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    For fifteen yea rs, the English pa st tense ha s been

    the subject of a debat e on the na ture of langua ge

    processing. The debat e began wit h t he report of a

    connectionist model by Rumelha rt a nd McClelland [1]

    and a crit ique by P inker a nd P rince [2], and h as

    since been the subject of ma ny pa pers, conferencesan d simula tion models [37] (see a lso McClella nd

    an d P at terson in th is issue [8]).

    The past tense is of theoretical int erest because it

    embra ces tw o striking ly different phenomena.

    Regular inflection, as in walk-walkeda nd

    play-played, applies predictably t o thousands of verbs

    an d is productively generalized to neologisms such a s

    spam-spammeda nd mosh-moshed, even by preschool

    children [9]. Irr egular inflection, a s in come-camea nd

    feel-felt, applies in unpredicta ble wa ys to some

    180 verbs, and is seldom generalized; rat her, the

    regula r suffix is often overgeneralized by children to

    these irregular forms, as in holdeda nd breaked[10,11].

    A simple explanat ion is tha t irregular forms must be

    stored in memory, wherea s regular forms can be

    generated by a rule that suffixes -edto t he st em [12,13].

    Rumelhart a nd McClelland challenged tha t

    explan at ion wit h a patt ern-a ssociat or model (RMM)

    tha t learn ed to associat e phonological featu res of the

    stem w ith phonological feat ures of the past-tense

    form. It thereby acquired several h undred regular

    and irregular forms and overgeneralized -edto some

    of the irregulars.

    The past t ense has served a s one of the ma in

    empirical phenomena used to contrast the strength s

    an d wea knesses of connectionist a nd rule-based

    models of langua ge a nd cognition [8]. More genera lly,

    becau se inflections like the past tense a re simple,

    frequent, and prevalent across langua ges, and

    because the regular an d irregular varia nts can be

    equated for complexity a nd mean ing, they ha ve

    served a s a test case for issues such a s the

    neurocognitive rea lity of rules and other

    symbol-man ipulat ing operat ions a nd the

    interaction betw een storage and computat ion in

    cognitive processing [57].

    In t his art icle we defend th e side of this debate t hat

    maint ains t hat rules are indispensable for explaining

    the past t ense, and by extension, langua ge and

    cognitive processes [35,14]. We review w ha t t he

    theory does and doesnt claim , the relevant evidence,the connectionist cha llenges, and our hopes for th e

    future of the debate.

    The Words-and-Rules theory

    The Words a nd R ules (WR) theory claims tha t t he

    regula rirregula r distinction is an epiphenomenon

    of the design of the hum an langua ge faculty, in

    part icular, the distin ction betw een lexicon an d

    gram mar made in most tra ditional th eories of

    lan gua ge. The lexicon is a subdivision of memory

    conta ining (am ong other th ings) the th ousands of

    arbitra ry soundmeaning pairings that underlie the

    morphemes and sim ple words of a la ngua ge. Thegram mar is a system of productive, combinatorial

    operations tha t assemble morphemes and simple

    words int o complex words, phra ses and sent ences.

    Irregular forms ar e just w ords, acquired a nd stored

    like other words, but w ith a gram mat ical featur e like

    past tensein corporat ed into their lexica l entries.

    Regular forms, by contra st, can be productively

    generated by a rule, just like phrases an d sentences.

    A stored inflected form of a verb blocks th e application

    of the rule to tha t verb (e.g. broughtpre-empts

    bringed). Elsew here (by defa ult) the rule a pplies:

    it conca tena tes -ed wit h th e symbol V, and thus ca n

    inflect a ny w ord categorized a s a verb (see Fig. 1).

    Irr egular forms, then, do not requir e an exception

    module. They a rise because the t wo subsyst ems

    overla p in th eir expressive power: a given

    combina tion of feat ures can be expressed by words or

    rules. Thus eith er a word (irregula r) or a rule-product

    (regular) can satisfy th e demand of a synta ctic or

    semantic representa tion that a feature such as past

    tense be overtly expressed. Diachr onically, an

    irregula r is born w hen (for va rious reasons) learn ers

    memorize a complex word outright, ra ther t han

    parsing it into a stem and a n affix that codes the

    feat ure a utonomously [3].

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com 1364-6613/02/$ see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1364-6613(02)01990-3

    456 Opinion

    The past and future ofthe past tenseSteven Pinker and Michael T. Ullman

    What is the interaction between storage and computation in language

    processing? What is the psychological status of grammatical rules? What are

    the relative strengths of connectionist and symbolic models of cognition?

    How are the components of language implemented in the brain? The English

    past tense has served as an arena for debates on these issues. We defend the

    theory that irregular past-tense forms are stored in the lexicon, a division ofdeclarative memory, whereas regular forms can be computed by a

    concatenation rule, which requires the procedural system. Irregulars have the

    psychological, linguistic and neuropsychological signatures of lexical memory,

    whereas regulars often have the signatures of grammatical processing.

    Furthermore,because regular inflection is rule-driven,speakers can apply it

    whenever memory fails.

