Investigating the Relation between Imagery and Perception: Evidence ...

TH E QUARTE R LY JOU RNAL O F E XPE R IM E NTAL PS YCH OL OG Y, 1997 , 50A (2), 274 ± 289

Investig ating the Relatio n between Im agery an d Perception: E vidence from Face P rim ing Roberto Cabeza Rotma n R esea rch I nstitute of B a ycrest Centre, U niversity of Toronto, Ca na da

A. M ike Burton and S tephen W. K elly Depa rtment of Psychology, U niversity of Gla sgow, Scotla nd

S higeru Akam atsu A TR H uma n I nforma tion P rocessing R esea rch La bora tories, K yoto, J a pa n

T h e relation b etw een im agery and percep tio n w as inve stig ated in face p rim ing. Two exp erim en ts are rep orted in w h ich su bjects eith er saw or im agined th e faces of celebrities. T h ey were later given a s peed ed p ercep tual test (fam iliarity judg em en t to p ictu res of celebrities) or a sp eed ed im agery test (in w h ich th ey w ere told th e n am es of celebrities an d asked to m ake a decision abou t th eir app ear an ce). S eeing faces pr im ed th e p ercep tu al test, an d im aging faces prim ed th e im age ry test; h ow ever, th ere w as n o prim ing betw een seeing an d im agin g faces. T h ese resu lts sh ow th at p erce ption an d im agery can be d issociated in no r m al su bje cts. In tw o fu rth er exp erim en ts, w e exam ined th e effects of im aging faces on a su bsequ ent face-n am ing task an d on a task requ irin g fam iliarity jud gem en ts to p artial faces. Both th ese tasks w ere facilitated by p rior im agin g of faces. T h ese resu lts are d iscus sed in relation to th os e of M cD er m ott & Roed iger (1994), w ho fou nd th at im agery pr om oted object p rim ing in a percep tu al test involving n am ing p ar tial lin e d raw ings. T he im p lication s for m od els of face re cogn itio n are also d iscus sed.

W hen o ne im agines a w ell-know n face, such as the face of a friend or a fam ous actor, the ex perien ce ap pears to b e sim ilar to that of actually seeing the face. E ven though the im age is no rm ally less clear, o ne can ``inspect’ ’ the im age of the face just as o ne can inspect the actual face. T his close resem blance betw een imagery an d p ercep tio n is the origin of the notion that im agery is a w eak form of perception. H ow ever, the difference b etw een imagery an d perception could b e qualitative, rather than m erely quan titative. T he objective of the present research w as to investig ate this issue by com parin g the effects of imagery an d perception o n face prim ing.

Requests for reprints sh ould be se nt to M ike Burto n, D epartme nt of Psyc hology, Un ive rsity of Glasgow, G lasgo w G 12 8Q Q , U.K . E m ail: m ike@ psy.gla.ac. uk We w ish to than k Vick i Bru ce, Sh lom o Bentin, and M orris M oscovitch for their valuable comm ents about this pape r. We are also indebted to Yoh’ ichi Tohkura for the provisio n of AT R ± HIP researc h facilitie s. This researc h was supported in part by an ES RC grant to Vick i Bruc e and M ike Burto n (ES RC G R 0002 3 4573) .

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199 7 The E xp erim ental Psyc holo g y S ociety

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T he notion th at im agery an d perception are closely related m ental processes is supported by cognitive an d neuropsychological evidence (for a review, see F inke, 1985). Fo r ex am ple, it has been dem onstrated that im agery can produce perceptual ad ap tation (e.g. Finke, 1979) an d that im agery can activate visu al areas of the cortex (e.g. Farah , Weisberg, M on heit, & Peronnet, 1988; K osslyn et al., 1993). A t the sam e tim e, there is evidence sug gesting that im agery an d perception involve different neural m echan ism s. O n the o ne h an d, th ere are brain-dam aged patients w ho are impaired in imagery b ut not in perception (e.g. Farah & L evine, 1988; Riddoch , 1990). Fo r ex am ple, Farah an d L evine (1988) reported the case of a patient w ith an infarct w ho had an isolate d de® cit in visual imag ery but displayed ``perfect’ ’ (p. 16 1) perfor m an ce in picture an d ob ject identi® catio n. O n the other han d, there are also b rain -dam aged patients w ho are im paired in perceptio n, but not in im agery. Behr m an n, W inocur, an d M oscovitch (1992), for in stance, have recently presen ted th e case of a patient w h o had su ffered a closed head injury w ho disp layed excellent m ental im agery but w as dram atically im paired in object recognition. T h us, even though im agery an d percep tio n are closely related processes, they can be dissociated in brain-dam aged patients. C an im agery an d perceptio n also be dissociated in nor m al subjects? To an swer this questio n, w e investig ated face p rim ing, a pheno m enon ® rst studied by Bruce an d Valen tine (1985), w ho found that seeing a face of a celebrity speeds a later decision ab out w hether or not the face is fam iliar (the face fam iliarity decision task). M ost face prim ing stu dies, like that of Bruce an d Valentine (198 5), have investig ated p rim ing of fam iliar faces in th e face fam iliarity d ecision task, p artly becau se face priming is dif® cult to obtain o n other tests or w ith unfam iliar faces. Fo r exam ple, face prim in g does not read ily occur on sex or exp ression decisio n tests (E llis, Young, & Flude, 1990), an d prim ing of unfam iliar faces is a pictu re-dependent an d short-lived phenom enon (e.g. Bentin & M oscovitch, 1988; Roberts & B ruce, 1989). T he m ain properties of face priming in the fam iliarity d ecisio n task can be sum m ariz ed as follow s (for a review, see Bruce, B ur ton, C arson, H an na, & M aso n, 1994): (1) it occurs between different photog rap hs of th e sam e person (Bruce & Valentine, 1985; E llis, Young, Flude & H ay, 1987); (2) it tran sfers betw een one part of an im age of a face (e.g. inter nal or ex ter nal features) an d the w hole image of the face (Brun as, Young, & E llis, 1990); (3) it does n ot cross dom ains: neith er nam es (B ru ce & Valentine, 1985; E llis et al., 1987) nor pictures of b odies (E llis et al., 1987) produce signi® can t prim ing of faces; (4) it is not affected by how the ® r st face is processed: fam iliarity, sex, or expressio n judgem ents prod uce sim ilar am oun ts of prim in g on a subseq uen t face fam iliarity task (E llis et al., 1990 ); an d (5) it does no t occur un less the face is identi® ed spontaneously at study (B runas-Wagstaff, Youn g, & E llis, 1992). Face prim ing has usually been interpreted in relation to activatio n m odels of face processing, such as those of H ay an d Youn g (1982) an d Bruce and Young (1986). A ccording to the latter m odel, recognizing a fam iliar face proceeds independently of expressio n an d facial speech processing an d involves three sequential step s: activatio n of its face recognitio n u nit (F RU ), activation of its person iden tity node (P IN ), an d n am e retrieval. T here is o ne F RU for each fam iliar face, it is view -independent, an d it responds only to faces. P IN s, in contrast, respo nd also to voices, nam es, an d so on, integ rating in this w ay different form s of perso nal infor m ation .

