d i t io n s a n d b o t h w e r e l a r g e r t h a nw h e n t h e t a r g e t o c c u r r e d f o l lo w i n g a m i s c u e . F o r t h isc o m p o n e n t , a c t iv i t y i s s u p p r e s s e d w h e n a t t e n t i o n isl oca t ed a t t he wrong loca t i on . Howeve r , fo r t hes u b s e q u e n t N 1 , a n e g a t i v e c o m p o n e n t t h a t a r i s e sa b o u t 1 5 0 - 2 0 0 m s a f t e r t h e s t i m u l u s , t h e r e w a s i n -c rea sed e l ec t r i c a l a c t i v i t y fo r co r rec t l y cued s t imul iwi th r e spec t t o t he neu t ra l o r m i scued cond i t i ons . Int h e N 1 c o m p o n e n t o f t h e e v o k e d p o t e n ti a l, t h ere la t ive ampl if ica tion app ears due to bo ost ing thea t t ended l oca t i on .

Accord ing t o ou r i n t e rp re t a t i on , t h i s suppre ss ion-e n h a n c e m e n t s e q u e n c e s h o u l d b e o b s e r v e d m o s tread i ly wi th s t imul i tha t a re su f f i c ien t l y i n t ense t opass ive ly ac t i va t e t he ea r ly v i sua l sys t em and the re -f o r e l e a v e l i t t l e r o o m f o r a n e a r l y e n h a n c e m e n t b ya t t en t i on . O n the con t ra ry , w i th l ow- in t ens i t y s t imul it ha t c ause l i t t l e pa ss ive ac t i va t i on , enhancementshou ld be seen ea r l i e r i n t ime and suppre ss ion shou ldha rd ly be pe rcep t ib l e a t a l l.

a de f i c i t o f d i sengag ing a t t en t i on f rom a cuedlocat ion16.R e c e n t P E T d a t a 17 f r o m n o r m a l s u b j e c t s s h o wtha t w hen a t t en t i on i s sh i f t ed f rom loca t i on to l oca t i one i t h e r v o l u n t a ri ly o r a s a r e s u l t o f b e i n g s u m m o n e d b ye x t e r n a l e v e n t s , t h e m a j o r f o cu s o f in c r e a s e d b l o o df low i s i n t he l e f t and r i gh t supe r io r pa r i e t a l l obes .Th i s i s t he on ly ac t i va ti on t o be spec if i ca l ly r e l a t ed t othe a t t en t i on sh i f t. As m en t ion ed ea r l ie r , c e l lu l a r

r e c o r d i n g s i n th e p a r ie t a l lo b e o f a w a k e m o n k e y s h a v ea l so suppor t ed t he i nv o lvem ent o f pa r i e t a l neur ons i nv i sua l a t t en t i on . Pa r i e t a l a rea s appea r t o be im-p l emen t ing a spec t s o f t he ac t o f sh i f t i ng a t t en t i onra the r t han be ing s i t e s a t wh ich a t t en t i on i s a f f ec t i ngt a rge t de t ec t i on 1'7 .The PET da t a a l so suppor t t he c l i n i ca l obse rva t i ont h a t t h e a t te n t i o n a l f u n c ti o n s o f t h e t w o h e m i s p h e r e sa re no t symmet r i c . The r i gh t supe r io r pa r i e t a l l obeinc rea se s i n b lood f l ow fo r a t t en t i on sh i f t s i n bo thf ie lds, whi le the le f t increas es only for r ight f ie ldshi f t s 17. Th is f inding might expla in th e c l inica l obse r-