    Steven Pinker*

    Dept of Brain and

    Cognitive Sciences,

    NE20-413, MassachusettsInstitute of Technology,

    Cambridge, MA 02139,

    USA.

    *e-mail:

    [email protected]

    Michael Ullman

    Dept of Neuroscience,

    Research Building EP-04,

    Georgetown University,

    3900 Reservoir Rd, NW,

    Washington DC 20007,

    USA.

    e-mail: michael@

    georgetown.edu

    The Past-Tense Debate

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    The WR th eory contr ast s wit h classical t heories of

    genera tive phonology and t heir descendent s, such as

    those of Chomsky an d Ha lle [1517], wh ich generat e

    irregular forms by a ffixing an abstra ct morpheme to

    the stem a nd a pplying rules that a lter the stems

    phonological composition. Such t heories ar e designed

    to account for the fa ct that most irregular forms a re

    not completely a rbitrary but fa ll into families

    displaying pat terns, as in r ing-rang, sink-sank,

    sit-sat, and feel-felt, sleep-slept, bleed-bled. A problem

    for this view is tha t irregular fa milies adm it

    numerous positive and n egat ive counterexam ples

    an d borderline cases, so an y set of rules will be

    complex and la den w ith exceptions, unless it posits

    implausibly a bstract un derlying representations

    (e.g. r i nfor run, which a llows t he verb to undergo the

    same rules as sing-sang-sung).

    The theory also contrast s with the

    Rumelhart McClelland model (RMM) and other

    connectionist m odels tha t posit a single pattern

    associat or, w ith n either lexical entries nor a

    combina toria l a ppara tus [1,18,19]. The key to

    these pat t ern associa tors is that ra ther t han linking

    a word to a word stored in memory, they link sounds

    to sounds. B ecause similar words sha re sounds,

    their representa tions are part ly superimposed, and

    any associat ion formed to one is automa tically

    genera lized to the others. This allow s such models

    to acquire fa milies of similar forms more easily

    than arbitrary sets, and t o generalize the pat terns

    to new similar words. Ha ving been tra ined on

    f l ing-f lunga nd cling-clung, they may generalize

    to spling-splung(as children and adult s

    occasiona lly do [20,21]); a nd h a ving been t ra ined

    on f l ip-f l ippeda nd clip-clipped, they generalize

    to pl ip p l ipped.

    WR is descended from a third approach: the

    lexicalist theories of J ackendoff, Lieber, and oth ers,

    who recognized tha t m any morphological

    phenomena a re neither arbitra ry lists nor fully

    syst ema tic a nd productive [2225]. They posited

    lexical redunda ncy rules, w hich do not freely

    generate new forms but merely captur e patterns ofredunda ncy in the lexicon, and a llow sporadic

    generalizat ion by ana logy. Pinker and P rince

    proposed tha t lexical r edundan cy rules are n ot

    rules at a ll, but consequences of the superpositional

    nat ure of memory: similar items are ea sier to learn

    tha n arbitra ry sets, and new items resembling old

    ones tend to inherit th eir properties. They a rgued

    tha t RMMs successes ca me from implementing

    this fea tur e of memory, and proposed the WR th eory

    as a lexicalist compromise between t he generat ive

    and connectionist extremes. Irregular s a re stored

    in a lexicon w ith t he superpositional property of

    patt ern associat ors; regulars can be generat ed orparsed by rules.

    Ullma n a nd colleagues ha ve recently extended the

    WR th eory to a hy pothesis about th e neurocognitive

    substra te of lexicon and gr am ma r. According t o the

    Decla ra tive/P rocedura l (DP ) hy pothesis [5,26], lexica l

    memory is a subdivision of declara tive memory, wh ich

    stores facts, events a nd a rbitra ry rela tions [27,28].

    The consolidat ion of new declar at ive memories

    requires media l-temporal lobe structures, in

    part icular the hippocam pus. Long-term ret ention

    depends lar gely on neocortex, especially tem poral

    an d temporo-pariet al regions; other str uctures are

    importa nt for a ctively retrieving a nd sear ching for

    these memories. Gr am ma tical processing, by

    contr a st, depends on the procedura l system, wh ich

    underlies the learn ing an d contr ol of motor and

    cognitive skills, part icularly t hose involving

    sequences [27,28]. It is subserved by the ba sal

    ganglia, a nd by the frontal cortex to which they

    project in th e case of langua ge, particula rly B rocas

    area and neighboring a nterior cortical regions.

    Irregu lar forms mu st be stored in the lexical portion

    of declara tive memory; regular pa st-tense forms ca n

    be computed in the gra mma tical portion of the

    procedura l system .