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Burton , Bruce, an d John ston (1990) im plem ented an d extended parts of the B ruce an d Young (1986) m odel u sing an interactive activation an d com petitio n architecture (IAC ). W ithin the IAC m od el, repetition priming is described as follow s. First, the presen tatio n of a fam iliar face activates its F RU, an d this passes activation to the associated PIN. If the PIN ’ s activatio n reaches a given threshold, the face is judged fam iliar. S eco nd, the sim ultaneous activatio n of an FRU an d a PIN strengthens the link between them , so that w hen the sam e face is presented ag ain, the activatio n ¯ ow s faster to the PIN an d the fam iliarity decision is sp eeded. T his is consistent w ith the fact that prim ing does not cross stim ulus dom ains. To recognize a perso n by (for exam ple) a nam e would involve a different route into th e P IN. H en ce, there w ill be no subsequent adv an tage for th e FRU to PIN route used in recognizing this p erso n from a face. T he IAC m odel, derived from the Bru ce an d Young (1986) fram ew ork, can account for m an y face priming results. F irst, it can explain w hy face priming is dif® cult to obtain o n sex an d expression judgem ents an d w ith unfam iliar faces: sex an d expression are processed in dependently of the identi® catio n route in w hich priming occurs, an d unfam iliar faces d o not have F RU s an d PIN s. Second, it can account for m ost of the p roperties of fam iliar face prim ing in the fam iliarity decisio n task: (1) it occurs between d ifferent photog rap h s o f the sam e perso n becau se they activate the sam e F RU an d PIN ; (2) it tran sfers between par t an d w hole face because the F RU ± P IN link is strengthened w henever there is sim ultaneous FRU an d P IN activatio n; (3) it does not cro ss dom ains, because nam es, pictures of bo dies, an d so o n activate th e PIN but not the FRU ; (4) it is not affected by the w ay in w hich the prim e face is p rocessed becau se the ¯ ow of activatio n from the FRU to the PIN is m an datory (an d so identi® cation of a presented fam iliar face is m an datory); an d (5) it does not occur w hen the ® rst face is not recognized spontaneo usly, as the effect depends on a simultaneou sly active FRU an d P IN , an d the u nreco gn iz ed face has not activated a PIN above threshold (by de® nition). W hat predictions are possible concer ning th e effects of im agery on face priming? A lthough there is a large literature on face recognition, there is rather little know n ab out face im agery. C raigie an d H an ley (1993) provide one of the few pap ers o n this topic. Working w ithin the B ruce an d Young fram ew ork, C raigie an d H an ley propose that there is a dedicated im agery m odule for recalling facial infor m ation. In th e rem aind er of this pap er, w e explore the p redictio ns of an alter native prop osalÐ that face imagery m ight rely on top-dow n activation of the sam e represen tatio ns that are used to recogniz e faces. Bruce an d Young (1986) an ticipated this proposal by sug gesting that FRU s can b e activated topdow n , for exam ple by exp ectation (p. 312). If this is true, it seem s reason able to assum e that a voluntary effort to form a m ental image of the perso n’ s face m ight also activate an FRU. W ithin the IAC m odel, all FRU ± PIN links are bi-directio nal, an d so it is possible that FRU s can be activated top-dow n from their correspond ing PIN s by processes su ch as imagery. If this proposal is consistent, face prim ing should occur not o nly as a result of perceiving a face, bu t also as a result of imagining a face. A n alternative IAC -like account is also possible. If FRU ± PIN links are sensitive not only to the simultaneous activation of an FRU an d its cor respond ing P IN but also to the direction in w hich activatio n ¯ ow s alo ng the link, then the effects of imagery an d perception sh ould be d issociable: perceptio n (bottom -up) would produce m ore prim ing than imagery (top-dow n) in a test em ph asizing perceptio n (b ottom -up), w hereas the op posite

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would occur in a test em p hasizin g im agery (top -dow n). T he sam e prediction can be derived from the tran sfer-ap propriate p rocessing p rinciple (M or ris, B ran sford, & Fran ks, 1977 ), as is discussed at the end of the p ap er. T he ob jective of E xperiments 1a an d 1b w as to test this prediction. Subjects saw or imagined faces of celebrities an d then received either a test involvin g perception (face fam iliarity decisio n te st) or a test involving imagery (face im agery decision test). In the face fam iliarity decision test, faces are presented an d subjects have to decide w hether or not each face is fam iliar, w hereas in the face imagery decision test nam es are presented an d su bjects have to im agine the cor resp onding faces in order to decid e w hether or not each face has a particular attribute. If imagery is a weak for m of perception (a qu an titative difference), the ``see’ ’ condition should produce m ore p rim ing than the ``im agine’ ’ cond itio n in b oth tests. In contrast, if imagery and p ercep tion are qualitatively different pro cesses, it is possible that th e ``im agine’ ’ co nditio n w ould surpass the ``see’ ’ cond ition w hen the test involves imagery. Such a resu lt w ould be consistent w ith a con® guration of the IAC m odel in w hich FRU s an d PIN s are con nected by separate bottom -up an d top -dow n links.

EX P ER IM E N T 1A M eth o d Subjects T h irty-six Jap an ese u n d erg r ad u ates p articipated in th e exp erim en t as volu n teers or in retu r n for a sm all p aym en t.