v a t io n t h a t r i g h t - h e m i s p h e r e l e s io n s p r o d u c e g r e a t e ra t t en t i on de f i c i t s t han do l e f t -hemisphe re l e s ions . Inthe norm a l sub j ec t how eve r , t he l e f t and r i gh t pa r i e t a la rea s a re i n t eg ra t ed i n to a s i ng l e mechan i sm, so t ha tcove r t a t t en t i on has a s i ng l e focus . The co rpusca l losum ap pea r s c ruc ia l in un i fy ing t he a t t en t i ona lfocus . In v i sua l sea rch t a sks , no rma l sub j ec t s a re no tf a s t e r w h e n t h e s a m e n u m b e r o f d i s t r a c t o r s i sd i s t r i bu t ed ac ross t he two v i sua l f i e lds t han when a l la re concen t ra t ed i n one f i e ld . Howeve r , pa t i en t swh ose co rpus ca l l osum is cu t can sea rch a t twice t her a t e w h e n d i s t r a c t o r s a r e s p l it e v e n l y b e t w e e n t h etwo v i sua l f i e lds i n compar i son t o when they a recon cen t ra t ed wi th in a s ing l e f i eld s . Th i s sug ges t s t ha ta t t en t i ona l mechan i sms in t he l e f t and r i gh t hemi -s p h e r e s c a n b e c o m e d e c o u p l e d a f te r c a l lo s o t o m y .

P o s i t r o n e m i s s i o n t o m o g r a p h y s t u d i e s s u g g e s t t h a tt he ana tomica l c i r cu i t ry which enab l e s t he pa r i e t a la rea s t o se l ec t i ve ly mod u la t e b ra in ac t i v i ty i n o the rp r e s t r i a t e a r e a s p a s s e s t h r o u g h t h e p u l v i na r n u c l e u sof t he t ha l amus . When one v i sua l f i e ld con t a ins ana r ray wi th on ly one l a rge t a rge t , wh i l e t he o the rc o n t a i n s a t a r g e t s u r r o u n d e d b y d i s t r a c t o r s , t h e r ei s ev idence o f i nc rea sed me tabo l i sm spec i f i c t o t hetha l amus o f t he s i de oppo s i t e t he complex a r ray . T heac t o f f i lt e r ing ou t t he d i s t r ac to r s o r am pl i fy ing t het a rg e t se em s to p rodu ce a l a rge r e f fec t i n t he pu lv ina ro f t he oppos i t e s i de t han i n o the r s t ruc tu re s 19.C o n t r o l o f o r i e n t i n g

In v i sua l sea rch , a t t en t i on moves f rom loca t i on t oloca t i on un t i l t he t a rge t i s de t ec t ed . How i s t hein t e rna l focus o f a t t en t i on m oved? In o rde r t o i so l a t et h e a t t en t i o n a l n e t w o r k r e s p o n s i b le f o r s e a r c h i n gloca ti ons , a cue can be u sed t o d i r ec t a t t en t i on t o t ha tl o c a t i o n . S u c h c u e i n g e x p e r i m e n t s h a v e s h o w n t h a ts t imul i t ha t a re p re sen t ed a t t he cued l oca t i on a rer e s p o n d e d t o m o r e r a p i d l y , h a v e a l o w e r p e r c e p t i o nth re sho ld and gene ra t e b ra in po t en t i a l s o f a l a rge rampl i t ude 7 '16 . W hen b ra in - l e s ioned pa t i en t s a ret e s t ed i n t h i s fo rm o f cued a t t en t i on , d i f f e ren t fo rms o fde f i c i t a re found depend ing on t he s i t e o f t he l e s ion .Fo r example , pa t i en t s w i th le s ions o f t he r i gh t pa r i e t all o be a r e s e v e r e l y i m p a i r ed w h e n c u e s a r e i n t h e g o o dright visual f ie ld and targ ets in the b ad lef t v isual f ie ld ,bu t no t much impa i red i n o the r s i t ua t i ons , sugges t i ng

G u i d i n g a n d c o n t r o l l in g s e a r c hS u p p o s e y o u a r e a s k e d to l oca t e t he ve r t i c a lr ec t ang l e . M us t y ou sea rch a l l t he t a rg e t l oca t ions o rcan you conf ine your sea rch t o t he ve r t i c a l ob j ec t son ly? The re i s cons ide rab l e ev idence t ha t sea rch canbe gu ided by i n fo rma t ion abou t t he co lo r o r o r i en t a t i onor o the r non- loca ti ona l f ea tu r e s o f t he t a rge t 2. H owis t h i s implemen ted i n t he b ra in? I t seems tha t t herec ru i tmen t and con t ro l o f pos t e r i o r b ra in a rea s , i nth i s c a se , i s supe rv i se d b y an ana tomica l l y d i s t i nc ts y s t e m w h i c h h a s b e e n c a l l e d t h e a n t e r i o r a t t e n t i o ns y s t e m 7. In the study of a t tent ional ampl i f ica t ion of

c o lo r , f o r m o r m o t io n , m e n t i o n e d a b o v e, t h e r e w a sev idence fo r ac t iva t i on o f a f ron t a l a tt en t i ona l sys t em ,bu t no pa r i e t a l a c t i va t i on was found9 . I t t hus appea r st h a t t w o d i f f e r e n t a t t en t i o n a l s y s t e m s s e r v e a ssou rces o f ac ti va t i on fo r co lo r o r fo rm ( f ron t a l a rea s )78 TINS, Vo / . "17, No. 2, 199 4