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com

    457Opinion

    TRENDS in Cognitive Sciences

    Word stem (e.g. walkor hold)

    Grammatical feature (e.g. past tense)

    Lexicon

    V

    walk

    Grammar

    suffix

    -edpast

    V

    walk

    suffix

    -edpast

    V

    V V

    hold

    VX suffix

    X

    heldpast

    heldpast

    Used for:

    Form of

    computation:

    Subdivision of:

    Associated

    with:

    Principal

    substrate:

    roots, idioms, irregulars,

    some regulars

    lookup, association

    declarative memory

    words, facts

    temporo-parietal cortex

    phrases, sentences, any

    regular form

    combination, unification

    procedural system

    rules, skills

    frontal cortex, basal ganglia

    Fig. 1. Simplified illustration of the Words-and-Rules (WR) theory and the Declarative/Procedural

    (DP) hypothesis. When a word must be inflected, the lexicon and grammar are accessed in parallel.

    If an inflected form for a verb (V) exists in memory, as with irregulars (e.g. held), it will be retrieved;

    a signal indicating a match blocks the operation of the grammatical suffixation process via an

    inhibitory link from lexicon to grammar, preventing the generation ofholded. If no inflected form

    is matched, the grammatical processor concatenates the appropriate suffix with the stem,

    generating a regular form.

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    What the words-and-rules theory does not say

    The WR th eory does not litera lly posit the discret e rule

    to form t he past t ense, add -edto t he verb. All it posits

    is the past -tense morpheme -ed, a va ria ble V(included

    both in t he at ta chment conditions for -edand the

    lexica l entry of every verb), and a genera l operat ion of

    merging or unify ing const ituent s. The regula r rule

    or past -tense ruleis shorth an d for th e unificat ion

    opera tion a pplied to the past -tense m orpheme. WR is

    thus compatible with constra int- and construction-

    based theories of langua ge, as long as t hey allow for

    var iables an d combina toria l operat ions [29].

    WR does not posit tha t r egular forms ar e never

    stored, only tha t t hey do not have tobe [3,3032].

    It w ould be difficult to prohibit regula r forms from

    ever being stored, given th at huma n memory can

    acquir e man y kinds of verbal ma teria l (e.g. idioms,

    clichs, poems). WR posits a pa ra llel-ra ce model,

    like those defended for inflection by Ba ay en and

    Ca ram azza a nd by many psycholinguists for visual

    wor d recognition [3339]. Whet her a r egula r

    form is st ored, an d w hether stored regular formsar e accessed, depends on w ord-, ta sk-, an d

    speak er-specific fact ors [5,4043]. For exa mple,

    regular forms tha t constitute doublets w ith

    irregulars, such a s dived/ dovea nd dreamed/ dreamt,

    must be st ored to escape blocking by t he irregula r.

    As predicted, judgments of the na tura lness of regula r

    doublet forms show st rong effects of frequency but

    other regula r forms d o not [30]. The sa me is t rue for

    regula r forms of verbs tha t resemble irregula rs (such

    a s blinkeda nd glided), becau se the forms m ust

    overcome a par tia l blocking effect exerted by th e

    similar irregula rs [30,32]. Tasks tha t require people

    to be sensitive to the phy sical form of words (such asprogressive demasking) or to th e prior existence of

    words (such a s lexical decision), as opposed to t asks

    tha t a sk people to judge possible forms, ar e likely to

    ta p stored representa tions for medium- an d

    high -freq uency reg ula r forms [3,35,44].

    Fina lly, WR is not a chimera of a connectionist

    pat tern a ssociat or glued onto a rule system . The

    lexicon ha s superpositional properties simila r to a

    pat tern a ssociat or, but lexical entr ies ha ve structured

    seman tic, morphological, phonological an d synt actic

    representa tions of a kind not current ly implemented

    in patt ern a ssociat ors.

    Empirical tests

    The key predictions of WR a re: (1) tha t ir regula rs

    should ha ve the psychological, linguistic an d

    neuropsychological signa tur es of lexical memory,

    whereas regular s will often ha ve the signatures of

    gra mma tical processing; and (2) tha t speakers should

    apply regular inflection whenever memory fails to

    supply a form for tha t cat egory. A stored form ma y be

    una vaila ble for ma ny rea sons: low or zero frequency,

    lack of a similar form t hat could inspire an ana logy,

    ina ccessibility becau se of a words exocentric

    structu re (see below), novelty of the form in childhood,

    and various kinds of da mage t o the neurological

    substra te of lexical mem ory. The het erogeneity of

    these regula r-eliciting circumst an ces offers

    converging evidence for distin guisha ble subsystems,

    including a productive defau lt tha t does not critically

    depend on the sta tistics of pat terns in m emory.

    Here w e discuss three ty pes of evidence for a

    distinction between lookup and concat enat ion, an d

    connectionistsat tempts to provide alterna tive

    a ccount s (for r eviews , see [3,4,14,31]).

    Generalization to unusual novel words

    The RMM m odel produced odd blend s

    (mai l-membled, tri lb-treeli lt), or no out put, for n ovel

    words u nlike those in its t ra ining set [2,20]. P eople,

    by contr ast , readily a pply regular inflections to novel

    unu sua l w ords [20]. Accordin g to WR, this is beca use -ed

    can at ta ch to any word classified as a verb,

    even if dissimila r to existing stored regula rs.