M aterials T h e target m aterials were p ho tog rap h s of 48 Jap an ese celebrities an d th eir n am es. For each of these celebrities, th ere w ere tw o ph otogr ap h s d iffering in view an gle an d /or h airstyle, on e to b e u sed at stud y an d on e at test. T h e 48 celebrities s elected h ad th e h igh est ratin gs in a p ilot stu dy in w h ich 15 su bjects w ere p resented w ith a list of 102 celebrities’ n am es an d h ad to r ate ho w w ell th ey cou ld im agine th eir faces u sin g a 5-point scale (0 = can n ot im agin e th e face or d o n ot kn ow th e p erso n; 1 = p oor im age; 2 = fair im age; 3 = good im age; 4 = excellen t im age). T h e set of 48 celebrities w as d ivided into 3 sets of 16 (8 m ale; 8 fem ale) m atch ed in im agery ratings (3.59, 3.60, an d 3.59, respe ctively). A dd ition ally, p h otogr ap h s of th e faces of a fur ther 28 celebrities an d of 68 u nfam iliar p eop le w ere also em p loye d. A ll th e ph otogr ap h s were cu t from m ag azines, an d the view an gle varied betw een fron t an d 3/4. T h e p ho tog rap h s w ere scan n ed at 72 d p i, an d th e im ages were en larged or red uced so th at th e im age d isp lay h ad a h eigh t of 6 cm on th e com p u ter screen u sed for th is experim en t. W idth varied slightly betw ee n im ages, but w as ap p roxim ately 4 cm . In all th e p h otogr ap h s, everyth ing excep t th e face and th e hair w as er ased u sin g g rap h ics so ftw are. D u ring th e experim en t, all im ages w ere d isp layed as grey-level. V iew ing d istan ce w as 60 cm .

Design and Procedure T h e exp erim ent w as c on du cted on a p ers on al com p u ter. T h e gener al instr uction s o n the screen told su bje cts th at th e object of the exp erim en t w as to investig ate th eir kno w led ge about celebrities, an d th at it co n sisted of th ree tas ks. T h ese th ree tasks cor respo n ded to th e ``im agine’ ’ an d ``see’ ’ stu d y

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tasks (a w ith in-su bjects m anip u lation ) an d th e test (fam iliarity d ecisio n or im agery d ecision , a betw een -su bjects m an ipu latio n). D u rin g th e s tu d y p h ase, n o m em ory test w as m ention ed , an d h en ce lear n ing w as inciden tal. D u rin g th e test p ha se, n o referen ce w as m ad e to infor m atio n p reviou sly p rese nted , an d con sequen tly th e tests w ere im p licit. E ach stu dy task con sisted of 1 critical set of 16 celebrities p lus 8 ® ller celeb rities (3 at the beginnin g of th e list and 5 at the en d of the list). O n e ach tr ial of b oth stu dy tasks, th e com p u ter p rese nted a stim u lus for 8 sec, b eep ed , and th en d isp layed a scale u n til a resp on se w as m ad e. In th e ``im agine’ ’ stu dy task, th e stim u lus w as a ce lebrity’ s n am e, an d th e su bjects’ task w as to close th eir eyes an d im agine th e celeb rity’ s face d urin g th e 8 sec. W hen th e com p u ter beep ed , they op ene d th eir eyes an d rated th e m en tal im age on an im age-qu ality scale (th e sam e u sed in th e p ilot stu d y). In th e ``see’ ’ stu dy task, the stim u lus w as a celebrity’ s face, an d th e su bjects’ task w as to insp ect the celebrities’ faces d u ring th e 8 sec. W h en th e com p uter beeped , th ey se lected th e occu p atio n of the celebrity am on g 5 alter n atives (u nk n ow n , actor, singer, com ed ian, oth er). G iven the fact th at all the celebrities u sed w ere very w ell kn ow n in the sub je ct p op u latio n, selecting a profession w as a trivial task th at w as u n likely to d istrac t th eir atten tion from insp ectin g th e p h otog rap h . T h is task w as includ ed on ly to h ave su bjects m ake a resp on se afte r the ``see’ ’ task. In the face fam iliarity d ecision test, 48 target p h otog r aph s of cele brities (16 im agined , 16 seen, an d 16 n on -stu died ), 16 ® ller p h otog rap h s of ce lebrities, an d 64 ph otogr ap h s of u n fam iliar faces w ere p rese nted m ix ed, in a differen t r an d om orde r for each su bject. In th e case of seen faces, stu dy an d test p ho tog rap h s w ere d ifferen t. For each face, su bjects h ad to ind icate as qu ickly as p ossible w h eth er th e face w as of a kn ow n per so n (press th e ``/’ ’ key w ith th e righ t h and ) or of an u n kn ow n p erso n (p ress the ``z’ ’ key w ith th e left h an d ). L aten cies of m ore th an 2 sec w ere n ot ana lysed . Su bjects w h o w ere n ot fam iliar en ou gh w ith th e celebrities (u su ally becau se th ey did n ot w atch televisio n) an d h en ce m ad e too m any er ro r s (m ore th an 12.5% ) w ere exclud ed an d rep laced. In th e face im agery d ecision test, su bjects w ere p resen ted w ith th e w ritten na m e of a celebrity an d asked to d ecide as qu ic kly as p ossible w h eth er th is p erso n h as a m ole on his/h er face. A s no celebrity in th e stud y list w as fam ou s for h avin g a m ole, sub jects could n ot m ake this d ecision o n th e basis of their gener al kn ow ledg e abou t th e celebrities; in order to m ake th e d ecision , they h ad to con stru ct a m en tal im age of the celebrity’ s face an d scan it for th e p resence of a m ole. It is very d if® cu lt to d eter m ine w h eth er or no t each cele brity in th e list actu ally ha s a m ole on th e face, an d h en ce w e d id n ot try to equ ate th e n um ber of cor rect ``yes’ ’ an d ``n o’ ’ resp on ses or to assess th e accu racy of su bjects’ resp on ses. Reaction tim e in this test re¯ ects th e tim e su bjects requ ired to for m an im age an d scan it u n til th ey felt con ® d ent en ou gh to m ake th e resp on se. A ccordin gly, reactio n tim es re¯ ect su bjects’ im agery p erfor m an ce accord ing to th eir ow n criteria. T h e im agery d ecision test d oes n ot involve u n fam iliar faces, an d h en ce it h ad on ly h alf as m an y trials as th e face fam iliarity d ecisio n test. In both test con d ition s, th e ord er of the tw o stu dy tasks and th e assign m en t of th e th ree critical sets of celebr ities to th e ``see’ ’ , ``im agin e’ ’ , an d n on -stu died con d itio ns wer e com p le tely cou n terbalan ced across su bjects.