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and fo r l oca t ion (pa r ie t a l) , a l t hough bo th migh t en t e rand ampl i fy ac t i v i t y wi th in t he v i sua l sy s t e m a t t hesam e s i t e ( fo r example , V4) .In gu ided sea rch , se l ec t i on by l oca ti on and se l ec t i onby co lo r o r fo rm occu r s imul t aneo us ly wi th r e l a t i ve lyl i tt le in terf ere nc e 2, un l ike the si tua t ion fo r locat ionwhen the co rpus ca l l osum i s i n t ac t i n wh ich a t t en t i onc a n n o t b e s h a r e d b e t w e e n t h e t w o f i e ld s s . O n especu la t i ve poss ib i l i t y wou ld be t ha t t ime sha r ing i sposs ib l e wh en tw o ana tom ica l ly d i s t i nc t a t t en t i ona lsources a re i nvo lved .T h e f r o n ta l a r e a s t h a t s e r v e t o g u i d e s e a r c h a p p e a rto i nvo lve a ne twork t ha t i nc ludes a t l e a s t po r t i onsof t he ba sa l gang l i a and o f t he an t e r io r c ingu la t eg y r u s 7 '21. The an t e r io r p o r t i on o f t he c ingu la t e gy rusappea r s t o be i nvo lved i n a wide r ange o f ac t i v i t i e st h a t h a v e b e e n t e r m e d c o l le c t iv e l y ' e x e c u t i v e f u n c-t i on ~1 . In PET l anguage s tud i e s , w hen su b j ec t s w e rerequ i red t o nam e the u se o f f ami li a r noun s ( fo rexample , p ound to hamm er ) , a c t i va t ion o f t he an t e r io rc ingu la t e a long wi th l e f t l a t e ra l a rea s we re mos tp r o m i n e n t4 '22 . When sub j ec t s we re r equ i red t o r e -spond to t he i nk co lo r in wh ich a conf l i c t ing co lo r nam ew a s p r e s e n t e d ( S t r o o p e f f e c t ) t h e r e w a s s t r o n gac t iva t i on o f t he an t e r io r c ingu la t e a long wi th p re -s t r i a t e co lo r a rea s 23'24. The de t ec t i on o f mul t i p le co lo rf o r m o r m o t i o n t a r g e t s i n c o m p a r i s o n t o p a s s i v ev i ewing o f t he sam e s t imul i a l so ac t i va t ed t he an t e r io rc ingula te9. Al l of these si tua t ions involve se lec t ion oft a r g e t s f r o m c o m p e t i n g i n p u ts , w h i c h is c o n s i d e r e d at r ad it i ona l ro l e o f a t t en t i on . In t he ca se o f t h i s a rea o ft h e b r ai n , t h e n a t u r e o f th e t a r g e t d o e s n o t s e e m t om a t t e r v e r y m u c h .