    One connectionist expla na tion of th e difficulties of

    the model is tha t t hey a re specific to RMM, which is an

    ear ly modeling exercise lacking a proper phonologica lrepresenta tion, a hidden la yer, an d a proper output

    decoder. However, a pa tt ern a ssociat or remedying all

    three deficiencies a lso had t rouble genera lizing to

    unusua l words [45]. More recent models tha t a re

    claimed to solve the problem do so, tellingly, by

    implementing or presupposing a rule. For example,

    Ha re, Elman a nd Da ugherty installed a clean-up

    netw orkin which the units for -edstrengthen the

    units for a n unchanged stem vowel and inhibit the

    units for a changed vowel [46] in effect, a n inna te

    mechanism dedicated to the English past tense. Many

    recent m odels ha ve given up on genera ting pa st-tense

    forms; their output layer conta ins one unit for everypast-tense cha nge, turning inflection into a multiple-

    choice test a mong a few in na te possibilities [4749].

    To convert t he choice int o an a ctua l form, some other

    mechanism w ould have to copy the stem and a pply the

    patt ern corresponding to the selected unit. Su ch a

    mechanism is simply a rule. Marcus has a rgued that

    patt ern associat orsdifficulty in generalizing to

    dissimila r forms is rooted in t heir design [4].

    Anoth er response is to claim th a t peoples success

    at genera lization depends on certa in stat istical

    patterns tha t a lso foster generalization in pattern

    associators. Many connectionists claimed t ha t robust

    generaliza tion depends on regular forms constitut ing

    the m ajority of forms in t he childs input [50]. How ever,

    the onset a nd ra te of over-regulariza tion errors in

    children do not correlate w ith chan ges in the n umber or

    proport ion of regula r verbs used by par ents [11,51,52].

    Moreover, th ere a re regula r inflections in other

    languages, such as th e German -splural, that apply to

    a m inority of nouns (~7%), but a re genera lized like

    English r egular inflection, na mely, to unusua l nouns,

    exocentr ic nouns, a nd in childh ood [50].

    Several modelers now argue tha t it is not the

    number or proportion of regular w ords tha t is

    crucial but th eir distribut ion in phonological

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

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    458 Opinion

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    spa ce [46,48,53,54]. If irreg ula rs fa ll into cluster s

    of simila r forms (sing, r i ng, spring; grow, throw,

    blow; etc.), but regula rs a re sprinkled through

    no-ma ns-la nd, (rhumbad, oinked, etc.), one ca n

    design pa ttern associators th at devote some of their

    units a nd connections to th e no-ma ns-lan d, an d th ey

    will generalize to new unusual w ords. P utting a side

    the problem tha t most of these models ha ve their

    inflections inna tely w ired in, the models cannot dea l

    with la nguages such as Hebrew, where regular a nd

    irregular nouns are intermingled in the sa me

    phonological neighborhoods. Nonetheless, H ebrew

    regula r plural suffixes behave like -sin English and

    Germa n: speakers apply them to unusual-sounding

    a nd exocent ric noun s [55,56].

    Systematic regularization

    Some irregulars show up in regular form in certa in

    contexts, such a s ri nged th e cit y(not rang),

    grandstandeda nd low-lifes[2,57] (see B ox 1 for

    furth er examples). This shows th at sound alone

    cannot be the input t o the inflection system: a given

    input, like r i ng, can be inflected either a s rangor

    ringed, depending on some other factor.

    The phenomenon fa lls out of the gra mma tical

    mechan ism governing how complex words a re formed

    [24,50,58,59]. Gener a lly a complex Eng lish w ord

    inherits its feat ures from its rightm ost morpheme,

    its head. For exam ple, the hea d of overeatis eat;

    therefore, overeatis a verb (it inherits t he Vcategory

    of eat), it refers to a kin d of eating (beca use it inherit s

    the semant ic feat ures of eat), and it has t he irregular

    past-tense overate(because it inherit s the st ored

    past -tense form of eat) (see F ig. 2).

    B ut th ere is a sm all fam ily of exceptions: head less

    (exocentric) words, w hich for va rious reasons ca nnot

    get their feat ures from t heir rightmost morpheme.

    For exam ple, unlike endocentr ic verbs such a s

    overeat-overatea nd outdo-outdid, which are verbs

    based on verbs, to r in ga nd to grandstandare verbs

    based on nouns (a r ing, a grandstand). In forming or

    parsing t he w ord, the head-inheritance mechanism

    must be circumvented. With tha t da ta pathw ay

    plugged, there is no wa y for the irregular forms rang

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com

    459Opinion

    An intriguing aspect of inflection is that irregular forms cansometimes turn up in regular form. Some of these regularizations areunsystematic for example, doublets such asdived/doveanddreamt/dreamed, in which the regular form is used sporadically becausethe irregular form is low in frequency and hence poorly remembered.But many are systematic: in particular contexts, the regular form isconsistently used, such as ringed the cityand low-lifes.