R esu lts an d D iscu ssion T he top pan el of Table 1 show s the m ean reaction tim es in the fam iliarity an d imagery decision tests of E xperim ent 1a. A s in previous research (e.g. Br uce & Valentine, 1985; E llis et al., 1 987), er ror rates in the fam iliarity decision test w ere sm all (see: 3.8% ; imagine: 6.6% ; no n-studied: 5 .9% ) an d were not an alysed further. First, as the reactio n tim es were co nsiderably longer in th e im agery decision test than in the fam iliarity decision test, a separate o ne-way A N OVA (see vs. imagine vs. non-stu died)

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TABLE 1 a E x p e rim en ts 1a a n d 1b : M e an R e sp o n s e L at en c ies in th e Fa c e Fa m ilia rity D ec is io n an d F ac e Im a g e ry D e cis io n T e s ts

Encoding Conditions Test Exp eriment 1a Exp eriment 1b

a

face fam iliarity decision face im agery dec ision face fam iliarity decision error s (% ) face im agery dec ision error s (% )

I ma ge

S ee

Non-studied

803 154 1 817 24 100 3 21

773 164 3 748 16 115 6 17

82 0 170 4 86 2 23 120 3 24

I n milliseconds.

w as perfor m ed on each test. In the fam iliarity decisio n test, the m ain effect of the see± imagine± no n-studied m an ipu lation w as signi® can t, F(2, 34) = 4.58, p < .0 5. Prim ing (studied ver sus no n-studied) w as sig ni® can t in the see con ditio n, F(1, 34) = 8.92, p < .01 , but not in the im agine cond ition , F(1, 34) = 1.1 5, p > 0.2. T he difference betw een these two condition s w as m argin ally signi® cant, F(1, 34) = 3.66, p < .065. In the imagery decision test, th e m ain effect of the see± imagine± no n-studied m an ipulatio n w as signi® can t, F(2, 34) = 9.07; p < .01. Prim in g w as signi® can t in the im agine co ndition. F(1, 34) = 17.75; p < .01, but not in the see condition, F(1, 34) = 2.46; p > .1. T he difference between the see an d imagine co nditions w as signi® can t, F(1, 34) = 6.99; p < .0 5. In sum , priming in th e face fam iliarity decision test w as robust fo r seen faces but non-signi® can t for imagined faces, w hereas prim ing in the im agery decisio n test show ed an o pp osite p atter n: it w as robust for imagined faces but no n-signi® can t for seen faces. Seco nd, a 2 (study task: see vs. im agine) 3 2 (test: fam iliarity decisio n, imagery decision) A N OVA w as co nducted o n priming scores (no n-studied RTsÐ stud ied RTs). T he m ain result of this an alysis w as a signi® can t interactio n between study task an d test factors, F(1, 34) = 8.8 9; p < .01, re¯ ecting the fact that perceptio n produced m ore prim ing than im agery in the face fam iliarity d ecision test, w hereas im agery produced m ore priming than perceptio n in th e im agery decision test. T hese results ap pear to support the id ea that seeing an d im aginin g faces involve separate processes. T he results show that seeing a face w ill prim e sub sequently seeing the face, an d imaging a face w ill prim e subsequent imaging of a face. Prim in g crossing between seeing an d imaging, in either directio n, w as not signi® cant in E xp erim ent 1a. T hese results seem to b e inconsistent w ith th e assum ption in the IAC m odel th at FRU ± PIN links are b i-directional an d eq uivalent. Instead , the data are consistent w ith separate, an d separately primeable, top-dow n an d bottom -up links b etw een PIN s an d FRU s. U nfortunately, the interpretation of the results of E xperim ent 1a is com plicated by the fact that the type of im agery decision test (the ``m ole’ ’ task) used in this ex periment does not allow an objective m easure of subjects’ accu rac y in im agining faces. We assum e that faster RTs in this test re¯ ect a facilitation to im agine

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previou sly encountered faces (priming), but w e cannot dem o nstr ate that sub jects were actually imagining target faces. A num ber of p ossible tests of imagery are available w hich m ight allow o ne to test the accur acy of the sub jects’ imagery. Fo r ex am ple, sub jects m ight be asked w hether celebrities have glasses or wear beards (Craigie & H an ley, 1993) or w hether their hair is light or d ark. N one o f these m an ipulations is plausible w ith Jap an ese celebrities an d subjects. T herefore, in E xperim ent 1b w e replicated E xp eriment 1a using C au casian sub jects an d celeb rities, an d a light hair/dark hair imagery task at test.

E XP E R IM E N T 1 B M eth o d Subjects Twen ty fou r su bjects w ere recru ited from the U n iver sity of G lasgow. S u bjects received a sm all p aym en t for th eir p ar ticipation .

M a terials T h e face s of 64 C au casian celebrities (48 critical an d 16 ® ller) an d 64 u n fam iliar C au casian face s w ere u sed in th is exp erim en t. T h e celebrities w ere selected from a p ilot stu d y of 92 celebrities. T h e m ost im ageable cele brities w ere chosen from th e 92 w ith the ad d ition al criterio n that p ilot su bjects be in a m inim um of 70% agreem en t as to w h ethe r a sp eci® c celebr ity’ s h air w as ``ligh t’ ’ or ``dark ’ ’ . H alf of th e targ et celebrities h ad d ark h air (e.g. H u gh G ran t, Jo an C ollin s) an d h alf h ad ligh t h air (e.g. D ar ry l H an n ah , Steve M artin), an d th ese lists w ere split into 50% m ale:50% fem ale. T hese 48 critical celebrities w ere d ivided into 3 lists of equal im ageability (rated 3.41, 3.42, an d 3.43 on th e sam e 0± 4 scale u sed in E xp erim en t 1a). P ho togr ap h s w ere scan n ed, and ver tical im age size w as ad justed to 6 cm . H orizon tal im age size varied sligh tly betw een im ages bu t w as ap p roxim ately 4 cm . E very th ing except the face an d hair w as er ased u sin g gr ap h ics so ftw are. D urin g th e experim en t, all im ages w ere d isp layed as g rey-leve l an d view ing distan ce w as 50 cm .

Design and Procedure T h e d esign and p ro ced u re w ere iden tical to E xp erim en t 1a, w ith the exc eption th at th e im agery task at test requ ired s ub jects to state w h ether th e n am ed celeb rity had ligh t or d ar k h air. A s in E xp erim ent 1a, laten cies above 2 sec w ere d iscarded , alth ou gh su bjects w ith er ror rates above 12.5% w ere n ot excluded .

