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7/27/2019 Pinker - The Past-Tense Debate
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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:
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
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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
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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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