    The Words-and-Rules theory explains this phenomenon using anindependently motivated theory of compositionality in word-formation[a,b] (see also Fig. 2 in main article). Irregular-sounding words areregularized if they lack a root in head position that can be marked for theinflectional feature (tense or number). The regular suffix applies as thedefault, as it does in other cases where memory access is disabled.This neatly explains a diverse set of systematic regularizations foundin actual usages, laboratory experiments with adults and children,and many languages [cf]:

    The word lacks a noun or a verb root

    onomatopoeia:dinged, pinged, zinged, peeped, beeped quotations: I found threemans on page 1; We to bed and not to bed

    in this room names: the Julia Childs, the Thomas Manns, the Shelby Footes truncations:synched, sysmans unassimilated borrowings: talismans, mongooses

    The root cannot be marked for the feature

    verbs with noun or adjective roots: ringed the city, steeled myself,spitted the pig, bared his soul, righted the boat, stringed the peas

    nouns with verb roots: a few loafs(episodes of loafing), a couple ofwolfs(wolfing down food)

    The words structure is exocentric

    verbs based on nouns based on verbs: grandstanded, flied out, costedout the grant, encasted his leg

    nouns based on names based on nouns: Mickey Mouses(simpletons),Renault Elfs, Top Shelfs(frozen food), Seawolfs(aircraft), TorontoMaple Leafs

    nouns whose referents are distinct from those of their roots: low-lifes,still lifes, sabre-tooths, Walkmans, tenderfoots

    nouns based on phrases: Bag-A-Leafs, Shear-A-Sheeps

    Although the meaning of the regularized forms differs from that of theirirregular counterparts, regularization is rarely triggered by differences in

    semantic features alone, as connectionists sometimes suggest [g,h].If an irregular-sounding word changes in meaning, but retains a rootin head position, it stays irregular, no matter how radical the changeor opaque the metaphor: compositional prefixing: overate, overshot, undid, preshrank,

    remade, outsold non-compositional prefixing: overcame, understood, withdrew,

    beheld, withstood, undertook compounding: bogeymen, superwomen, muskoxen, stepchildren,

    milkteeth

    metaphors:straw men, chessmen, snowmen, sawteeth, metrical feet,six feet tall, brainchildren, children of a lesser god, beewolves,wolves in sheeps clothing

    idioms: went out with (dated), went nuts(demented), went in for(chose),went off(exploded), went off(spoiled);took in(swindled), took off(launched), took in(welcomed), took over(usurped), took up, (commenced), took a leak(urinated), took a bath(lost money), took a bath (bathed), took a walk(walked);blew over(ended), blew away(assassinated), blew away(impressed),blew up(exploded), blew up(inflated), blew off(dismissed),blew in(arrived)

    [scores of other examples with come, do, have, get, set, put, stand,throw, etc.]

    Referencesa Williams, E. (1981) On the notions of lexically rela teda nd head of a w ord.

    Li nguist. Inq. 12, 245274

    b Selkirk, E.O. (1982)Th e Synt ax of Words, MIT Press

    c Pinker, S. (1999)Words and Rul es: The Ingredients of L anguage,

    HarperCollins

    d Kim, J .J . et a l. (1994) Sensitivity of childrens inflection to morphological

    structure. J. Chi ld Lang. 21, 173209

    e Marcus, G.F. et al . (1995) German inflection: the exception tha t proves t he

    rule. Cogn. Psychol. 29, 189256

    f Beren t , I . et al . (1999) Defa ult nomin al inflection in H ebrew : evidence for

    mental va riables. Cognition72, 144

    g Daugherty, K.G. et al . (1993) Why no mere mort al ha s ever flown out t o

    center field but people often say they do. In 15th A nnu. Conf. Cogn. Sci. Soc.,

    Erlbaum

    h Har ris, C.L. (1992) Understanding E nglish past-tense formation: the

    shared mean ing hypothesis. In Proc. 14th An nu. Conf. Cogn. Sci. Soc.,Erlbaum

    Box 1. Systematic regularization

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    or stoodto percolat e up from the ent ries for r i ngor

    stand. With t he irregula r form sealed in m emory, the

    suffixation rule steps in as the default , yielding ringed

    a nd grandstanded. Man y examples, involving diverse

    constr uctions from severa l langua ge families, ha ve

    been documented from nat ura listic sources and

    experiment ally elicited from children an d adult s

    [3,50,60,61]. Appar ent counter exam ples exist ,

    but virtua lly all can independently be shown to be

    cases wh ere people do not a ssign a n exocentric

    structu re t o the w ord [3,60].

    There ha ve been th ree connectionist explan at ions.

    One is that if a pattern a ssociator had semant ic as w ell

    as phonological input units, a complex word with an

    altered mean ing would dilute the associations to

    irregula r forms, fa voring th e competing r egular [62,63].

    However, in almost every case in which an irregular

    words mean ing changes, the irregular form is in fact

    retained, such a s meta phors (straw men/ *m ans,

    sawteeth , Gods chil dr en) an d idioms (cut/ *cutted a

    deal, took a leak, hi t the fan, put th em down) [2,3,50].