R esu lts an d D iscu ssion T he RT results of E xperiment 1a are clear an d echo th ose of E xperim ent 1b (see Table 1). H owever, before w e co nsider the RT data, we should ® rst exam ine the er ror rates in this ex periment. E rror rate s were h igh, though relatively consistent in this study. Fo r the fam iliarity decision test, erro r r ates in each of the conditio ns w ere: see, 16% ; im agine, 24% ; not

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stu died, 23% . Fo r the im agery decision test, er ror rates w ere: see 17% ; im agine, 21% , not stu died, 24% . Separate A N OVAs w ere conducted on er rors fro m the two tests, each revealing no signi® can t differences (on the fam iliarity decision, F(2, 22) = 3.26, p > 0.0 5; on the imagery decisio n, F(2, 22) = 2.15, p > 0.05). Both the nature of the experim ent an d the restrictio n on stimulus choice lead to these high error rates. Er ror s are calculated fro m test phase only. It w ould be possible to co nditionalize er rors at S tage 2 by kn ow n-only item s at Stage 1, an d this reduces the er ror rates to those m ore norm ally reported in face recognitio n experim ents. H owever, standard izing in th is w ay lead s to disproportionate errors in the ``not studied’ ’ g roup, as all unknow n celebrities ap pear as er rors at Stage 2. Fo r this reason w e have chosen to present er rors at test, ignoring perfor m an ce on the item s at study. N ote also th at the restrictions on attributed hair colour also lead to er rors. It is relatively dif® cult to ® n d fam ous faces w hose hair is consistently held to be light or dark, an d hence the fam iliarity of the faces w as low er than w ould ideally b e the case. H aving said this, there is n o evidence that errors w ere unevenly distribu ted between the cond ition s, an d so w e now tur n to an alysis of reactio n tim es. RT data for cor rect decisions o nly are show n in th e bottom pan el of Table 1. T hese results show the sam e patte rn as those of Ex perim ent 1a. F irst, a separate A N OVA w as co nducted on each test. In the fam iliarity decisio n test, the m ain effect of the see± imagine± no n-studied m an ipulation w as signi® can t, F(2, 22) = 8.2, p < .01. Prim in g (studied vs. no n-studied) w as signi® can t in the se e co ndition, F(1, 22) = 16.2, p < .01, but not in the im agine cond ition , F(1, 22) = 2.62, p > 0.05. T he d ifference between these two co nditions w as reliable, F(1, 22) = 5.74, p < .05. In the im agery decisio n test, the m ain effect of the see± imagine± n on-studied m an ipulation w as signi® can t, F(2, 22) = 5.02; p < .05. Prim ing w as reliable in the im agine condition, F(1, 22) = 9.12; p < .01, bu t not in the see co nditio n, F < 1. T he difference between the see an d im agine conditions w as signi® can t, F(1, 22) = 5.4; p < .05. Seco nd, a 2 (study task: see vs. im agine) 3 2 (test: fam iliarity decisio n, imagery decision) A N OVA w as conducted on priming scores (RTs for no n-studied item s m inus RTs for studied item s). In this A N OVA, the m ain effect o f study task, F(1 , 22) < 1, an d test, F(1, 22) < 1, were non-signi® can t. T hese results indicate th at, over all, the am ou nt of prim ing w as sim ilar in the two study conditions an d in the two tests. H owever, the effect of the study m an ipulatio n on th e two tests w as very differen t, as indicated by a highly signi® can t interaction between study task an d test, F(1, 22) = 6.56; p < .01. T h is signi® cant interaction re¯ ects a cross-over dissociation betw een face fam iliarity an d face imagery decision tests as an effect of the see-im age study m an ipulation. In a test involving perceptio n of faces (face fam iliarity decision), priming w as larger for faces that were seen at study than for faces th at w ere im agined at study. In a test req uiring su bjects to m ake a decision requiring a face to be imagined, the opposite patter n of results w as found, w ith prim ing b eing larger for im agined faces th an for faces th at w ere actually seen at study. T he results of E xperim ent 1b are co nsistent w ith the results of E xperim en t 1a an d su pp ort the idea that there is a qualitative difference between im agery an d perceptio n. If imagery w ere m erely a w eak for m of perception, the see co nditio n should have p roduced m ore prim ing in both tests, but it did not. In terpreted in ter m s of the IAC m odel of face processing (Burto n et al., 1990), the results of E xperim ent 1a an d 1b sup port the n otio n

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that between an F RU an d a PIN there are two separate links: a bottom -up FRU ® P IN link, w hich is th e one primarily involved in identi® cation, an d a top-dow n PIN ® F RU link, w hich is the o ne prim arily tapped by imagery; these two links seem to be independen tly primeable.