T h e t e r m ' e x e c u t i v e ' s u g g e s t s t w o i m p o r t a n t o v e r -a ll func t i ons . F i r s t , an execu t ive i s i n fo rmed abou t t heprocesse s t ak ing p l ace wi th in t he o rgan i za t i on . As y s t e m t h a t w o u l d b e r e l a t e d t o o u r s u b j e c t i v eexpe r i ence o f foca l a t t en t i on wou ld c l ea r ly p l ay t h i sf u n c ti o n f o r a s u b s e t o f c u r r e n t ( s e n s o r y ) a n d s t o r e d( m e m o r y ) i n f o rm a t io n . T h e r e a r e r e a s o n s f o r r e l a ti n gan t e r io r c ingu la t e func t i on t o foca l awareness o f t het a rge t 25 . The s t ro nge s t r ea son i s t ha t t he i n t ens i t y o fc ingu la te ac ti v i t y t end s t o i nc rea se wi th t he num ber o ft a rge t s i n a se t o f s t imul i3 a n d d e c r e a s e s w i t h p r a c ti c eon any s ing l e s t imulus se t 4 . The se f i nd ings co r re -spond to cogn i t i ve t heor i e s l i nk ing foca l a t t en t i on t onum ber and d i f fi cu lt y o f t a rge t de t ec t i on 25.A seco nd func t ion o f an exe cu t ive i s t o exe rc i ses o m e c o n t ro l o v e r t h e s y s t e m . T h e a n a t o m y o f t h ean t e r io r c ingu la t e p rov ides pa thways fo r connec t ing i tt o bo th t he pos t e r io r pa r i e t a l a rea and t o an t e r io ra rea s ac t i ve dur ing l anguage t a sks 26 . W ork ingm e m o r y i s g e n e r a l l y t h o u g h t t o i n v o l v e b o t h ar e p r e s e n t a t i o n o f p a s t e v e n t s a n d a n ex e c u t i v es y s t e m i n v o l v e d i n s u s t a i n i n g a n d t r a n s f o r m i n g t h i srep re sen t a t i on 27. Rece n t P ET (Re fs 28 and 29) andneurophys io log i ca l3'31 s tud i e s show tha t l a t e ra l a rea so f t he p re f ron t a l co r t e x p l ay a key ro l e i n ho ld ing on-l ine a r ep re sen t a t i on o f pa s t even t s . Ce l lu la r r eco rd -ings in t he aw ake m on key ind i ca t e tha t c e l l s wi th in t hedorso l a t e ra l p re f ron t a l co r t ex ma in t a in a r ep re sen-t a t io n o f th e s p a ti a l e n v i r o n m e n t w h e n m o n k e y s h a v eto ho ld in mind a l oca ti on t o which t o mo ve t h e i r ey esa f t e r t h e s t imulus d i sappea r s 24 . La t e ra l a rea s o f t hef ron t a l and pos t e r io r co r t ex a re a l so ac t i ve i n s t ud i e swhen peop le mus t ob t a in a qu i ck a ssoc i a t i on t o wordst imul i3 '4 '22. Whi le specia l ized areas of the prefronta l

co r t ex appea r t o ho ld t he r e l ev an t i n fo rma t ion on- li ne ,t he an t e r io r c ingu la te i s p lay ing a ro l e i n t he ex ecu t ivef u n c t i o n s o f a w a r e n e s s a n d c o n t r o l d i s c u s s e d i ncogn i t i ve s t ud i e s and o f t en found impa i red i n sub j ec t swi th f ron t a l damage .C o n c l u d in g r e m a r k sRecen t b ra in - imag ing and neurophys io log i ca l da t aind i cat e t ha t a t t en t i on i s ne i t he r a p r ope r ty o f a s i ng l eb ra in a rea , no r t ha t o f t he en t i r e b ra in . Whi lea t t e n t i o n a l e f f e c t s s e e m m e d i a t e d b y a c o m m o nprinciple of attenti on al amplification at all leve ls of thecor t ica l c i rcui t ry , the or igins of these ampl i f ica t ione f fec t s a re t o be found in spec ia l i z ed co r t i c a l a rea s o fthe f ron t a l and pa ri e t a l l obes . Th ese r e su l t s r e p re sen ts u b s t a n t i a l p r o g r e s s i n t h e e f f o r t t o d e t e r m i n e h o wbra in ac t iv i t y i s r egu l a t ed t h roug h a t t en t i on . Whi l em any philosophica l and prac t ica l i ssues r em ain ind e v e lo p i n g s u c h a n u n d e r s t a n d in g , t h e n e w t o o ls n o wavai lable should provide the basis for the effor ts s t i l lahead .Selected references

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AdmowledEementsThis esearch wassupportedby ONEGra nt NOOO14-89-J3013 and by theJamesS. McDonn dlFoundation and P ewCharitable Truststhrough he Centerfor the C ognitiveNeuroscienceo fAttention. StamslasDehaene wassponsored n part byINSERM, CNRS ndthe NationalScienceFoundation.

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