    Accordingly, experiments ha ve shown th at just

    changing th e meaning of an irregular verb does not

    cause people to swit ch to t he regu lar [60,61]. Although

    all complex and derived w ords are sema ntically

    different from their bases, wh en semant ic similarity

    an d exocentric structur e are unconfounded in a

    regression, exocentric stru cture a ccounts for a

    significant proport ion of th e var ian ce in choice of

    inflectional form, an d sema nt ic simila rity d oes not [60].

    Equa lly unpromising is t he suggestion t hat people

    regula rize words t o avoid am biguity [6365]. Man y

    idioms a re ambiguous betw een literal a nd idiomatic

    senses, such a s bought t he farma nd threw i t up, or

    among different idiomatic senses a s w ell, such as

    blew away(impressed, assa ssina ted), but t his does

    not lead people to switch to a regular to disambiguat e

    one of them (buyed t he farm, thr owed u p). Conversely,

    grandstooda nd low-livesare una mbiguous,but people still find them ungram ma tical.

    One connectionist model add ed nodes representin g

    the semant ic similarity of the verb to the

    homophonous noun (e.g. to r i nga nd a r ing) [64].

    The netw ork can then be tra ined to have th ese nodes

    turn off irregular patt erns and tur n on the regular

    one. But these unusual nodes are not part of the

    semantic representat ion of a verb itself; they a re an

    explicit encoding of t he verbs rela tion t o the noun

    tha t heads ittha t is, a crude implementation of

    morphological stru cture. In a ddition, the modelers

    had to train t he network on regular past tenses of

    denomina l verbs homophonous wit h irregula rs.But such homophones a re virtually a bsent from

    speech a ddressed t o children, w ho nonetheless tend

    to regu la rize exocentr ic form s [61].

    Neuropsychological dissociations

    According to WR an d DP, da ma ge to the neura l

    substra te for lexical memory should cau se a great er

    impairment of irregular forms (and a ny regular forms

    tha t a re dependent on memory storage), and a

    diminut ion of the tendency t o ana logize novel

    irregula r-sounding forms according t o stored pat terns

    (as in spling-splung). In comparison, dam ag e to the

    substrate for gra mma tical combination should cause

    a greater impairment of the use of the rule in regular

    forms, and of its genera lizat ion to novel forms.

    Anomia is a n impairment in w ord finding often

    associated w ith da ma ge to left tempora l/temporo-

    pariet al regions (see Fig. 3a ). Pa tient s often produce

    fluent a nd la rgely gramm at ical speech, suggesting

    tha t the lexicon is more impaired than gra mma tical

    combina tion [66]. In elicited pas t-tense production

    ta sks, patients (compared w ith controls) do worse with

    irregular tha n w ith regula r verbs (Fig. 3b), produce

    regulariza tion errors like swimmed(which occur wh en

    no memorized form comes to mind a nd t he rule a pplies

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com

    460 Opinion

    TRENDS in Cognitive Sciences

    (a)

    (c)

    (b)

    (d)

    N

    work

    V

    --- men plural

    --- men plural

    man

    N

    ---

    ---V

    over-

    prefix

    --- atepast tense

    eat

    V

    ---

    ---

    --- atepast tense

    V|

    fly

    N|

    V|

    -ed

    N

    |

    life

    A

    |

    low

    N

    -s

    Fig. 2. Systematic regularization. Complex words are assembled out ofsimple morphemes according to a righthand-head rule: the rightmost

    morpheme, the head, contributes its syntactic, semantic and

    morphological properties to the word as a whole. Thus in (a), the

    combination ofover-and eatis a verb, because its head (circled), eat,

    is a verb (V); its meaning is a kind of eating (eating too much), because

    that is the meaning ofeat, and its past-tense form is overate, because

    the irregular past-tense ofeatis ate. All three kinds of information

    percolate up from the lexical entry for the head in memory along the

    rightmost edge of the words tree structure (thick arrows). Similarly in

    (b), the combination ofworkand manis a noun (N), it refers to a kind of

    man, and its plural is workmen, the result of its inheriting all three

    properties from its head, man. However, a handful of derived words in

    English (headless or exocentric words) have to disable this inheritance

    mechanism. A low-life(c) is not a kind of life (in the way a workmanis a

    kind of man) but a person who has a low li fe; for the word to work this

    way the usual data pipeline has to be blocked (depicted by the no entry

    sign). This leaves the irregular plural form (lives), trapped in memory,and the regular suffix -sapplies as the default. The baseball term to fly

    out(d) comes from the noun a fly(as in an infield fly), which itself came

    from the simple verb root to fly(at the bottom of the tree). The words

    structure requires the inheritance mechanism to be blocked twice: to

    allow the verb root flyto be converted to the noun (because verbs

    ordinarily beget verbs, not nouns) and again to allow the noun to be

    converted back into a verb (because nouns ordinarily beget nouns).

    The irregular past-tense forms flewand flownare sealed in memory,

    and -edis suffixed as the default, generating flied out.