E X PE R IM EN TS 2A A N D 2B T he ® nd ing of E xperim ents 1a an d 1 b that imagery produced non-signi® can t priming o n the fam iliarity decisio n test is ap parently inconsistent w ith the results of M cD er m ott an d Roediger (1994), w h o found that an imagery study condition produced rob ust prim ing in a percep tual test. In the study phase of their experim ents, sub jects read w ords (e.g. read ``b utter¯ y’ ’ ), saw line draw ings (e.g. see a picture of a butter¯ y ), or read w ords an d imagined their referents (e.g. read ``butter¯ y’ ’ an d im agine a lin e dr aw in g of a butter¯ y). T hey also rated the p leasan tness of the w ords, drawings, an d im ages. A t test, subjects had to nam e brie¯ y presented fragm ented pictu res of the referent ob jects. P rim ing w as signi® cant in th e see condition but not in the read condition. M ore important, prim ing in the imagery cond ition w as ap proximately m idw ay between the see an d read co nditions an d w as sign i® can t (E xperiments 1, 2a, 2b, an d 4). T he image study cond ition in M cD er m ott an d Roediger’ s (1994) exp eriments w as very sim ilar to the one in E xperim ents 1a an d 1b. In b oth cases, subjects read a w ord (th e nam e of an object or the nam e of a celebrity), constructed a m en tal im age (an image of a line draw ing of the object or an image of a celebrity’ s face), an d rated the im age (a pleasan tness or vividn ess rating of the object image, or a q uality rating of th e face im age). It is therefore su rprising that an essentially iden tical imagery condition produced a co nsiderable am ount of priming in M cD erm ott an d Roediger’ s experiments bu t not in our E xperiments 1a an d 1b. O n e sim ple explanation is that line drawings of o bjects are easier to im agine than are faces o f celebrities, an d h ence subjects generated better images at encoding in M cD er m ott an d Roediger’ s exp eriments than in E xperim ents 1a an d 1b. Yet, the m ean rating in the imagery study task in E xperim ent 1a w as 3.59 an d in Exp erim ent 1b w as 3.42 (3 = good image, 4 = excellent image), w hich ind icates that subjects evaluated their ow n im ages as very good. M oreover, the sam e im agery condition yielded a rob ust am ount of prim ing in the im agery decision test. If the n atu re of the test can produce such a rem arkable chan ge on the effects of imagery, it is m ore reaso nable to attribute the inconsistency between M cD er m ott an d Roediger’ s results an d ours to differences between th e picture fragm ent identi® cation test an d the face fam iliarity decision test. T h ere are two differences th at m igh t be relevan t. First, the pictu re fragm ent identi® cation test involves nam ing, w hereas the face fam iliarity decision test d oes not. Becau se the imagine condition involves seeing a nam e, it is possible that part o f the priming generated by this cond ition in the pictu re fragm ent identi® catio n test re¯ ects verb al tran sfer. In this case, the difference between the effects of the im agine co ndition in the fam iliarity decision test an d in picture fragm ent identi® catio n test w ould not be in imaginal priming but in verb al priming. Seco nd, the test stim uli in the picture fragm ent id enti® cation test are deg rad ed, w hereas in the face fam iliarity decision test th ey are not. T he reason w hy this difference could be relevan t is that identifying fragm ented stimuli m ay req uire a m ental reconstruction of the

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fragm ents, a process that m ay be similar to imagery. T h us, it is possible that the im age co ndition produced robust prim ing in th e picture fragm ent identi® cation test b ut not in the face fam iliarity decisio n test, becau se th e ® rst test involves an important im aginal co m ponent w hereas the seco nd is m ore pu rely perceptual. T he face fam iliarity decisio n test w ould primarily involve a bottom -up activation ¯ ow, w hereas the picture fragm ent co m pletio n would involve bo th bo ttom -u p an d top-dow n ¯ ow s of infor m ation. In sum , it is possible that im agery can generate su bstantial prim ing on a perceptual test if the test involves nam ing or stim ulus deg rad ation. A ccord ingly, E xperim ent 2a exam ined imaginal prim ing in a percep tual test involving nam in g an d E xperiment 2b in a perceptual test involvin g identi® catio n of deg rad ed stim uli. T he test investig ated in E xperiment 2a w as a face identi® cation test an d similar to th e w ord id enti® cation test that is w idely used in the verb al priming dom ain: subjects have to nam e brie¯ y p resented faces of celebrities, an d the dependent variable is nam ing accuracy. T his test differs from the face fam iliarity decision test not only in nam ing, but also in that stim ulus exposure is very brief. In order to deter m ine w heth er this second difference is relevan t, exposu re tim e w as also m an ipulated (20 m sec vs. 5 0 m sec). T he test inv estig ated in Ex periment 2b w as a ver sion of the face fam iliarity decision test in w hich the test p ictures included o nly the inter nal features of th e face (eyebrow s, eyes, nose, an d m outh). T he focus of E xperim ents 2a an d 2b w as w hether im agery can produce signi® can t prim ing o n perceptual tests involving nam ing or stim ulus deg rad ation. It w as therefore necessary to introduce a co ntrol con ditio n in w h ich the nam es of celebrities are read b ut no im ages are for m ed (labelled the ``read ’ ’ co ndition).

M eth o d E igh teen Jap an ese un d erg rad u ates w ere p aid to p articip ate in E xp erim en t 2a, an d a fu rth er 18 in E xp erim ent 2b. T h e m aterials w ere th e sa m e as in E xp erim en t 1a. In bo th experim en ts, th e stu dy tasks w ere: (1) th e ``im age’ ’ tas k, u sed in E xperim en ts 1a an d 1b; an d (2) a ``re ad’ ’ task, in w h ich su bjects are asked to m ake an occu p ation d ecision to a cele br ity’ s n am e. Task (2) rep laces th e ``see’ ’ task from E xp erim ents 1a an d 1b. To keep th ese tasks as sim ilar as p ossible, celeb rity n am es w ere presen ted for 8 sec, follow ing w h ich an occu p ation d ecisio n w as m ad e. O n ce again , read /im agine p rim ing con d ition s w as a w ith in-su bjects m an ipu latio n. T h e face iden ti® catio n test (E xp erim en t 2a) includ ed th e 48 c ritical ph otog r ap hs of celebrities (16 im agine con dition , 16 read con d ition , an d 16 n o n-stu d ied ) p lus 16 ® ller p h otog rap h s of celebrities m ixed in a ran d om ord er. S u bjects w ere told th at faces of celebrities w ou ld ap p ear brie¯ y on th e screen and that th ey h ad to sa y th eir n am es, gu essing w h en n ot su re. E ach trial con sisted of a w ar n ing sign al (an oval of h eigh t 7.5 cm ) for 500 m sec, the p ictu re of a face for 20 or 50 m sec, a m ask (a 7.5cm oval ® lled w ith a h atch ing p atter n available w ith in th e g rap h ics so ftw are u sed ) for 500 m sec, an d the sen ten ce ``A n swer p lease’ ’ u n til th e resp on se w as m ad e. A ll the se even ts ap p eared in th e sam e central p osition of th e screen . S u bjects said th e n am es, and the exp erim en ter en ter ed the respo ns e in the com p u ter as eith er cor rect or incor rect. T h e inter val betw een trials w as 1.5 sec. B efore the test there w ere 4 p ractice tr ials w ith ® ller celebrities, an d th e ® rst 4 trials of th e test w ere also ® llers. T h e face fam iliarity d ecision test (E xp e rim en t 2b) w as as in E xp erim ent 1a, w ith tw o excep tion s. F irst, th e p ictu res of celebrities an d u n fam iliar faces in th e test includ e d o nly the inter nal featu res of the face. U sing a gr ap hics prog ram , an oval inclu d ing on ly the eyebro w s, eyes, n ose, an d m ou th w as cu t from each face. T h e p recise d im en sio ns of th is oval varied from face to face, bu t it w as c on -

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str u cted to be of th e sm allest siz e po ssible th at w ou ld includ e eyes, no se, an d m ou th. S econ d , in th e test each face w as p reced ed by a be ep so u n d an d rem ain ed on the screen u n til su bjects pressed th e / key (fam iliar) or th e Z key (u n fam iliar). A 1500-m sec ISI follow ed th e respo n se.

R esu lts an d D iscu ssion T he results of the face nam ing test in E xperiment 2a are show n in Table 2. T he m ain effect of exposure w as reliable, F(1, 31) = 3 9.74, p < .01, re¯ ecting overall higher accuracy in the 50-m sec cond ition th an in the 20-m sec co nditio n. T he m ain effect of the im agine± read ± no n-studied factor w as signi® can t, F(2, 62) = 3.72, p < .05, but the E xposure 3 Item Type interaction w as n ot, F < 1. Prim ing w as signi® can t in the im age co ndition, F(1, 70) = 5.69, p < .05, but not in the read condition, F(1 , 70) = 2.37, p > .1. T he difference between the image an d read conditions w as non-signi® can t, F < 1. T he accur acy an d RT data of the inter nal-feature face fam iliarity d ecision test in E xperiment 2b are show n in Table 3. In the accuracy data, there w as a signi® can t effect of the image± read ± non-studied factor, F(2, 34) = 3.2, p < .05, an d pairw ise com parisons indicated th at prim ing in th e image co ndition w as reliable, F(1, 34 ) = 3.9, p < .05, w hereas priming in the read condition w as not, F < 1. T he difference between the im age an d read conditions w as signi® cant, F(1, 34) = 5.7, p < .05. A nalysis of th e RT data prod uced a signi® cant m ain effect of the im age± read ± n on-studied factor, F(2, 34) = 5.8, p < .01. Priming w as reliable in the image co nditio n, F(1, 34) = 11.47, p < .01, but not in the read condition, F(1, 34) = 2.2, p > .1, w ith no reliable difference between them , F(1, 34) = 3.63, p > .05 . In E xperim ent 2a, the sam e im age co ndition that produced alm ost no priming in the face fam iliarity decisio n test in E xperiments 1a an d 1b yielded a signi® can t am oun t of prim ing in the face identi® cation test. P rim ing by reading a nam e w as not signi® can t, though it suggests a trend in th at direction. T hese results sugg est that nam ing a t test is affected by prior im agery, th ough this result m ay depend in pa rt o n the fact that su bjects read a nam e as part of the imagery prim in g task. In E xp eriment 2b, the image co nditio n generated a robust am oun t of prim ing in the inter nal-feature fam iliarity decision test, w hereas the sam e condition produced alm ost no prim ing in the full-face fam iliarity decisio n te st in E xperim ents 1a an d 1b. T h is difference is especially striking becau se th e two tests w ere essentially identical, w ith the o nly relevan t difference b eing that the test stim uli w ere deg rad ed in o ne case but not in the other. T his result is consistent w ith th e hypoth esis that stim ulus d eg radatio n can enhan ce im aginal TA B LE 2 M e a n A c c u ra c y o f N am in g Fa c es as a Fu n c tio n o f E n c o d in g T a sk a n d E xp o s u re o f T e st Ite m

Encoding Condition Test 20-m sec exposure 50-m sec exposure

I ma ge

R ea d

N on-studied

.32 .55

.2 9 .5 3

.24 .49

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T A B LE 3 M e a n A c c u r ac y an d R e s p o n s e La ten c ie s fo r a F am ilia rity D ec is io n T ak e n to the In te rn al F e atu res o f Fa c es

Encoding Conditions Test

I ma ge

decision accurac y reaction times

.76 141 0

Rea d

N on-studied

.66 1538

.6 8 1638

prim ing o n perceptual tests, b ecau se processing deg rad ed stimuli involves a process of m ental reconstructio n that is close to imagery operatio ns. In sum , the results of E xperim ents 2a an d 2b suggest that both nam ing an d stim ulus deg rad ation at test can enhan ce the am ount of priming obtained by prior imagery generated from a nam e. We postulate that the reasons for th is effect m ight b e different for nam ing (2a) an d stim ulus d eg rad atio n (2b). N am ing m ay receive enhan ced prim ing from imagery in part due to som e verbal prim ing, th ough this rem ains speculative. Identifying deg rad ed stimuli, o n the oth er han d , m ay receive priming from imagery b ecau se imagery is itself involved in this particular percep tual test. T he results of E xp eriments 2a an d 2b can account for the ap paren t inco nsistenc y between the effect of the im agery condition o n the face fam iliarity decisio n test in E xperim ents 1a an d 1b an d o n the pictu re fragm ent identi® cation test in M cD er m ott an d Roediger (1994), because the latter test involves both nam ing an d stim ulu s deg rad ation.

G EN E R A L D IS C U S S IO N To sum m arize the p resent research, the results of E xperim ents 1a an d 1b show ed a crossover dissociatio n between imagery an d perception o n face priming tests: perceptio n prod uced prim ing in a test involving perception, w hereas im agery generated prim ing in a test involving im agery. T he resu lts of E xperim en ts 2a an d 2b indicate that th e am ount of p rim ing generated by image g eneration from a nam e to a perceptual test increases w hen the test involves n am ing or image deg rad ation. T he presen t results have direct im plications for theoretical accounts of face priming. T he IAC m odel of Burto n et al. (1990) postulates that priming re¯ ects strengthening of the con nections between FRU s an d PIN s. T he cross-over dissociatio n foun d in E xperim ents 1a an d 1b is co nsisten t w ith the hypothesis that each F RU ± PIN pair is connected by two separate links: a bottom -up F RU ® PIN link an d a top-dow n PIN ® F RU link. Perceiving a face primarily strengthens th e bo ttom -u p FRU ® PIN link an d prod uces a co nsiderable am ount of p rim ing on tests involving the sam e link, such as the fam iliarity decision test, but little prim ing o n tests tapping th e top-dow n PIN ® FRU link. In contrast, im agining a face stren gth ens in particular the top-dow n PIN ® FRU link an d yields robu st prim in g on tasks tapping this co nnection , like the imag ery decisio n test, but little prim ing o n tasks b ased on the bottom -up FRU ® PIN link. If this interpretation is correct, there are im portant im plications for the structu re of the IAC m odel. A t present links are strengthened th rough a H eb bian up date m echan ism :