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    as t he default), rarely a nalogize irregular patt erns to

    novel words (e.g. spling-splung), an d a re relatively

    unimpaired at generating novel regular forms like

    plammed[26,67,68]. Agramm at ism, by contr ast ,

    is an impairment in producing fluent gramm at ical

    sequences, and is a ssociat ed with dam age to a nterior

    perisylv ia n reg ions of the left h emisph ere [69,70]. As

    predicted, agra mma tic patients show the opposite

    patt ern: more trouble inflecting regular t ha n irregular

    verbs, a la ck of errors like swimmed, and great difficulty

    suffixing novel w ords [26,67]. S imila r effects ha ve been

    documented in rea ding a loud, writing to dictation,

    repeat ing a nd judging w ords (even when controlling for

    frequency a nd lengt h) [67],an d in a regula r/irregula r

    contr ast w ith J apa nese-speaking patient s [71].

    The predicted double dissociat ion patt erns a re also

    seen in a comparison of neurodegenerat ive diseases.

    Alzheimers disease (AD) is ma rked by gr eat er

    degenera tion of media l and n eocortical temporal lobe

    structu res tha n of fronta l cortex (part icularly B rocas

    area ) and t he basa l ganglia, and greater impairment

    of lexical an d conceptual know ledge tha n of motor

    an d cognitive skills, including aspects of gram ma tical

    processing [72]. P ar kinsons d isease (P D), a ssociat ed

    with ba sal gan glia degenerat ion, is marked by

    grea ter impa irment of motor and cognitive skills

    (including gra mma tical processing) tha n use of words

    an d fa cts [72,73]. As predicted, AD pat ients h ave

    more trouble inflecting irregular t han regular verbs,

    are r elatively unimpaired a t suffixing novel words,

    generate few irregular a nalogies for novel w ords,

    an d produce over-regula riza tion errors; P D pat ients

    show t he contra sting pat terns [26,32]. Moreover, the

    performance patterns correlate w ith t he severity of

    the associated processing impairm ents in the

    tw o populations: anomia in AD, a nd right-side

    hypokinesia (an ind ex of left-hemisphere basa l

    ga nglia degenera tion) in PD [26,32].

    Int riguingly, Huntin gtons Disease (HD), cau sed

    by degeneration of different ba sal ga nglia structures,results in disinhibition of the projected fronta l area s,

    leading to unsupressible movements [73]. When H D

    patients inflect verbs, they show a t hird patt ern:

    producing extra suffixes for regular a nd novel words

    like wa l keded, pl aggededa nd dugged, but not

    an alogous errors on irregulars like dugugor keptet

    suggesting that these errors a re instances of

    unsuppressed r egular suffixat ion [26,32].

    Converging find ings come from other

    methodologies. In norma l subjects, both r egular

    and irregular inflected forms can prime their stems.

    By hypothesis, a regular form is parsed into affix

    an d stem (which primes itself); an irregula r form isassociated w ith its stem, somewha t like sema ntic

    priming. Pa tients with left inferior frontal da mage do

    not show regular priming (walked-walk), alt hough

    they retain irregular priming (found-f ind) an d

    semantic priming (swan-goose). Apatient w ith

    temporal-lobe dama ge showed the opposite pat tern

    [68,74,75]. In studies tha t ha ve recorded event-

    related potentials (ERP s) to printed w ords, when a

    regula r suffix is placed on an irregula r word (e.g. the

    German Muskels) or omitted w here it is obligatory

    (e.g. Yester da y I walk), the electr ophysiological

    response is similar to the Left Anterior Negativit y

    (LAN) commonly seen w ith sy nta ctic violat ions.

    When irr egular inflection is illicitly a pplied (e.g. the

    German Karusellen) or omitt ed (e.g. Yester da y I dig),

    the response is a central n egativity similar t o the

    N400 elicited by lexical an omalies, including

    pronouncea ble non-words [40,7679]. This sugg ests

    tha t t he brain processes regular forms like synta ctic

    combina tions and irregula r forms like words.

    Double dissociations a re difficult to explain in

    patt ern a ssociat ors, because except for art ificially

    sma ll networks, lesioningt he networks hurt s

    irregular forms more tha n regula r ones [80]. Arecent

    interesting model by J oanisse and Seidenberg

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com

    461Opinion

    TRENDS in Cognitive Sciences

    20%

    98%

    Agrammaticnon-fluent aphasic

    FCL

    Control subjects Control subjectsAnomicfluent aphasic

    JLU

    90%

    99%

    Irregular (e.g.dug)

    Regular (e.g.

    looked) or over-

    regularized(e.g.digged)

    69%

    96%

    1%

    63%

    19%

    98%

    100

    80

    60

    %c

    or

    rect

    40

    20

    0

    (a)

    (b)

    Fig. 3. Dissociating regular and irregular processing in aphasia. (a) The approximate lesion sites of

    patient FCL (red area, left anterior perisylvian regions), who had symptoms of agrammatism, and patient

    JLU (green area, left temporo-parietal region), who had symptoms of anomia. (b) Results of verb-

    inflection tests showed that the agrammatic patient had more trouble inflecting regular verbs (lighter

    bars) than irregular verbs (darker bars), whereas the anomic patient had more trouble inflecting irregular

    verbs and overapplied the regular suffix to many of the irregulars (light green bar on top of dark green

    bar). The performance of age- and education-matched control subjects is shown in the grey bars.