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w henever two co nnected units are simultaneously active, the strength of their connecting link is increased (Burton & Bruce, 1993; Burto n, 1994). H owever, if F RU ± P IN links are not sym m etrical, then a new m echan ism m ust be introdu ced: it is not suf® cient to know that two units are sim ultan eously active, o ne needs also to kn ow w hich b ecam e active ® rst. A lthough com putationally quite tractable, th is proposal w ill req uire extra resources not available to the m odel as it cur rently exists. T he results of E xperim ents 1a and 1b are also consistent w ith processing accounts of m em ory perfor m an ce (G raf & Ryan , 1990; M oscovitch, Vrie z en, & G oshe-G ottstein, 1993 ; Roediger, Weldon , & C hallis, 1989). T hese accounts are based on the tran sferap propriate processing principle (M or ris et al., 1977), w hich states that m em ory perfor m an ce is a functio n of the overlap between study an d test operatio ns. O n the b asis of this principle, the dissociation between the fam iliarity decisio n test an d the im agery decisio n task as an effect of the see-im age m an ipulatio n can be explained as follow s: the m ental oper ation s tap ped by the fam iliarity d ecisio n test have a larger overlap w ith the operations involved in the ``see’ ’ study task than w ith the ones eng aged by the imag ery study task, w hereas th e converse is true fo r the imagery decisio n te st. O ne problem for this explanatio n, how ever, is that M cD er m ott an d Roediger (1 994) em ployed th e sam e p rinciple to account for signi® cant prim in g effects between the im age stu dy task an d the picture fragm ent identi® catio n test. A ccording to these au thor s, these signi® can t priming effects re¯ ect the im portant overlap between imagery an d perceptio n. In fact, w e have show n that these results can be explain ed by overlap in th e particular tasks used to exam ine these ph enom en a. First, th e results of E xperim ent 2a an d 2b su ggest that a signi® can t am ount of the priming prod uced by imagery o n picture fragm ent identi® cation tests is p artly a co nsequence of nam ing at test an d partly a co nsequence of using a perceptual test designed to incor porate som e imagery. Seco nd, the idea that imagery an d p erception share com m on operatio ns an d the evidence that they can som etim es prim e each oth er does not imply that they can not be dissociated w hen they are pitted ag ainst each other. In fact, differen t types of perceptual processing (C ab eza, 1995; Weldon & Roediger, 1987) an d different types of conceptual processing (Cab eza, 1 994) can be dissociated w hen the precise discrepan cies between th em are directly m an ipulated in th e experim ent. A lthough b oth processing an d structural accounts can be brought to bear on the data presented here, we have focused on a structural interpretation, an d in particular on the IAC account of prim ing. It seem s to us that converging evidence favours a structural interp retatio n of these effects. T he resu lts of E llis et al. (1990) clearly dem onstrated that fam iliarity d ecision s cou ld be p rim ed equally w ell by a prior fam iliarity, sex, or expressio n judgem ent. H ow ever, sex or expression jud gem ents do not prime subsequent sex or ex pression judgem ents. Further, E llis, B urton, Young, an d F lude (in press) dem onstrated that recognition of a part-face primes subsequent recognitio n of a no n-overlapping partface of the sam e personÐ a result that seem s m ore naturally to favour a structural account of p erson recognition prim ing. O f course, we do not claim to have elim inate d the possibility of a processing accoun t of these p henom ena, an d data from an y individ ual experim ent are explicable in these ter m s. H ow ever, w ithin the theoretical fram ew ork of face recognitio n the co nverg ing eviden ce ap p ears to favou r stru ctural theories, an d w e have

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therefore co ncentrated on the im plications of these results for the structural th eory developed around the IAC architectu re. Finally, the presen t results are also relevan t for neuropsychological research co ncer ning impair m ents o n face p rocessing. Burto n, Young, Bruce, Johnston, an d E llis (1 991) used the IAC m odel to prop ose an explanatio n for covert recognitio n in prosopagnosia. Brie¯ y, this is a disorder in w hich a patient is unable to recogn iz e an y faces explicitly b ut can be show n to recogniz e them w hen tested imp licitly. E xam ples of im plicit tests include sem an tic prim ing an d interference effects (e.g. Young, H ellawell, & D e H aan, 1988). Burto n et al. (1991) proposed th at this set of phenom ena could be explained if the links between F RU s an d PIN s w ere attenuated. So, although the F RU becom es active, it can only pass som e of its activation on to the PIN. In particular, activation at the PIN m ay be insuf® cient to trig ger a recognition response b ut m ay nevertheless be suf® ciently large to in¯ uence other sim ultaneous processes. If the IAC account of th e data in the p resent pap er is accurate, th en the links in two directio ns (FRU ® PIN an d PIN ® FRU ) are dissociable an d could selectively be d am aged. T herefore, it should b e p ossible to ® nd patients w ho, desp ite b eing unable to recogn iz e faces, are no netheless able to im age them an d, co nversely, patients w ho are impaired in imagery but not in perception of faces. C onsistent w ith this prediction , Young, H um phreys, Riddoch, H ellaw ell, an d de H aan (1994 ) describe a p rosop agnosic patient w ho is severely impaired in face recognition but show s relatively p reserved imagery of faces. A lthough w e are unaw are of any patient w ho is impaired in face imagery but not in face recognition, if su ch a patient w ere fou nd, this w ould lend ad ditio nal support to the structur al m odel of repetitio n prim ing w e have describ ed here. In sum m ary, w e h ave argued that recog nitio n an d im agery of faces do share som e of the sam e representatio ns. In ter m s of the IAC m odel, w e have im plicated the sam e (F RU an d PIN ) represen tational units in both processes. H ow ever, w e have argued (here an d elsew here) that prim ing re¯ ects ch an ges to the connections b etw een representational units. T he experim ents described here suggest that these connectio ns m ay be separate an d dissociable in th e case of face recognition an d im agery.

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Young, A.W., H ellawell, D., & D e H aan, E.H.F. (1988) . Cross-dom ain sem an tic prim ing in nor m al sub jects an d a prosop agnos ic patient. Qua rterly J ourna l of E xperimenta l P sychology, 40A , 561± 580. Young, A.W., Hum phreys, G.W., Riddoch, M .J., Hellawell, D.J., & de Haan, E.H.F. (199 4). Recognition impair m ents and face imagery. N europsychologia , 32, 693 ± 702. Origina l ma nuscript received 1 Ma y 199 5 Accepted revision receiv ed 25 J uly 199 6