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    conceded tha t dist inct subsystems h ave t o be lesioned

    to produce double dissociat ions [81]. Although th ey

    called th ese modules phonological an d seman tic,

    the sema ntic module wa s in fact a lexicon: it ha d one

    unit dedicated to each word, with no representa tion

    of meaning. The finding tha t lesioning a lexicon

    differentially impairs irregular in flection is exactly

    wha t WR predicts. Moreover, the model failed to

    duplicate the finding that a gramma tic pat ients have

    more trouble wit h regula r tha n irregula r verbs [26,67].

    Lesioning the phonology module caused a consistent

    selective deficit only w ith n ovel verbs; regulars were

    no harder th an irr egulars. The report a lso claims tha t

    because a novel form ha s no meaning, the only wa y

    to generate its past t ense is by ana logy to known

    phonological forms [81]. This predicts t ha t pa tient

    groups should ha ve para llel tendencies to generalize

    regular a nd irregula r inflection to novel words

    (plammeda nd splung, respectively), w hereas in fa ct

    th ese tenden cies dissocia te [32,67]. Fina lly, the m odel

    predicts th at selective difficulty w ith irregula r forms

    should depend on sema ntic deficits. Miozzo reports anan omic pat ient wh o had difficulty accessing word

    forms but not word mean ings; nonetheless, he had

    trouble with irr egulars but not wit h regula rs [82].

    The future of the past-tense debate

    The Rumelha rtMcClellan d model wa s deservedly

    influentia l, we believe, becau se it capt ured a rea l

    phenomenon. The persistence of fam ilies of irregular

    verbs with overlapping part ial similarities, and

    peoples use a nd occasional genera lizat ion of these

    family pat terns a ccording to similarity and frequency,

    can be simply explained by t he assumption tha t

    human memory is partly superpositional a ndassociative. Theories that try to explain every

    instance of redunda ncy among w ords using the

    sam e combinat orial mechanism used for productive

    synta x a nd r egular morphology require needless

    complexity an d esoteric representa tions, and fa il to

    capture the ma ny linguistic, psychological and

    neuropsychological phenomena in wh ich irregular

    forms behave like words.

    At t he sa me tim e, the post-RMM connectionist

    models have revealed the problems in try ing to

    explain al llinguistic phenomena with a single

    pat tern-associat or architecture. Ea ch model has been

    ta ilored to account for one phenomenon explained

    by th e WR th eory; unlike RMM, few models a ccount

    for more tha n one phenomenon or predict new ones.

    And m odelers repea tedly build in or presuppose

    surroga tes for the linguistic phenomena t hey claim

    to eschew, such a s lexical items, m orphological

    structu re an d concat enat ion operat ions. We predict

    tha t th e need for structured representat ions an d

    combina toria l operat ions would assert itself even

    more strongly if m odelers included phenomena

    tha t a re currently ignored in current simulat ions,

    such a s synta x and its interaction with inflection,

    the ma ssively productive combina torial inflection of

    polysynt hetic la ngua ges, an d the psychological

    events concealed by providing t he models wit h correct

    past -tense forms dur ing tr ain ing (i.e. childrens

    ability t o recognize an in put as a past -tense form,

    retrieve its st em from memory, compute t heir ownform, and compare th e two).

    As an increasing number of linguistic and

    neuropsychological phenomena a re a ddressed,

    especially the complex data from neuroimaging,

    inad equa cies will no doubt be revealed in both kinds of

    models. Nothing in linguistics prevents t heories from

    appealing t o richer conceptions of memory t ha n simple

    rote storage. Neither does neural netw ork modeling

    prohibit structured or abstra ct representa tions,

    combina torial operations, and subsystems for

    different kinds of computa tion. The ad versaria l

    na ture of scientific debate might sometimes have

    prevented both sides from acknowledging tha tfeatur es of one model ma y correspond to constructs

    of the other, described a t a different level of an alysis.

    We suspect tha t a llowing a full range of data to tell us

    wh ich processes are most nat ura lly explained by

    wh ich kinds of mecha nisms, rat her tha n shoehorning

    all phenomena into a single mechanism fa vored by one

    or another cam p, holds the best hope for a n eventua l

    resolution of the past-tense debate.

    TRENDS in Cognitive Sciences Vol.6 No.11 November 2002

    http://tics.trends.com

    462 Opinion

    Acknowledgements

    We dedicate this paper to

    David Rumelhart and

    JayMcClelland, in

    acknowledgement of

    the stimulating effect of

    their model on

    psycholinguistic and

    neurolinguistic research.S.P. was supported by NIH

    grant HD-18381. M.T.U.

    was supported by a

    McDonnell-Pew grant in

    Cognitive Neuroscience,

    NSF SBR-9905273, NIH

    MH58189, and Army

    DAMD-1793-V-

    3018/3019/3020 and

    DAMD-17992-9007.

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